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Android - Performing stop of activity that is not resumed

由 寵の児 提交于 2019-11-27 11:19:50

Just giving my 50 cents on the issue. Catching the exception is indeed one possibility, but the correct way to deal with the issue of an activity being killed by the system for its resources in background is a common problem in android and according to Google the solution for this is:

onPause() is where you deal with the user leaving your activity. Most importantly, any changes made by the user should at this point be committed (usually to the ContentProvider holding the data).

Emphasis is mine. But what this means is that the Android lifecycles are designed so that under normal conditions onPause should be called as an Activity or Fragment is sent to the background. They hint at this in several of the android documentation pages:

As your activity enters the paused state, the system calls the onPause() method on your Activity, which allows you to stop ongoing actions that should not continue while paused (such as a video) or persist any information that should be permanently saved in case the user continues to leave your app. If the user returns to your activity from the paused state, the system resumes it and calls the onResume() method.

Note: When your activity receives a call to onPause(), it may be an indication that the activity will be paused for a moment and the user may return focus to your activity. However, it's usually the first indication that the user is leaving your activity.

But the resource that could most likely help you are these two:

http://developer.android.com/training/basics/activity-lifecycle/stopping.html

http://developer.android.com/training/basics/activity-lifecycle/recreating.html

What's probably happening with your lost resources is this:

When your activity receives a call to the onStop() method, it's no longer visible and should release almost all resources that aren't needed while the user is not using it. Once your activity is stopped, the system might destroy the instance if it needs to recover system memory. ... By default, the system uses the Bundle instance state to save information about each View object in your activity layout (such as the text value entered into an EditText object). So, if your activity instance is destroyed and recreated, the state of the layout is restored to its previous state with no code required by you. However, your activity might have more state information that you'd like to restore, such as member variables that track the user's progress in the activity.

Note: In order for the Android system to restore the state of the views in your activity, each view must have a unique ID, supplied by the android:id attribute.

To save additional data about the activity state, you must override the onSaveInstanceState() callback method. The system calls this method when the user is leaving your activity and passes it the Bundle object that will be saved in the event that your activity is destroyed unexpectedly. If the system must recreate the activity instance later, it passes the same Bundle object to both the onRestoreInstanceState() and onCreate() methods.

The correct solution for this is to override and implement the lifecycle methods of the Activity / Fragment as needed.

Two examples given by Google:

    static final String STATE_SCORE = "playerScore";
static final String STATE_LEVEL = "playerLevel";
...

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state
    savedInstanceState.putInt(STATE_SCORE, mCurrentScore);
    savedInstanceState.putInt(STATE_LEVEL, mCurrentLevel);

    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}
Caution: Always call the superclass implementation of onSaveInstanceState() so the default implementation can save the state of the view hierarchy.

And the reverse restore operation:

public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);

    // Restore state members from saved instance
    mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
    mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
}
Whisky

One Line: It seems some of your activity variables were freed from memory as Android OS needed memory for the Facebook application.

Explanation: When an application in foreground needs more memory than that is available, Android frees some memory from the apps that are running in background. Foreground tasks always have higher priority than background applications.

So, what might have happened to your application while it was in background is that some of its variables have lost their values which you are using in your onResume(). Because of this they are holding wrong values or default values (you can check by using Sysout) as they are re-created when you again bring your app to foreground and due to which some of your code is not working right.

Diarrhio

Hopefully you've addressed the issue already, but there is an error in your code, perhaps related:

if (backgroundThreadRunning = true) {
    backgroundThreadRunning = false;
}

you are assigning instead of comparing in the "if" statement. Should be:

if (backgroundThreadRunning == true) {
    backgroundThreadRunning = false;
}

or 

if (backgroundThreadRunning) {
    backgroundThreadRunning = false;
}
标签
android
android-activity
background
android-lifecycle
onresume
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