Fix tests.

Add ImageActivity to show image attachment in full-screen

See merge request briar/briar!999

bramble-api/src/main/java/org/briarproject/bramble/api/io/BlockSink.java

@@ -0,0 +1,20 @@
+package org.briarproject.bramble.api.io;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
+
+import java.util.List;
+
+public interface BlockSink {
+
+	/**
+	 * Stores a block of the message with the given temporary ID.
+	 */
+	void putBlock(HashingId h, int blockNumber, byte[] data) throws DbException;
+
+	/**
+	 * Sets the hash tree path of a previously stored block.
+	 */
+	void setPath(HashingId h, int blockNumber, List<TreeHash> path)
+			throws DbException;
+}

bramble-api/src/main/java/org/briarproject/bramble/api/io/HashingId.java

@@ -0,0 +1,27 @@
+package org.briarproject.bramble.api.io;
+
+import org.briarproject.bramble.api.UniqueId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Type-safe wrapper for a byte array that uniquely identifies a
+ * {@link Message} while it's being hashed and the {@link MessageId} is not
+ * yet known.
+ */
+@ThreadSafe
+@NotNullByDefault
+public class HashingId extends UniqueId {
+
+	public HashingId(byte[] id) {
+		super(id);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		return o instanceof HashingId && super.equals(o);
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/MessageFactory.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.api.sync;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
 
 @NotNullByDefault
 public interface MessageFactory {
@@ -10,4 +11,6 @@ public interface MessageFactory {
 	Message createMessage(byte[] raw);
 
 	byte[] getRawMessage(Message m);
+
+	MessageId getMessageId(GroupId g, long timestamp, TreeHash rootHash);
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/MessageId.java

@@ -24,6 +24,12 @@ public class MessageId extends UniqueId {
 	public static final String BLOCK_LABEL =
 			"org.briarproject.bramble/MESSAGE_BLOCK";
 
+	/**
+	 * Label for hashing two tree hashes to produce a parent.
+	 */
+	public static final String TREE_LABEL =
+			"org.briarproject.bramble/MESSAGE_TREE";
+
 	public MessageId(byte[] id) {
 		super(id);
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncConstants.java

@@ -35,4 +35,9 @@ public interface SyncConstants {
 	 * The maximum number of message IDs in an ack, offer or request record.
 	 */
 	int MAX_MESSAGE_IDS = MAX_RECORD_PAYLOAD_BYTES / UniqueId.LENGTH;
+
+	/**
+	 * The maximum length of a message block in bytes.
+	 */
+	int MAX_BLOCK_LENGTH = 32 * 2014; // 32 KiB
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class LeafNode extends TreeNode {
+
+	public LeafNode(TreeHash hash, int blockNumber) {
+		super(hash, 0, blockNumber, blockNumber);
+	}
+
+	@Override
+	public TreeNode getLeftChild() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public TreeNode getRightChild() {
+		throw new UnsupportedOperationException();
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java

@@ -0,0 +1,26 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class ParentNode extends TreeNode {
+
+	private final TreeNode left, right;
+
+	public ParentNode(TreeHash hash, TreeNode left, TreeNode right) {
+		super(hash, Math.max(left.getHeight(), right.getHeight()) + 1,
+				left.getFirstBlockNumber(), right.getLastBlockNumber());
+		this.left = left;
+		this.right = right;
+	}
+
+	@Override
+	public TreeNode getLeftChild() {
+		return left;
+	}
+
+	@Override
+	public TreeNode getRightChild() {
+		return right;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/StreamHasher.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.io.BlockSink;
+import org.briarproject.bramble.api.io.HashingId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+@NotNullByDefault
+public interface StreamHasher {
+
+	/**
+	 * Reads the given input stream, divides the data into blocks, stores
+	 * the blocks and the resulting hash tree using the given block sink and
+	 * temporary ID, and returns the hash tree.
+	 */
+	TreeNode hash(InputStream in, BlockSink sink, HashingId h)
+			throws IOException, DbException;
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeHash.java

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.UniqueId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Type-safe wrapper for a byte array that uniquely identifies a sequence of
+ * one or more message blocks.
+ */
+@ThreadSafe
+@NotNullByDefault
+public class TreeHash extends UniqueId {
+
+	public TreeHash(byte[] id) {
+		super(id);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		return o instanceof TreeHash && super.equals(o);
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeHasher.java

@@ -0,0 +1,11 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public interface TreeHasher {
+
+	LeafNode hashBlock(int blockNumber, byte[] data);
+
+	ParentNode mergeTrees(TreeNode left, TreeNode right);
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java

@@ -0,0 +1,38 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public abstract class TreeNode {
+
+	private final TreeHash hash;
+	private final int height, firstBlockNumber, lastBlockNumber;
+
+	TreeNode(TreeHash hash, int height, int firstBlockNumber,
+			int lastBlockNumber) {
+		this.hash = hash;
+		this.height = height;
+		this.firstBlockNumber = firstBlockNumber;
+		this.lastBlockNumber = lastBlockNumber;
+	}
+
+	public TreeHash getHash() {
+		return hash;
+	}
+
+	public int getHeight() {
+		return height;
+	}
+
+	public int getFirstBlockNumber() {
+		return firstBlockNumber;
+	}
+
+	public int getLastBlockNumber() {
+		return lastBlockNumber;
+	}
+
+	public abstract TreeNode getLeftChild();
+
+	public abstract TreeNode getRightChild();
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/MessageFactoryImpl.java

@@ -7,6 +7,7 @@ import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageFactory;
 import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
 import org.briarproject.bramble.util.ByteUtils;
 
 import javax.annotation.concurrent.Immutable;
@@ -39,13 +40,19 @@ class MessageFactoryImpl implements MessageFactory {
 		if (body.length == 0) throw new IllegalArgumentException();
 		if (body.length > MAX_MESSAGE_BODY_LENGTH)
 			throw new IllegalArgumentException();
-		MessageId id = getMessageId(g, timestamp, body);
+		MessageId id = getMessageIdFromBody(g, timestamp, body);
 		return new Message(id, g, timestamp, body);
 	}
 
