[android] enable image shared element transition for API 21+22

There's an Android framework bug (#224270) on these APIs that causes a NPE
when the shared element is not visible anymore when returning.
Since we know restore the list position, the shared element should be
visible and thus not produce NPEs anymore.

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

@@ -20,6 +20,7 @@ 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.conversation.ImageFragment;
 import org.briarproject.briar.android.forum.CreateForumActivity;
 import org.briarproject.briar.android.forum.ForumActivity;
 import org.briarproject.briar.android.forum.ForumListFragment;
@@ -218,4 +219,6 @@ public interface ActivityComponent {
 
 	void inject(AliasDialogFragment aliasDialogFragment);
 
+	void inject(ImageFragment imageFragment);
+
 }

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

@@ -2,6 +2,7 @@ package org.briarproject.briar.android.conversation;
 
 import android.os.Parcel;
 import android.os.Parcelable;
+import android.support.annotation.Nullable;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.MessageId;
@@ -111,4 +112,10 @@ public class AttachmentItem implements Parcelable {
 		dest.writeByte((byte) (hasError ? 1 : 0));
 	}
 
+	@Override
+	public boolean equals(@Nullable Object o) {
+		return o instanceof AttachmentItem &&
+				messageId.equals(((AttachmentItem) o).messageId);
+	}
+
 }

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

@@ -8,6 +8,7 @@ import android.content.DialogInterface;
 import android.content.Intent;
 import android.net.Uri;
 import android.os.Bundle;
+import android.os.Parcelable;
 import android.support.annotation.Nullable;
 import android.support.annotation.UiThread;
 import android.support.design.widget.Snackbar;
@@ -107,7 +108,6 @@ import static android.support.v4.app.ActivityOptionsCompat.makeSceneTransitionAn
 import static android.support.v4.view.ViewCompat.setTransitionName;
 import static android.support.v7.util.SortedList.INVALID_POSITION;
 import static android.view.Gravity.RIGHT;
-import static android.widget.Toast.LENGTH_LONG;
 import static android.widget.Toast.LENGTH_SHORT;
 import static java.util.Collections.emptyList;
 import static java.util.Collections.sort;
@@ -121,7 +121,8 @@ import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 import static org.briarproject.briar.android.TestingConstants.FEATURE_FLAG_IMAGE_ATTACHMENTS;
 import static org.briarproject.briar.android.activity.RequestCodes.REQUEST_ATTACH_IMAGE;
 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.ATTACHMENTS;
+import static org.briarproject.briar.android.conversation.ImageActivity.ATTACHMENT_POSITION;
 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;
@@ -193,6 +194,8 @@ public class ConversationActivity extends BriarActivity
 	ViewModelProvider.Factory viewModelFactory;
 
 	private volatile ContactId contactId;
+	@Nullable
+	private Parcelable layoutManagerState;
 
 	private final Observer<String> contactNameObserver = name -> {
 		requireNonNull(name);
@@ -308,6 +311,21 @@ public class ConversationActivity extends BriarActivity
 		list.stopPeriodicUpdate();
 	}
 
+	@Override
+	protected void onSaveInstanceState(Bundle outState) {
+		super.onSaveInstanceState(outState);
+		if (layoutManager != null) {
+			layoutManagerState = layoutManager.onSaveInstanceState();
+			outState.putParcelable("layoutManager", layoutManagerState);
+		}
+	}
+
+	@Override
+	protected void onRestoreInstanceState(Bundle savedInstanceState) {
+		super.onRestoreInstanceState(savedInstanceState);
+		layoutManagerState = savedInstanceState.getParcelable("layoutManager");
+	}
+
 	@Override
 	public boolean onCreateOptionsMenu(Menu menu) {
 		// Inflate the menu items for use in the action bar
@@ -427,8 +445,12 @@ public class ConversationActivity extends BriarActivity
 				List<ConversationItem> items = createItems(headers);
 				adapter.addAll(items);
 				list.showData();
-				// Scroll to the bottom
-				list.scrollToPosition(adapter.getItemCount() - 1);
+				if (layoutManagerState == null) {
+					// Scroll to the bottom
+					list.scrollToPosition(adapter.getItemCount() - 1);
+				} else {
+					layoutManager.onRestoreInstanceState(layoutManagerState);
+				}
 			} else {
 				LOG.info("Concurrent update, reloading");
 				loadMessages();
@@ -470,7 +492,8 @@ public class ConversationActivity extends BriarActivity
 					adapter.getMessageItem(m);
 			if (pair != null) {
 				pair.getSecond().setText(text);
-				boolean bottom = adapter.isScrolledToBottom(layoutManager);
+				boolean bottom = layoutManagerState == null &&
+						adapter.isScrolledToBottom(layoutManager);
 				adapter.notifyItemChanged(pair.getFirst());
 				if (bottom) list.scrollToPosition(adapter.getItemCount() - 1);
 			}
@@ -501,7 +524,8 @@ public class ConversationActivity extends BriarActivity
 					adapter.getMessageItem(m);
 			if (pair != null) {
 				pair.getSecond().setAttachments(items);
-				boolean bottom = adapter.isScrolledToBottom(layoutManager);
+				boolean bottom = layoutManagerState == null &&
+						adapter.isScrolledToBottom(layoutManager);
 				adapter.notifyItemChanged(pair.getFirst());
 				if (bottom) list.scrollToPosition(adapter.getItemCount() - 1);
 			}
@@ -552,7 +576,8 @@ public class ConversationActivity extends BriarActivity
 
