Improve UI hiding behaviour.

briar-android/src/main/java/org/briarproject/briar/android/BackgroundMonitor.java

@@ -0,0 +1,54 @@
+package org.briarproject.briar.android;
+
+import android.app.Activity;
+import android.app.Application.ActivityLifecycleCallbacks;
+import android.os.Bundle;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+class BackgroundMonitor implements ActivityLifecycleCallbacks {
+
+	private final AtomicInteger foregroundActivities = new AtomicInteger(0);
+
+	boolean isRunningInBackground() {
+		return foregroundActivities.get() == 0;
+	}
+
+	@Override
+	public void onActivityCreated(Activity a, @Nullable Bundle state) {
+	}
+
+	@Override
+	public void onActivityStarted(Activity a) {
+		foregroundActivities.incrementAndGet();
+	}
+
+	@Override
+	public void onActivityResumed(Activity a) {
+	}
+
+	@Override
+	public void onActivityPaused(Activity a) {
+	}
+
+	@Override
+	public void onActivityStopped(Activity a) {
+		foregroundActivities.decrementAndGet();
+	}
+
+	@Override
+	public void onActivitySaveInstanceState(Activity a,
+			@Nullable Bundle outState) {
+	}
+
+	@Override
+	public void onActivityDestroyed(Activity a) {
+	}
+}

briar-android/src/main/java/org/briarproject/briar/android/BriarApplication.java

@@ -16,4 +16,6 @@ public interface BriarApplication {
 	AndroidComponent getApplicationComponent();
 
 	SharedPreferences getDefaultSharedPreferences();
+
+	boolean isRunningInBackground();
 }

briar-android/src/main/java/org/briarproject/briar/android/BriarApplicationImpl.java

@@ -1,5 +1,7 @@
 package org.briarproject.briar.android;
 
+import android.app.ActivityManager;
+import android.app.ActivityManager.RunningAppProcessInfo;
 import android.app.Application;
 import android.content.Context;
 import android.content.SharedPreferences;
@@ -30,6 +32,8 @@ import java.util.logging.Handler;
 import java.util.logging.LogRecord;
 import java.util.logging.Logger;
 
+import static android.app.ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND;
+import static android.os.Build.VERSION.SDK_INT;
 import static java.util.logging.Level.FINE;
 import static java.util.logging.Level.INFO;
 import static org.acra.ReportField.ANDROID_VERSION;
@@ -79,6 +83,7 @@ public class BriarApplicationImpl extends Application
 			Logger.getLogger(BriarApplicationImpl.class.getName());
 
 	private final CachingLogHandler logHandler = new CachingLogHandler();
+	private final BackgroundMonitor backgroundMonitor = new BackgroundMonitor();
 
 	private AndroidComponent applicationComponent;
 	private volatile SharedPreferences prefs;
@@ -115,6 +120,9 @@ public class BriarApplicationImpl extends Application
 
 		applicationComponent = createApplicationComponent();
 		EmojiManager.install(new GoogleEmojiProvider());
+
+		if (SDK_INT < 16)
+			registerActivityLifecycleCallbacks(backgroundMonitor);
 	}
 
 	protected AndroidComponent createApplicationComponent() {
@@ -173,4 +181,15 @@ public class BriarApplicationImpl extends Application
 	public SharedPreferences getDefaultSharedPreferences() {
 		return prefs;
 	}
+
+	@Override
+	public boolean isRunningInBackground() {
+		if (SDK_INT >= 16) {
+			RunningAppProcessInfo info = new RunningAppProcessInfo();
+			ActivityManager.getMyMemoryState(info);
+			return (info.importance != IMPORTANCE_FOREGROUND);
+		} else {
+			return backgroundMonitor.isRunningInBackground();
+		}
+	}
 }

briar-android/src/main/java/org/briarproject/briar/android/BriarService.java

@@ -1,7 +1,5 @@
 package org.briarproject.briar.android;
 