-	private MessageId getMessageId(GroupId g, long timestamp, byte[] body) {
+	private MessageId getMessageIdFromBody(GroupId g, long timestamp,
+			byte[] body) {
 		// There's only one block, so the root hash is the hash of the block
 		byte[] rootHash = crypto.hash(BLOCK_LABEL, FORMAT_VERSION_BYTES, body);
+		return getMessageIdFromRootHash(g, timestamp, rootHash);
+	}
+
+	private MessageId getMessageIdFromRootHash(GroupId g, long timestamp,
+			byte[] rootHash) {
 		byte[] timeBytes = new byte[INT_64_BYTES];
 		ByteUtils.writeUint64(timestamp, timeBytes, 0);
 		byte[] idHash = crypto.hash(ID_LABEL, FORMAT_VERSION_BYTES,
@@ -65,7 +72,7 @@ class MessageFactoryImpl implements MessageFactory {
 		long timestamp = ByteUtils.readUint64(raw, UniqueId.LENGTH);
 		byte[] body = new byte[raw.length - MESSAGE_HEADER_LENGTH];
 		System.arraycopy(raw, MESSAGE_HEADER_LENGTH, body, 0, body.length);
-		MessageId id = getMessageId(g, timestamp, body);
+		MessageId id = getMessageIdFromBody(g, timestamp, body);
 		return new Message(id, g, timestamp, body);
 	}
 
@@ -78,4 +85,10 @@ class MessageFactoryImpl implements MessageFactory {
 		System.arraycopy(body, 0, raw, MESSAGE_HEADER_LENGTH, body.length);
 		return raw;
 	}
+
+	@Override
+	public MessageId getMessageId(GroupId g, long timestamp,
+			TreeHash rootHash) {
+		return getMessageIdFromRootHash(g, timestamp, rootHash.getBytes());
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/sync/tree/HashTree.java

@@ -0,0 +1,16 @@
+package org.briarproject.bramble.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.tree.LeafNode;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
+
+import javax.annotation.concurrent.NotThreadSafe;
+
+@NotThreadSafe
+@NotNullByDefault
+interface HashTree {
+
+	void addLeaf(LeafNode leaf);
+
+	TreeNode getRoot();
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/tree/HashTreeImpl.java

@@ -0,0 +1,46 @@
+package org.briarproject.bramble.sync.tree;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.tree.LeafNode;
+import org.briarproject.bramble.api.sync.tree.TreeHasher;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
+
+import java.util.Deque;
+import java.util.LinkedList;
+
+import javax.inject.Inject;
+
+@NotNullByDefault
+class HashTreeImpl implements HashTree {
+
+	private final TreeHasher treeHasher;
+	private final Deque<TreeNode> nodes = new LinkedList<>();
+
+	@Inject
+	HashTreeImpl(TreeHasher treeHasher) {
+		this.treeHasher = treeHasher;
+	}
+
+	@Override
+	public void addLeaf(LeafNode leaf) {
+		TreeNode add = leaf;
+		int height = leaf.getHeight();
+		TreeNode last = nodes.peekLast();
+		while (last != null && last.getHeight() == height) {
+			add = treeHasher.mergeTrees(last, add);
+			height = add.getHeight();
+			nodes.removeLast();
+			last = nodes.peekLast();
+		}
+		nodes.addLast(add);
+	}
+
+	@Override
+	public TreeNode getRoot() {
+		TreeNode root = nodes.removeLast();
+		while (!nodes.isEmpty()) {
+			root = treeHasher.mergeTrees(nodes.removeLast(), root);
+		}
+		return root;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/tree/StreamHasherImpl.java

@@ -0,0 +1,85 @@
+package org.briarproject.bramble.sync.tree;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.io.BlockSink;
+import org.briarproject.bramble.api.io.HashingId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.tree.StreamHasher;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
+import org.briarproject.bramble.api.sync.tree.TreeHasher;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.LinkedList;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+import javax.inject.Provider;
+
+import static java.util.Arrays.copyOfRange;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_BLOCK_LENGTH;
+
+@Immutable
+@NotNullByDefault
+class StreamHasherImpl implements StreamHasher {
+
+	private final TreeHasher treeHasher;
+	private final Provider<HashTree> hashTreeProvider;
+
+	@Inject
+	StreamHasherImpl(TreeHasher treeHasher,
+			Provider<HashTree> hashTreeProvider) {
+		this.treeHasher = treeHasher;
+		this.hashTreeProvider = hashTreeProvider;
+	}
+
+	@Override
+	public TreeNode hash(InputStream in, BlockSink sink, HashingId h)
+			throws IOException, DbException {
+		HashTree tree = hashTreeProvider.get();
+		byte[] block = new byte[MAX_BLOCK_LENGTH];
+		int read;
+		for (int blockNumber = 0; (read = read(in, block)) > 0; blockNumber++) {
+			byte[] data;
+			if (read == block.length) data = block;
+			else data = copyOfRange(block, 0, read);
+			sink.putBlock(h, blockNumber, data);
+			tree.addLeaf(treeHasher.hashBlock(blockNumber, data));
+		}
+		TreeNode root = tree.getRoot();
+		setPaths(sink, h, root, new LinkedList<>());
+		return root;
+	}
+
+	/**
+	 * Reads a block from the given input stream and returns the number of
+	 * bytes read, or 0 if no bytes were read before reaching the end of the
+	 * stream.
+	 */
+	private int read(InputStream in, byte[] block) throws IOException {
+		int offset = 0;
+		while (offset < block.length) {
+			int read = in.read(block, offset, block.length - offset);
+			if (read == -1) return offset;
+			offset += read;
+		}
+		return offset;
+	}
+
+	private void setPaths(BlockSink sink, HashingId h, TreeNode node,
+			LinkedList<TreeHash> path) throws DbException {
+		if (node.getHeight() == 0) {
+			// We've reached a leaf - store the path
+			sink.setPath(h, node.getFirstBlockNumber(), path);
+		} else {
+			// Add the right child's hash to the path and traverse the left
+			path.addFirst(node.getRightChild().getHash());
+			setPaths(sink, h, node.getLeftChild(), path);
+			// Add the left child's hash to the path and traverse the right
+			path.removeFirst();
+			path.addFirst(node.getLeftChild().getHash());
+			setPaths(sink, h, node.getRightChild(), path);
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/tree/TreeHasherImpl.java