 	private void addConversationItem(ConversationItem item) {
 		runOnUiThreadUnlessDestroyed(() -> {
-			boolean bottom = adapter.isScrolledToBottom(layoutManager);
+			boolean bottom = layoutManagerState == null &&
+					adapter.isScrolledToBottom(layoutManager);
 			adapter.incrementRevision();
 			adapter.add(item);
 			if (bottom) list.scrollToPosition(adapter.getItemCount() - 1);
@@ -812,19 +837,18 @@ public class ConversationActivity extends BriarActivity
 		} else {
 			name = getString(R.string.you);
 		}
+		ArrayList<AttachmentItem> attachments =
+				new ArrayList<>(messageItem.getAttachments());
 		Intent i = new Intent(this, ImageActivity.class);
-		i.putExtra(ATTACHMENT, item);
+		i.putParcelableArrayListExtra(ATTACHMENTS, attachments);
+		i.putExtra(ATTACHMENT_POSITION, attachments.indexOf(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);
-		}
+		// restoring list position should not trigger android bug #224270
+		String transitionName = item.getTransitionName();
+		ActivityOptionsCompat options =
+				makeSceneTransitionAnimation(this, view, transitionName);
+		ActivityCompat.startActivity(this, i, options.toBundle());
 	}
 
 	@DatabaseExecutor

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

@@ -4,15 +4,18 @@ import android.arch.lifecycle.ViewModelProvider;
 import android.arch.lifecycle.ViewModelProviders;
 import android.content.DialogInterface.OnClickListener;
 import android.content.Intent;
-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.design.widget.Snackbar;
+import android.support.v4.app.Fragment;
+import android.support.v4.app.FragmentManager;
+import android.support.v4.app.FragmentStatePagerAdapter;
 import android.support.v4.content.ContextCompat;
 import android.support.v4.graphics.drawable.DrawableCompat;
+import android.support.v4.view.ViewPager;
 import android.support.v7.app.AlertDialog.Builder;
 import android.support.v7.widget.Toolbar;
 import android.transition.Fade;
@@ -20,23 +23,18 @@ import android.transition.Transition;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
+import android.view.ViewTreeObserver.OnGlobalLayoutListener;
 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 java.text.SimpleDateFormat;
 import java.util.Date;
+import java.util.List;
 import java.util.Locale;
 
 import javax.inject.Inject;
@@ -53,16 +51,15 @@ 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.activity.RequestCodes.REQUEST_SAVE_ATTACHMENT;
 import static org.briarproject.briar.android.util.UiUtils.formatDateAbsolute;
 
 public class ImageActivity extends BriarActivity
-		implements PullDownLayout.Callback {
+		implements PullDownLayout.Callback, OnGlobalLayoutListener {
 
-	final static String ATTACHMENT = "attachment";
+	final static String ATTACHMENTS = "attachments";
+	final static String ATTACHMENT_POSITION = "position";
 	final static String NAME = "name";
 	final static String DATE = "date";
 
@@ -72,8 +69,8 @@ public class ImageActivity extends BriarActivity
 	private ImageViewModel viewModel;
 	private PullDownLayout layout;
 	private AppBarLayout appBarLayout;
-	private PhotoView photoView;
-	private AttachmentItem attachment;
+	private ViewPager viewPager;
+	private List<AttachmentItem> attachments;
 
 	@Override
 	public void injectActivity(ActivityComponent component) {
@@ -85,7 +82,7 @@ public class ImageActivity extends BriarActivity
 		super.onCreate(state);
 