-import android.app.ActivityManager;
-import android.app.ActivityManager.RunningAppProcessInfo;
 import android.app.Notification;
 import android.app.NotificationChannel;
 import android.app.NotificationManager;
@@ -35,7 +33,6 @@ import java.util.logging.Logger;
 import javax.annotation.Nullable;
 import javax.inject.Inject;
 
-import static android.app.ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND;
 import static android.app.NotificationManager.IMPORTANCE_DEFAULT;
 import static android.app.NotificationManager.IMPORTANCE_NONE;
 import static android.app.PendingIntent.FLAG_UPDATE_CURRENT;
@@ -74,6 +71,7 @@ public class BriarService extends Service {
 
 	@Nullable
 	private BroadcastReceiver receiver = null;
+	private BriarApplication app;
 
 	@Inject
 	AndroidNotificationManager notificationManager;
@@ -93,8 +91,8 @@ public class BriarService extends Service {
 	public void onCreate() {
 		super.onCreate();
 
-		BriarApplication application = (BriarApplication) getApplication();
+		app = (BriarApplication) getApplication();
-		application.getApplicationComponent().inject(this);
+		app.getApplicationComponent().inject(this);
 
 		LOG.info("Created");
 		if (created.getAndSet(true)) {
@@ -220,8 +218,8 @@ public class BriarService extends Service {
 	public void onLowMemory() {
 		super.onLowMemory();
 		LOG.warning("Memory is low");
-		// Clear the UI - this is done in onTrimMemory() if SDK_INT >= 16
+		// If we're not in the foreground, clear the UI to save memory
-		if (SDK_INT < 16) hideUi();
+		if (app.isRunningInBackground()) hideUi();
 	}
 
 	@Override
@@ -235,20 +233,16 @@ public class BriarService extends Service {
 			LOG.info("Trim memory: near middle of LRU list");
 		} else if (level == TRIM_MEMORY_COMPLETE) {
 			LOG.info("Trim memory: near end of LRU list");
-		} else if (SDK_INT >= 16) {
+		} else if (level == TRIM_MEMORY_RUNNING_MODERATE) {
-			if (level == TRIM_MEMORY_RUNNING_MODERATE) {
+			LOG.info("Trim memory: running moderately low");
-				LOG.info("Trim memory: running moderately low");
+		} else if (level == TRIM_MEMORY_RUNNING_LOW) {
-			} else if (level == TRIM_MEMORY_RUNNING_LOW) {
+			LOG.info("Trim memory: running low");
-				LOG.info("Trim memory: running low");
+		} else if (level == TRIM_MEMORY_RUNNING_CRITICAL) {
-			} else if (level == TRIM_MEMORY_RUNNING_CRITICAL) {
+			// This level may be received if SDK_INT < 16, although the
-				LOG.info("Trim memory: running critically low");
+			// constant isn't declared until API level 16
-				// If we're not in the foreground, clear the UI to save memory
+			LOG.warning("Trim memory: running critically low");
-				RunningAppProcessInfo info = new RunningAppProcessInfo();
+			// If we're not in the foreground, clear the UI to save memory
-				ActivityManager.getMyMemoryState(info);
+			if (app.isRunningInBackground()) hideUi();
-				if (info.importance != IMPORTANCE_FOREGROUND) hideUi();
-			} else if (LOG.isLoggable(INFO)) {
-				LOG.info("Trim memory: unknown level " + level);
-			}
 		} else if (LOG.isLoggable(INFO)) {
 			LOG.info("Trim memory: unknown level " + level);
 		}

briar-android/src/test/java/org/briarproject/briar/android/TestBriarApplication.java

@@ -63,4 +63,9 @@ public class TestBriarApplication extends Application
 	public SharedPreferences getDefaultSharedPreferences() {
 		return prefs;
 	}
+
+	@Override
+	public boolean isRunningInBackground() {
+		return false;
+	}
 }