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.sync.tree;
+
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.tree.LeafNode;
+import org.briarproject.bramble.api.sync.tree.ParentNode;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
+import org.briarproject.bramble.api.sync.tree.TreeHasher;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.sync.Message.FORMAT_VERSION;
+import static org.briarproject.bramble.api.sync.MessageId.BLOCK_LABEL;
+import static org.briarproject.bramble.api.sync.MessageId.TREE_LABEL;
+
+@Immutable
+@NotNullByDefault
+class TreeHasherImpl implements TreeHasher {
+
+	private static final byte[] FORMAT_VERSION_BYTES =
+			new byte[] {FORMAT_VERSION};
+
+	private final CryptoComponent crypto;
+
+	@Inject
+	TreeHasherImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public LeafNode hashBlock(int blockNumber, byte[] data) {
+		byte[] hash = crypto.hash(BLOCK_LABEL, FORMAT_VERSION_BYTES, data);
+		return new LeafNode(new TreeHash(hash), blockNumber);
+	}
+
+	@Override
+	public ParentNode mergeTrees(TreeNode left, TreeNode right) {
+		byte[] hash = crypto.hash(TREE_LABEL, FORMAT_VERSION_BYTES,
+				left.getHash().getBytes(), right.getHash().getBytes());
+		return new ParentNode(new TreeHash(hash), left, right);
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/test/TestMessageFactory.java

@@ -4,6 +4,8 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageFactory;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
 
 import static org.briarproject.bramble.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
 
@@ -27,4 +29,10 @@ public class TestMessageFactory implements MessageFactory {
 		System.arraycopy(body, 0, raw, MESSAGE_HEADER_LENGTH, body.length);
 		return raw;
 	}
+
+	@Override
+	public MessageId getMessageId(GroupId g, long timestamp,
+			TreeHash rootHash) {
+		throw new UnsupportedOperationException();
+	}
 }

briar-android/build.gradle

@@ -123,6 +123,7 @@ dependencies {
 		exclude group: 'com.android.support'
 		exclude module: 'disklrucache' // when there's no disk cache, we can't accidentally use it
 	}
+	implementation 'com.github.chrisbanes:PhotoView:2.1.4' // later versions already use androidx
 
 	annotationProcessor 'com.google.dagger:dagger-compiler:2.19'
 	annotationProcessor "com.github.bumptech.glide:compiler:$glideVersion"

briar-android/src/main/AndroidManifest.xml

@@ -113,6 +113,15 @@
 				/>
 		</activity>
 
+		<activity
+			android:name=".android.conversation.ImageActivity"
+			android:parentActivityName="org.briarproject.briar.android.conversation.ConversationActivity"
+			android:theme="@style/BriarTheme.Transparent.NoActionBar">
+			<meta-data
+				android:name="android.support.PARENT_ACTIVITY"
+				android:value="org.briarproject.briar.android.conversation.ConversationActivity"/>
+		</activity>
+
 		<activity
 			android:name="org.briarproject.briar.android.privategroup.creation.CreateGroupActivity"
 			android:label="@string/groups_create_group_title"

briar-android/src/main/java/org/briarproject/briar/android/activity/ActivityComponent.java

@@ -15,10 +15,11 @@ import org.briarproject.briar.android.blog.ReblogFragment;
 import org.briarproject.briar.android.blog.RssFeedImportActivity;
 import org.briarproject.briar.android.blog.RssFeedManageActivity;
 import org.briarproject.briar.android.blog.WriteBlogPostActivity;
-import org.briarproject.briar.android.conversation.AliasDialogFragment;
 import org.briarproject.briar.android.contact.ContactListFragment;
 import org.briarproject.briar.android.contact.ContactModule;
+import org.briarproject.briar.android.conversation.AliasDialogFragment;
 import org.briarproject.briar.android.conversation.ConversationActivity;
+import org.briarproject.briar.android.conversation.ImageActivity;
 import org.briarproject.briar.android.forum.CreateForumActivity;
 import org.briarproject.briar.android.forum.ForumActivity;
 import org.briarproject.briar.android.forum.ForumListFragment;
@@ -110,6 +111,8 @@ public interface ActivityComponent {
 
 	void inject(ConversationActivity activity);
 
+	void inject(ImageActivity activity);
+
 	void inject(ForumInvitationActivity activity);
 
 	void inject(BlogInvitationActivity activity);

briar-android/src/main/java/org/briarproject/briar/android/activity/BriarActivity.java

@@ -2,12 +2,11 @@ package org.briarproject.briar.android.activity;
 
 import android.annotation.SuppressLint;
 import android.content.Intent;
+import android.support.annotation.RequiresApi;
 import android.support.v7.app.ActionBar;
 import android.support.v7.app.AlertDialog;
 import android.support.v7.widget.Toolbar;
-import android.transition.Slide;
 import android.transition.Transition;
-import android.view.Gravity;
 import android.view.Window;
 import android.widget.CheckBox;
 
@@ -33,6 +32,7 @@ import static android.os.Build.VERSION.SDK_INT;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_DOZE_WHITELISTING;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_PASSWORD;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_UNLOCK;
+import static org.briarproject.briar.android.util.UiUtils.excludeSystemUi;
 import static org.briarproject.briar.android.util.UiUtils.getDozeWhitelistingIntent;
 import static org.briarproject.briar.android.util.UiUtils.isSamsung7;
 
@@ -111,21 +111,28 @@ public abstract class BriarActivity extends BaseActivity {
 		lockManager.onActivityStop();
 	}
 