 		// Transitions
-		supportPostponeEnterTransition();
+		if (state == null) supportPostponeEnterTransition();
 		Window window = getWindow();
 		if (SDK_INT >= 21) {
 			Transition transition = new Fade();
@@ -102,6 +99,7 @@ public class ImageActivity extends BriarActivity
 		layout = findViewById(R.id.layout);
 		layout.getBackground().setAlpha(255);
 		layout.setCallback(this);
+		layout.getViewTreeObserver().addOnGlobalLayoutListener(this);
 
 		// Status Bar
 		if (SDK_INT >= 21) {
@@ -118,59 +116,29 @@ public class ImageActivity extends BriarActivity
 		TextView dateView = toolbar.findViewById(R.id.dateView);
 
 		// Intent Extras
-		attachment = getIntent().getParcelableExtra(ATTACHMENT);
-		String name = getIntent().getStringExtra(NAME);
-		long time = getIntent().getLongExtra(DATE, 0);
+		Intent i = getIntent();
+		attachments = i.getParcelableArrayListExtra(ATTACHMENTS);
+		int position = i.getIntExtra(ATTACHMENT_POSITION, -1);
+		if (position == -1) throw new IllegalStateException();
+		String name = i.getStringExtra(NAME);
+		long time = i.getLongExtra(DATE, 0);
 		String date = formatDateAbsolute(this, time);
 		contactName.setText(name);
 		dateView.setText(date);
 
-		// Image View
-		photoView = findViewById(R.id.photoView);
+		// Set up image ViewPager
+		viewPager = findViewById(R.id.viewPager);
+		ImagePagerAdapter pagerAdapter =
+				new ImagePagerAdapter(getSupportFragmentManager());
+		viewPager.setAdapter(pagerAdapter);
+		viewPager.setCurrentItem(position);
+
 		if (SDK_INT >= 16) {
-			photoView.setOnClickListener(view -> toggleSystemUi());
+			viewModel.getOnImageClicked().observe(this, this::onImageClicked);
 			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
@@ -193,11 +161,23 @@ public class ImageActivity extends BriarActivity
 		}
 	}
 
+	@Override
+	public void onGlobalLayout() {
+		viewModel.setToolbarPosition(
+				appBarLayout.getTop(), appBarLayout.getBottom()
+		);
+		if (SDK_INT >= 16) {
+			layout.getViewTreeObserver().removeOnGlobalLayoutListener(this);
+		} else {
+			layout.getViewTreeObserver().removeGlobalOnLayoutListener(this);
+		}
+	}
+
 	@Override
 	protected void onActivityResult(int request, int result, Intent data) {
 		super.onActivityResult(request, result, data);
 		if (request == REQUEST_SAVE_ATTACHMENT && result == RESULT_OK) {
-			viewModel.saveImage(attachment, data.getData());
+			viewModel.saveImage(getVisibleAttachment(), data.getData());
 		}
 	}
 
@@ -225,6 +205,14 @@ public class ImageActivity extends BriarActivity
 		supportFinishAfterTransition();
 	}
 
+	@RequiresApi(api = 16)
+	private void onImageClicked(@Nullable Boolean clicked) {
+		if (clicked != null && clicked) {
+			toggleSystemUi();
+			viewModel.onOnImageClickSeen();
+		}
+	}
+
 	@RequiresApi(api = 16)
 	private void toggleSystemUi() {
 		View decorView = getWindow().getDecorView();
@@ -259,29 +247,13 @@ public class ImageActivity extends BriarActivity
 				.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();
-	}
-
 	private void showSaveImageDialog() {
 		OnClickListener okListener = (dialog, which) -> {
 			if (SDK_INT >= 19) {
 				Intent intent = getCreationIntent();
 				startActivityForResult(intent, REQUEST_SAVE_ATTACHMENT);
 			} else {
-				viewModel.saveImage(attachment);
+				viewModel.saveImage(getVisibleAttachment());
 			}
 		};
 		Builder builder = new Builder(this, R.style.BriarDialogTheme);
@@ -303,7 +275,7 @@ public class ImageActivity extends BriarActivity
 		String fileName = sdf.format(new Date());
 		Intent intent = new Intent(ACTION_CREATE_DOCUMENT);
 		intent.addCategory(CATEGORY_OPENABLE);
-		intent.setType(attachment.getMimeType());
+		intent.setType(getVisibleAttachment().getMimeType());
 		intent.putExtra(EXTRA_TITLE, fileName);
 		return intent;
 	}
@@ -320,4 +292,31 @@ public class ImageActivity extends BriarActivity
 		viewModel.onSaveStateSeen();
 	}
 