-	public void setSceneTransitionAnimation() {
-		if (SDK_INT < 21) return;
+	/**
+	 * Sets the transition animations.
+	 * @param enterTransition used to move views into initial positions
+	 * @param exitTransition used to move views out when starting a <b>new</b> activity.
+	 * @param returnTransition used when window is closing, because the activity is finishing.
+	 */
+	@RequiresApi(api = 21)
+	public void setSceneTransitionAnimation(
+			@Nullable Transition enterTransition,
+			@Nullable Transition exitTransition,
+			@Nullable Transition returnTransition) {
 		// workaround for #1007
 		if (isSamsung7()) {
 			return;
 		}
-		Transition slide = new Slide(Gravity.RIGHT);
-		slide.excludeTarget(android.R.id.statusBarBackground, true);
-		slide.excludeTarget(android.R.id.navigationBarBackground, true);
+		if (enterTransition != null) excludeSystemUi(enterTransition);
+		if (exitTransition != null) excludeSystemUi(exitTransition);
+		if (returnTransition != null) excludeSystemUi(returnTransition);
 		Window window = getWindow();
-		window.requestFeature(Window.FEATURE_CONTENT_TRANSITIONS);
-		window.setExitTransition(slide);
-		window.setEnterTransition(slide);
-		window.setTransitionBackgroundFadeDuration(getResources()
-				.getInteger(android.R.integer.config_longAnimTime));
+		window.setEnterTransition(enterTransition);
+		window.setExitTransition(exitTransition);
+		window.setReturnTransition(returnTransition);
 	}
 
 	/**

briar-android/src/main/java/org/briarproject/briar/android/blog/ReblogActivity.java

@@ -18,7 +18,6 @@ public class ReblogActivity extends BriarActivity implements
 
 	@Override
 	public void onCreate(Bundle savedInstanceState) {
-		setSceneTransitionAnimation();
 		super.onCreate(savedInstanceState);
 
 		Intent intent = getIntent();

briar-android/src/main/java/org/briarproject/briar/android/conversation/AttachmentItem.java

@@ -1,5 +1,8 @@
 package org.briarproject.briar.android.conversation;
 
+import android.os.Parcel;
+import android.os.Parcelable;
+
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.MessageId;
 
@@ -7,13 +10,26 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public class AttachmentItem {
+public class AttachmentItem implements Parcelable {
 
 	private final MessageId messageId;
 	private final int width, height;
 	private final int thumbnailWidth, thumbnailHeight;
 	private final boolean hasError;
 
+	public static final Creator<AttachmentItem> CREATOR =
+			new Creator<AttachmentItem>() {
+				@Override
+				public AttachmentItem createFromParcel(Parcel in) {
+					return new AttachmentItem(in);
+				}
+
+				@Override
+				public AttachmentItem[] newArray(int size) {
+					return new AttachmentItem[size];
+				}
+			};
+
 	AttachmentItem(MessageId messageId, int width, int height,
 			int thumbnailWidth, int thumbnailHeight, boolean hasError) {
 		this.messageId = messageId;
@@ -24,6 +40,17 @@ public class AttachmentItem {
 		this.hasError = hasError;
 	}
 
+	protected AttachmentItem(Parcel in) {
+		byte[] messageIdByte = new byte[MessageId.LENGTH];
+		in.readByteArray(messageIdByte);
+		messageId = new MessageId(messageIdByte);
+		width = in.readInt();
+		height = in.readInt();
+		thumbnailWidth = in.readInt();
+		thumbnailHeight = in.readInt();
+		hasError = in.readByte() != 0;
+	}
+
 	public MessageId getMessageId() {
 		return messageId;
 	}
@@ -48,4 +75,24 @@ public class AttachmentItem {
 		return hasError;
 	}
 
+	// TODO use counter instead, because in theory one attachment can appear in more than one messages
+	String getTransitionName() {
+		return String.valueOf(messageId.hashCode());
+	}
+
+	@Override
+	public int describeContents() {
+		return 0;
+	}
+
+	@Override
+	public void writeToParcel(Parcel dest, int flags) {
+		dest.writeByteArray(messageId.getBytes());
+		dest.writeInt(width);
+		dest.writeInt(height);
+		dest.writeInt(thumbnailWidth);
+		dest.writeInt(thumbnailHeight);
+		dest.writeByte((byte) (hasError ? 1 : 0));
+	}
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationActivity.java

@@ -9,11 +9,15 @@ import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.annotation.UiThread;
 import android.support.design.widget.Snackbar;
+import android.support.v4.app.ActivityCompat;
+import android.support.v4.app.ActivityOptionsCompat;
 import android.support.v4.content.ContextCompat;
 import android.support.v7.app.AlertDialog;
 import android.support.v7.widget.ActionMenuView;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.Toolbar;
+import android.transition.Slide;
+import android.transition.Transition;
 import android.util.SparseArray;
 import android.view.Menu;
 import android.view.MenuInflater;
@@ -96,8 +100,11 @@ import im.delight.android.identicons.IdenticonDrawable;
 import uk.co.samuelwall.materialtaptargetprompt.MaterialTapTargetPrompt;
 import uk.co.samuelwall.materialtaptargetprompt.MaterialTapTargetPrompt.PromptStateChangeListener;
 
+import static android.os.Build.VERSION.SDK_INT;
+import static android.support.v4.app.ActivityOptionsCompat.makeSceneTransitionAnimation;
 import static android.support.v4.view.ViewCompat.setTransitionName;
 import static android.support.v7.util.SortedList.INVALID_POSITION;
+import static android.view.Gravity.END;
 import static android.widget.Toast.LENGTH_SHORT;
 import static java.util.Collections.emptyList;
 import static java.util.Collections.sort;
@@ -108,6 +115,9 @@ import static org.briarproject.bramble.util.LogUtils.logDuration;
 import static org.briarproject.bramble.util.LogUtils.logException;
 import static org.briarproject.bramble.util.LogUtils.now;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_INTRODUCTION;
+import static org.briarproject.briar.android.conversation.ImageActivity.ATTACHMENT;
+import static org.briarproject.briar.android.conversation.ImageActivity.DATE;
+import static org.briarproject.briar.android.conversation.ImageActivity.NAME;
 import static org.briarproject.briar.android.settings.SettingsFragment.SETTINGS_NAMESPACE;
 import static org.briarproject.briar.android.util.UiUtils.getAvatarTransitionName;
 import static org.briarproject.briar.android.util.UiUtils.getBulbTransitionName;
@@ -186,7 +196,10 @@ public class ConversationActivity extends BriarActivity
 
 	@Override
 	public void onCreate(@Nullable Bundle state) {
-		setSceneTransitionAnimation();
+		if (SDK_INT >= 21) {
+			Transition slide = new Slide(END);
+			setSceneTransitionAnimation(slide, null, slide);
+		}
 		super.onCreate(state);
 
 		Intent i = getIntent();
@@ -802,6 +815,31 @@ public class ConversationActivity extends BriarActivity
 		startActivity(i);
 	}
 
+	@Override
+	public void onAttachmentClicked(View view,
+			ConversationMessageItem messageItem, AttachmentItem item) {
+		String name;
+		if (messageItem.isIncoming()) {
+			// must be available when items are being displayed
+			name = viewModel.getContactDisplayName().getValue();
+		} else {
+			name = getString(R.string.you);
+		}
+		Intent i = new Intent(this, ImageActivity.class);
+		i.putExtra(ATTACHMENT, item);
+		i.putExtra(NAME, name);
+		i.putExtra(DATE, messageItem.getTime());
+		if (SDK_INT >= 23) {
+			String transitionName = item.getTransitionName();
+			ActivityOptionsCompat options =
+					makeSceneTransitionAnimation(this, view, transitionName);
+			ActivityCompat.startActivity(this, i, options.toBundle());
+		} else {
+			// work-around for android bug #224270
+			startActivity(i);
+		}
+	}
+
 	@DatabaseExecutor
 	private void respondToIntroductionRequest(SessionId sessionId,
 			boolean accept, long time) throws DbException {
@@ -845,4 +883,5 @@ public class ConversationActivity extends BriarActivity
 		}
 		return attachments;
 	}
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationListener.java

@@ -1,6 +1,7 @@
 package org.briarproject.briar.android.conversation;
 
 import android.support.annotation.UiThread;
+import android.view.View;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -14,4 +15,7 @@ interface ConversationListener {
 
 	void openRequestedShareable(ConversationRequestItem item);
 
+	void onAttachmentClicked(View view, ConversationMessageItem messageItem,
+			AttachmentItem attachmentItem);
+
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationMessageViewHolder.java

@@ -85,7 +85,7 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 		if (item.getAttachments().isEmpty()) {
 			bindTextItem();
 		} else {
-			bindImageItem(item);
+			bindImageItem(item, listener);
 		}
 	}
 
@@ -98,7 +98,8 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 		textConstraints.applyTo(layout);
 	}
 
-	private void bindImageItem(ConversationMessageItem item) {
+	private void bindImageItem(ConversationMessageItem item,
+			ConversationListener listener) {
 		// TODO show more than just the first image
 		AttachmentItem attachment = item.getAttachments().get(0);
 
@@ -127,17 +128,18 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 			clearImage();
 			imageView.setImageResource(ERROR_RES);
 		} else {
-			loadImage(item, attachment);
+			loadImage(item, attachment, listener);
 		}
 	}
 
 	private void clearImage() {
 		GlideApp.with(imageView)
 				.clear(imageView);
+		imageView.setOnClickListener(null);
 	}
 
 	private void loadImage(ConversationMessageItem item,
-			AttachmentItem attachment) {
+			AttachmentItem attachment, ConversationListener listener) {
 		boolean leftCornerSmall =
 				(isIncoming() && !isRtl) || (!isIncoming() && isRtl);
 		boolean bottomRound = item.getText() == null;
@@ -152,6 +154,8 @@ class ConversationMessageViewHolder extends ConversationItemViewHolder {
 				.transition(withCrossFade())
 				.into(imageView)
 				.waitForLayout();
+		imageView.setOnClickListener(
+				view -> listener.onAttachmentClicked(view, item, attachment));
 	}
 
 }

briar-android/src/main/java/org/briarproject/briar/android/conversation/ImageActivity.java