+	AttachmentItem getVisibleAttachment() {
+		return attachments.get(viewPager.getCurrentItem());
+	}
+
+	private class ImagePagerAdapter extends FragmentStatePagerAdapter {
+
+		private boolean isFirst = true;
+
+		private ImagePagerAdapter(FragmentManager fm) {
+			super(fm);
+		}
+
+		@Override
+		public Fragment getItem(int position) {
+			Fragment f = ImageFragment
+					.newInstance(attachments.get(position), isFirst);
+			isFirst = false;
+			return f;
+		}
+
+		@Override
+		public int getCount() {
+			return attachments.size();
+		}
+
+	}
+
 }

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

@@ -0,0 +1,131 @@
+package org.briarproject.briar.android.conversation;
+
+import android.arch.lifecycle.ViewModelProvider;
+import android.arch.lifecycle.ViewModelProviders;
+import android.graphics.drawable.Animatable;
+import android.graphics.drawable.Drawable;
+import android.os.Bundle;
+import android.support.annotation.Nullable;
+import android.support.v4.app.Fragment;
+import android.view.LayoutInflater;
+import android.view.View;
+import android.view.ViewGroup;
+
+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.bramble.api.nullsafety.MethodsNotNullByDefault;
+import org.briarproject.briar.R;
+import org.briarproject.briar.android.activity.BaseActivity;
+import org.briarproject.briar.android.conversation.glide.GlideApp;
+
+import javax.annotation.ParametersAreNonnullByDefault;
+import javax.inject.Inject;
+
+import static android.os.Build.VERSION.SDK_INT;
+import static android.widget.ImageView.ScaleType.FIT_START;
+import static com.bumptech.glide.load.engine.DiskCacheStrategy.NONE;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.briar.android.conversation.ImageActivity.ATTACHMENT_POSITION;
+
+@MethodsNotNullByDefault
+@ParametersAreNonnullByDefault
+public class ImageFragment extends Fragment {
+
+	private final static String IS_FIRST = "isFirst";
+
+	@Inject
+	ViewModelProvider.Factory viewModelFactory;
+
+	private AttachmentItem attachment;
+	private boolean isFirst;
+	private ImageViewModel viewModel;
+	private PhotoView photoView;
+
+	static ImageFragment newInstance(AttachmentItem a, boolean isFirst) {
+		ImageFragment f = new ImageFragment();
+		Bundle args = new Bundle();
+		args.putParcelable(ATTACHMENT_POSITION, a);
+		args.putBoolean(IS_FIRST, isFirst);
+		f.setArguments(args);
+		return f;
+	}
+
+	@Override
+	public void onActivityCreated(@Nullable Bundle savedInstanceState) {
+		super.onActivityCreated(savedInstanceState);
+		BaseActivity a = (BaseActivity) requireNonNull(getActivity());
+		a.getActivityComponent().inject(this);
+	}
+
+	@Override
+	public void onCreate(@Nullable Bundle savedInstanceState) {
+		super.onCreate(savedInstanceState);
+
+		Bundle args = requireNonNull(getArguments());
+		attachment = requireNonNull(args.getParcelable(ATTACHMENT_POSITION));
+		isFirst = args.getBoolean(IS_FIRST);
+	}
+
+	@Nullable
+	@Override
+	public View onCreateView(LayoutInflater inflater,
+			@Nullable ViewGroup container,
+			@Nullable Bundle savedInstanceState) {
+		View v = inflater.inflate(R.layout.fragment_image, container,
+				false);
+
+		viewModel = ViewModelProviders.of(requireNonNull(getActivity()),
+				viewModelFactory).get(ImageViewModel.class);
+
+		photoView = v.findViewById(R.id.photoView);
+		photoView.setOnClickListener(view -> viewModel.clickImage());
+
+		// Request Listener
+		RequestListener<Drawable> listener = new RequestListener<Drawable>() {
+			@Override
+			public boolean onLoadFailed(@Nullable GlideException e,
+					Object model, Target<Drawable> target,
+					boolean isFirstResource) {
+				if (getActivity() != null && isFirst)
+					getActivity().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 (viewModel.isOverlappingToolbar(photoView, resource)) {
+					photoView.setScaleType(FIT_START);
+				}
+				if (getActivity() != null && isFirst) {
+					getActivity().supportStartPostponedEnterTransition();
+				}
+				return false;
+			}
+		};
+
+		// Load Image
+		GlideApp.with(this)
+				.load(attachment)
+				.diskCacheStrategy(NONE)
+				.error(R.drawable.ic_image_broken)
+				.dontTransform()
+				.addListener(listener)
+				.into(photoView);
+
+		return v;
+	}
+
+}

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

@@ -15,6 +15,7 @@ import org.briarproject.briar.android.conversation.glide.BriarImageTransformatio
 import org.briarproject.briar.android.conversation.glide.GlideApp;
 import org.briarproject.briar.android.conversation.glide.Radii;
 