@@ -0,0 +1,233 @@
+package org.briarproject.briar.android.conversation;
+
+import android.graphics.drawable.Animatable;
+import android.graphics.drawable.Drawable;
+import android.os.Bundle;
+import android.support.annotation.Nullable;
+import android.support.annotation.RequiresApi;
+import android.support.design.widget.AppBarLayout;
+import android.support.v7.widget.Toolbar;
+import android.transition.Fade;
+import android.transition.Transition;
+import android.view.MenuItem;
+import android.view.View;
+import android.view.Window;
+import android.widget.TextView;
+
+import com.bumptech.glide.load.DataSource;
+import com.bumptech.glide.load.engine.GlideException;
+import com.bumptech.glide.request.RequestListener;
+import com.bumptech.glide.request.target.Target;
+import com.github.chrisbanes.photoview.PhotoView;
+
+import org.briarproject.briar.R;
+import org.briarproject.briar.android.activity.ActivityComponent;
+import org.briarproject.briar.android.activity.BriarActivity;
+import org.briarproject.briar.android.conversation.glide.GlideApp;
+import org.briarproject.briar.android.view.PullDownLayout;
+
+import static android.graphics.Color.TRANSPARENT;
+import static android.os.Build.VERSION.SDK_INT;
+import static android.view.View.GONE;
+import static android.view.View.SYSTEM_UI_FLAG_FULLSCREEN;
+import static android.view.View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN;
+import static android.view.View.SYSTEM_UI_FLAG_LAYOUT_STABLE;
+import static android.view.View.VISIBLE;
+import static android.view.WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS;
+import static android.widget.ImageView.ScaleType.FIT_START;
+import static com.bumptech.glide.load.engine.DiskCacheStrategy.NONE;
+import static java.util.Objects.requireNonNull;
+import static org.briarproject.briar.android.util.UiUtils.formatDateAbsolute;
+
+public class ImageActivity extends BriarActivity
+		implements PullDownLayout.Callback {
+
+	final static String ATTACHMENT = "attachment";
+	final static String NAME = "name";
+	final static String DATE = "date";
+
+	private PullDownLayout layout;
+	private AppBarLayout appBarLayout;
+	private PhotoView photoView;
+
+	@Override
+	public void injectActivity(ActivityComponent component) {
+		component.inject(this);
+	}
+
+	@Override
+	public void onCreate(@Nullable Bundle state) {
+		super.onCreate(state);
+
+		// Transitions
+		supportPostponeEnterTransition();
+		Window window = getWindow();
+		if (SDK_INT >= 21) {
+			Transition transition = new Fade();
+			setSceneTransitionAnimation(transition, null, transition);
+		}
+
+		// inflate layout
+		setContentView(R.layout.activity_image);
+		layout = findViewById(R.id.layout);
+		layout.getBackground().setAlpha(255);
+		layout.setCallback(this);
+
+		// Status Bar
+		if (SDK_INT >= 21) {
+			window.setStatusBarColor(TRANSPARENT);
+		} else if (SDK_INT >= 19) {
+			// we can't make the status bar transparent, but translucent
+			window.addFlags(FLAG_TRANSLUCENT_STATUS);
+		}
+
+		// Toolbar
+		appBarLayout = findViewById(R.id.appBarLayout);
+		Toolbar toolbar = requireNonNull(setUpCustomToolbar(true));
+		TextView contactName = toolbar.findViewById(R.id.contactName);
+		TextView dateView = toolbar.findViewById(R.id.dateView);
+
+		// Intent Extras
+		AttachmentItem attachment = getIntent().getParcelableExtra(ATTACHMENT);
+		String name = getIntent().getStringExtra(NAME);
+		long time = getIntent().getLongExtra(DATE, 0);
+		String date = formatDateAbsolute(this, time);
+		contactName.setText(name);
+		dateView.setText(date);
+
+		// Image View
+		photoView = findViewById(R.id.photoView);
+		if (SDK_INT >= 16) {
+			photoView.setOnClickListener(view -> toggleSystemUi());
+			window.getDecorView().setSystemUiVisibility(
+					SYSTEM_UI_FLAG_LAYOUT_STABLE |
+							SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN);
+		}
+
+		// Request Listener
+		RequestListener<Drawable> listener = new RequestListener<Drawable>() {
+			@Override
+			public boolean onLoadFailed(@Nullable GlideException e,
+					Object model, Target<Drawable> target,
+					boolean isFirstResource) {
+				supportStartPostponedEnterTransition();
+				return false;
+			}
+
+			@Override
+			public boolean onResourceReady(Drawable resource, Object model,
+					Target<Drawable> target, DataSource dataSource,
+					boolean isFirstResource) {
+				if (SDK_INT >= 21 && !(resource instanceof Animatable)) {
+					// set transition name only when not animatable,
+					// because the animation won't start otherwise
+					photoView.setTransitionName(
+							attachment.getTransitionName());
+				}
+				// Move image to the top if overlapping toolbar
+				if (isOverlappingToolbar(resource)) {
+					photoView.setScaleType(FIT_START);
+				}
+				supportStartPostponedEnterTransition();
+				return false;
+			}
+		};
+
+		// Load Image
+		GlideApp.with(this)
+				.load(attachment)
+				.diskCacheStrategy(NONE)
+				.error(R.drawable.ic_image_broken)
+				.dontTransform()
+				.addListener(listener)
+				.into(photoView);
+	}
+
+	@Override
+	public boolean onOptionsItemSelected(MenuItem item) {
+		switch (item.getItemId()) {
+			case android.R.id.home:
+				onBackPressed();
+				return true;
+			default:
+				return super.onOptionsItemSelected(item);
+		}
+	}
+
+	@Override
+	public void onPullStart() {
+		appBarLayout.animate()
+				.alpha(0f)
+				.start();
+	}
+
+	@Override
+	public void onPull(float progress) {
+		layout.getBackground().setAlpha(Math.round((1 - progress) * 255));
+	}
+
+	@Override
+	public void onPullCancel() {
+		appBarLayout.animate()
+				.alpha(1f)
+				.start();
+	}
+
+	@Override
+	public void onPullComplete() {
+		supportFinishAfterTransition();
+	}
+
+	@RequiresApi(api = 16)
+	private void toggleSystemUi() {
+		View decorView = getWindow().getDecorView();
+		if (appBarLayout.getVisibility() == VISIBLE) {
+			hideSystemUi(decorView);
+		} else {
+			showSystemUi(decorView);
+		}
+	}
+
+	@RequiresApi(api = 16)
+	private void hideSystemUi(View decorView) {
+		decorView.setSystemUiVisibility(SYSTEM_UI_FLAG_LAYOUT_STABLE
+				| SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+				| SYSTEM_UI_FLAG_FULLSCREEN
+		);
+		appBarLayout.animate()
+				.translationYBy(-1 * appBarLayout.getHeight())
+				.alpha(0f)
+				.withEndAction(() -> appBarLayout.setVisibility(GONE))
+				.start();
+	}
+
+	@RequiresApi(api = 16)
+	private void showSystemUi(View decorView) {
+		decorView.setSystemUiVisibility(
+				SYSTEM_UI_FLAG_LAYOUT_STABLE
+						| SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+		);
+		appBarLayout.animate()
+				.translationYBy(appBarLayout.getHeight())
+				.alpha(1f)
+				.withStartAction(() -> appBarLayout.setVisibility(VISIBLE))
+				.start();
+	}
+
+	private boolean isOverlappingToolbar(Drawable drawable) {
+		int width = drawable.getIntrinsicWidth();
+		int height = drawable.getIntrinsicHeight();
+		float widthPercentage = photoView.getWidth() / (float) width;
+		float heightPercentage = photoView.getHeight() / (float) height;
+		float scaleFactor = Math.min(widthPercentage, heightPercentage);
+		int realWidth = (int) (width * scaleFactor);
+		int realHeight = (int) (height * scaleFactor);
+		// return if photo doesn't use the full width,
+		// because it will be moved to the right otherwise
+		if (realWidth < photoView.getWidth()) return false;
+		int drawableTop = (photoView.getHeight() - realHeight) / 2;
+		return drawableTop < appBarLayout.getBottom() &&
+				drawableTop != appBarLayout.getTop();
+	}
+
+}

briar-android/src/main/java/org/briarproject/briar/android/util/UiUtils.java

@@ -16,6 +16,7 @@ import android.support.annotation.ColorInt;
 import android.support.annotation.ColorRes;
 import android.support.annotation.MainThread;
 import android.support.annotation.Nullable;
+import android.support.annotation.RequiresApi;
 import android.support.design.widget.TextInputLayout;
 import android.support.v4.app.FragmentManager;
 import android.support.v4.content.ContextCompat;
@@ -31,6 +32,7 @@ import android.text.method.LinkMovementMethod;
 import android.text.style.ClickableSpan;
 import android.text.style.ForegroundColorSpan;
 import android.text.style.URLSpan;
+import android.transition.Transition;
 import android.util.TypedValue;
 import android.view.KeyEvent;
 import android.view.View;
@@ -60,12 +62,16 @@ import static android.support.v7.app.AppCompatDelegate.MODE_NIGHT_NO;
 import static android.support.v7.app.AppCompatDelegate.MODE_NIGHT_YES;
 import static android.support.v7.app.AppCompatDelegate.setDefaultNightMode;
 import static android.text.format.DateUtils.DAY_IN_MILLIS;
+import static android.text.format.DateUtils.FORMAT_ABBREV_ALL;
 import static android.text.format.DateUtils.FORMAT_ABBREV_MONTH;
 import static android.text.format.DateUtils.FORMAT_ABBREV_RELATIVE;
 import static android.text.format.DateUtils.FORMAT_ABBREV_TIME;
 import static android.text.format.DateUtils.FORMAT_SHOW_DATE;
+import static android.text.format.DateUtils.FORMAT_SHOW_TIME;
+import static android.text.format.DateUtils.FORMAT_SHOW_YEAR;
 import static android.text.format.DateUtils.MINUTE_IN_MILLIS;
 import static android.text.format.DateUtils.WEEK_IN_MILLIS;
+import static android.text.format.DateUtils.YEAR_IN_MILLIS;
 import static android.view.KeyEvent.ACTION_DOWN;
 import static android.view.KeyEvent.KEYCODE_ENTER;
 import static android.view.inputmethod.EditorInfo.IME_NULL;
@@ -117,6 +123,13 @@ public class UiUtils {
 				MIN_DATE_RESOLUTION, flags).toString();
 	}
 
+	public static String formatDateAbsolute(Context ctx, long time) {
+		int flags = FORMAT_SHOW_TIME | FORMAT_SHOW_DATE | FORMAT_ABBREV_ALL;
+		long diff = System.currentTimeMillis() - time;
+		if (diff >= YEAR_IN_MILLIS) flags |= FORMAT_SHOW_YEAR;
+		return DateUtils.formatDateTime(ctx, time, flags);
+	}
+
 	public static int getDaysUntilExpiry() {
 		long now = System.currentTimeMillis();
 		long daysBeforeExpiry = (EXPIRY_DATE - now) / 1000 / 60 / 60 / 24;
@@ -318,6 +331,12 @@ public class UiUtils {
 				keyEvent.getKeyCode() == KEYCODE_ENTER;
 	}
 
+	@RequiresApi(api = 21)
+	public static void excludeSystemUi(Transition transition) {
+		transition.excludeTarget(android.R.id.statusBarBackground, true);
+		transition.excludeTarget(android.R.id.navigationBarBackground, true);
+	}
+
 	/**
 	 * Observes the given {@link LiveData} until the first change.
 	 * If the LiveData's value is available, the {@link Observer} will be

briar-android/src/main/java/org/briarproject/briar/android/view/PullDownLayout.java

@@ -0,0 +1,161 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015 XiNGRZ <xxx@oxo.ooo>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.briarproject.briar.android.view;
+
+import android.content.Context;
+import android.support.annotation.Nullable;
+import android.support.v4.view.ViewCompat;
+import android.support.v4.widget.ViewDragHelper;
+import android.util.AttributeSet;
+import android.view.MotionEvent;
+import android.view.View;
+import android.view.ViewConfiguration;
+import android.widget.FrameLayout;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class PullDownLayout extends FrameLayout {
+
+	private final ViewDragHelper dragger;
+
+	private final int minimumFlingVelocity;
+
+	@Nullable
+	private Callback callback;
+
+	public PullDownLayout(Context context) {
+		this(context, null);
+	}
+
+	public PullDownLayout(Context context, @Nullable AttributeSet attrs) {
+		this(context, attrs, 0);
+	}
+
+	public PullDownLayout(Context context, @Nullable AttributeSet attrs,
+			int defStyleAttr) {
+		super(context, attrs, defStyleAttr);
+		dragger = ViewDragHelper.create(this, 1f / 8f, new ViewDragCallback());
+		minimumFlingVelocity =
+				ViewConfiguration.get(context).getScaledMinimumFlingVelocity();
+	}
+
+	public void setCallback(@Nullable Callback callback) {
+		this.callback = callback;
+	}
+
+	@Override
+	public boolean onInterceptTouchEvent(MotionEvent ev) {
+		return dragger.shouldInterceptTouchEvent(ev);
+	}
+
+	@Override
+	public boolean onTouchEvent(MotionEvent event) {
+		dragger.processTouchEvent(event);
+		return true;
+	}
+
+	@Override
+	public void computeScroll() {
+		if (dragger.continueSettling(true)) {
+			ViewCompat.postInvalidateOnAnimation(this);
+		}
+	}
+
+	public interface Callback {
+
+		void onPullStart();
+
+		void onPull(float progress);
+
+		void onPullCancel();
+
+		void onPullComplete();
+
+	}
+
+	private class ViewDragCallback extends ViewDragHelper.Callback {
+
+		@Override
+		public boolean tryCaptureView(View child, int pointerId) {
+			return true;
+		}
+
+		@Override
+		public int clampViewPositionHorizontal(View child, int left, int dx) {
+			return 0;
+		}
+
+		@Override
+		public int clampViewPositionVertical(View child, int top, int dy) {
+			return Math.max(0, top);
+		}
+
+		@Override
+		public int getViewHorizontalDragRange(View child) {
+			return 0;
+		}
+
+		@Override
+		public int getViewVerticalDragRange(View child) {
+			return getHeight();
+		}
+
+		@Override
+		public void onViewCaptured(View capturedChild, int activePointerId) {
+			if (callback != null) {
+				callback.onPullStart();
+			}
+		}
+
+		@Override
+		public void onViewPositionChanged(View changedView, int left, int top,
+				int dx, int dy) {
+			if (callback != null) {
+				callback.onPull((float) top / (float) getHeight());
+			}
+		}
+
+		@Override
+		public void onViewReleased(View releasedChild, float xvel, float yvel) {
+			int slop = yvel > minimumFlingVelocity ? getHeight() / 6 :
+					getHeight() / 3;
+			if (releasedChild.getTop() > slop) {
+				if (callback != null) {
+					callback.onPullComplete();
+				}
+			} else {
+				if (callback != null) {
+					callback.onPullCancel();
+				}
+
+				dragger.settleCapturedViewAt(0, 0);
+				invalidate();
+			}
+		}
+
+	}
+
+}