+import static android.os.Build.VERSION.SDK_INT;
 import static com.bumptech.glide.load.engine.DiskCacheStrategy.NONE;
 import static com.bumptech.glide.load.resource.drawable.DrawableTransitionOptions.withCrossFade;
 
@@ -42,6 +43,9 @@ class ImageViewHolder extends ViewHolder {
 		} else {
 			setImageViewDimensions(attachment, single, needsStretch);
 			loadImage(attachment, r);
+			if (SDK_INT >= 21) {
+				imageView.setTransitionName(attachment.getTransitionName());
+			}
 		}
 	}
 

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

@@ -4,9 +4,11 @@ import android.app.Application;
 import android.arch.lifecycle.AndroidViewModel;
 import android.arch.lifecycle.LiveData;
 import android.arch.lifecycle.MutableLiveData;
+import android.graphics.drawable.Drawable;
 import android.net.Uri;
 import android.support.annotation.Nullable;
 import android.support.annotation.UiThread;
+import android.view.View;
 
 import org.briarproject.bramble.api.db.DatabaseExecutor;
 import org.briarproject.bramble.api.db.DbException;
@@ -48,7 +50,10 @@ public class ImageViewModel extends AndroidViewModel {
 	@IoExecutor
 	private final Executor ioExecutor;
 
+	private final MutableLiveData<Boolean> imageClicked =
+			new MutableLiveData<>();
 	private final MutableLiveData<Boolean> saveState = new MutableLiveData<>();
+	private int toolbarTop, toolbarBottom;
 
 	@Inject
 	ImageViewModel(Application application,
@@ -61,6 +66,45 @@ public class ImageViewModel extends AndroidViewModel {
 		this.ioExecutor = ioExecutor;
 	}
 
+	void clickImage() {
+		imageClicked.setValue(true);
+	}
+
+	/**
+	 * A LiveData that is true if the image was clicked,
+	 * false if it wasn't.
+	 *
+	 * Call {@link #onOnImageClickSeen()} after consuming an update.
+	 */
+	LiveData<Boolean> getOnImageClicked() {
+		return imageClicked;
+	}
+
+	@UiThread
+	void onOnImageClickSeen() {
+		imageClicked.setValue(false);
+	}
+
+	void setToolbarPosition(int top, int bottom) {
+		toolbarTop = top;
+		toolbarBottom = bottom;
+	}
+
+	boolean isOverlappingToolbar(View screenView, Drawable drawable) {
+		int width = drawable.getIntrinsicWidth();
+		int height = drawable.getIntrinsicHeight();
+		float widthPercentage = screenView.getWidth() / (float) width;
+		float heightPercentage = screenView.getHeight() / (float) height;
+		float scaleFactor = Math.min(widthPercentage, heightPercentage);
+		int realWidth = (int) (width * scaleFactor);
+		int realHeight = (int) (height * scaleFactor);
+		// return if image doesn't use the full width,
+		// because it will be moved to the right otherwise
+		if (realWidth < screenView.getWidth()) return false;
+		int drawableTop = (screenView.getHeight() - realHeight) / 2;
+		return drawableTop < toolbarBottom && drawableTop != toolbarTop;
+	}
+
 	/**
 	 * A LiveData that is true if the image was saved,
 	 * false if there was an error and null otherwise.

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

@@ -49,7 +49,8 @@
 		android:id="@+id/conversationView"
 		android:layout_width="match_parent"
 		android:layout_height="0dp"
-		android:layout_weight="2"/>
+		android:layout_weight="2"
+		app:scrollToEnd="false"/>
 
 	<org.briarproject.briar.android.view.ImagePreview
 		android:id="@+id/imagePreview"

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

@@ -8,12 +8,11 @@
 	android:background="@color/briar_black"
 	tools:context=".android.conversation.ImageActivity">
 
-	<com.github.chrisbanes.photoview.PhotoView
-		android:id="@+id/photoView"
+	<android.support.v4.view.ViewPager
+		android:id="@+id/viewPager"
 		android:layout_width="match_parent"
 		android:layout_height="match_parent"
-		tools:ignore="ContentDescription"
-		tools:srcCompat="@tools:sample/backgrounds/scenic"/>
+		tools:background="@color/briar_green_light"/>
 
 	<android.support.design.widget.AppBarLayout
 		android:id="@+id/appBarLayout"

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

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<com.github.chrisbanes.photoview.PhotoView
+	android:id="@+id/photoView"
+	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"
+	tools:ignore="ContentDescription"
+	tools:srcCompat="@tools:sample/backgrounds/scenic"/>