briar-android/src/main/res/layout/activity_image.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="utf-8"?>
+<org.briarproject.briar.android.view.PullDownLayout
+	android:id="@+id/layout"
+	xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:tools="http://schemas.android.com/tools"
+	android:layout_width="match_parent"
+	android:layout_height="match_parent"
+	android:background="@color/briar_black"
+	tools:context=".android.conversation.ImageActivity">
+
+	<com.github.chrisbanes.photoview.PhotoView
+		android:id="@+id/photoView"
+		android:layout_width="match_parent"
+		android:layout_height="match_parent"
+		tools:ignore="ContentDescription"
+		tools:srcCompat="@tools:sample/backgrounds/scenic"/>
+
+	<android.support.design.widget.AppBarLayout
+		android:id="@+id/appBarLayout"
+		android:layout_width="match_parent"
+		android:layout_height="wrap_content"
+		android:background="@color/msg_status_bubble_background">
+
+		<android.support.v7.widget.Toolbar
+			android:id="@+id/toolbar"
+			style="@style/BriarToolbar"
+			android:layout_width="match_parent"
+			android:layout_height="wrap_content"
+			android:background="@color/msg_status_bubble_background"
+			android:fitsSystemWindows="true">
+
+			<LinearLayout
+				android:layout_width="wrap_content"
+				android:layout_height="match_parent"
+				android:orientation="vertical">
+
+				<com.vanniktech.emoji.EmojiTextView
+					android:id="@+id/contactName"
+					style="@style/TextAppearance.AppCompat.Widget.ActionBar.Title.Inverse"
+					android:layout_width="wrap_content"
+					android:layout_height="match_parent"
+					android:ellipsize="end"
+					android:maxLines="1"
+					android:textColor="@color/action_bar_text"
+					tools:text="Contact Name of someone who chose a long name"/>
+
+				<TextView
+					android:id="@+id/dateView"
+					android:layout_width="wrap_content"
+					android:layout_height="wrap_content"
+					android:textColor="@color/action_bar_text"
+					tools:text="date"/>
+
+			</LinearLayout>
+
+		</android.support.v7.widget.Toolbar>
+
+	</android.support.design.widget.AppBarLayout>
+
+</org.briarproject.briar.android.view.PullDownLayout>

briar-android/src/main/res/values/strings.xml

@@ -134,6 +134,8 @@
 	<string name="dialog_title_delete_contact">Confirm Contact Deletion</string>
 	<string name="dialog_message_delete_contact">Are you sure that you want to remove this contact and all messages exchanged with this contact?</string>
 	<string name="contact_deleted_toast">Contact deleted</string>
+	<!-- This is shown in the action bar when opening an image in fullscreen that the user sent -->
+	<string name="you">You</string>
 
 	<!-- Adding Contacts -->
 	<string name="add_contact_title">Add a Contact</string>

briar-android/src/main/res/values/themes.xml

@@ -18,6 +18,12 @@
 		<item name="toolbarStyle">@style/BriarToolbar</item>
 	</style>
 
+	<style name="BriarTheme.Transparent.NoActionBar" parent="BriarTheme.NoActionBar">
+		<item name="android:windowBackground">@android:color/transparent</item>
+		<item name="android:windowIsTranslucent">true</item>
+		<item name="android:windowActionBarOverlay">true</item>
+	</style>
+
 	<style name="ActivityAnimation" parent="@android:style/Animation.Activity">
 		<item name="android:activityOpenEnterAnimation">@anim/screen_new_in</item>
 		<item name="android:activityOpenExitAnimation">@anim/screen_old_out</item>

briar-android/witness.gradle

@@ -89,6 +89,7 @@ dependencyVerification {
         'com.github.bumptech.glide:compiler:4.8.0:compiler-4.8.0.jar:1fa93dd0cf7ef0b8b98a59a67a1ee84915416c2d677d83a771ea3e32ad15e6bf',
         'com.github.bumptech.glide:gifdecoder:4.8.0:gifdecoder-4.8.0.aar:b00c5454a023a9488ea49603930d9c25e09192e5ceaadf64977aa52946b3c1b4',
         'com.github.bumptech.glide:glide:4.8.0:glide-4.8.0.aar:5ddf08b12cc43332e812988f16c2c39e7fce49d1c4d94b7948dcde7f00bf49d6',
+        'com.github.chrisbanes:PhotoView:2.1.4:PhotoView-2.1.4.aar:04cb397fcb3df0757c8aed6927ebdd247930b5c78ee9acc59cd07dccdaaf3460',
         'com.google.android.apps.common.testing.accessibility.framework:accessibility-test-framework:2.0:accessibility-test-framework-2.0.jar:cdf16ef8f5b8023d003ce3cc1b0d51bda737762e2dab2fedf43d1c4292353f7f',
         'com.google.android.apps.common.testing.accessibility.framework:accessibility-test-framework:2.1:accessibility-test-framework-2.1.jar:7b0aa6ed7553597ce0610684a9f7eca8021eee218f2e2f427c04a7fbf5f920bd',
         'com.google.code.findbugs:jsr305:1.3.9:jsr305-1.3.9.jar:905721a0eea90a81534abb7ee6ef4ea2e5e645fa1def0a5cd88402df1b46c9ed',

build.gradle

@@ -5,6 +5,7 @@ allprojects {
 		jcenter()
 		mavenLocal()
 		google()
+		maven { url "https://jitpack.io" }
 	}
 	afterEvaluate {
 		tasks.withType(Test) {