Android：如何避免启动已经在堆栈中的活动？

Question

Lets try to explain my question: I got an application and a service. The application is started with activity A. The service sends a broadcast that will let the application start Activity B Now the user starts activity C.

Now the service wants to start activity B again. But how do I let him know that the activity is still on the stack, or is there an intent flag for this?

How do I avoid that it will launch activity B because its already in the stack?

Answer 1

I think you need to make your activity B singleInstance that if it's already create you don't want to create again, that is launch mode of the activity can be defined in manifest android:launchMode that defines how the activity will be instanciated .

in your case use android:launchMode="singleInstance"

Answer 2

You can use flag Intent.FLAG_ACTIVITY_NEW_TASK . If the activity is already running it will bring that to front instead of creating new activity.

If you add Intent.FLAG_ACTIVITY_CLEAR_TOP with this, then all the activities after this activity in the backstack will be cleared.

Answer 3

If the activity will be on the top if already started, set the FLAG_ACTIVITY_SINGLE_TOP flag

intent.setFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
mContext.startActivity(intent);

Answer 4

Approaches android:launchMode="singleInstance" and just adding flags to the Intent do not work for me. What works is that:

In the code where activity gets started:

Intent intent = new Intent(activity, ActivityThatHasToBeStarted.class);
intent.setFlags(Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT);
startActivity(intent);

In the ActivityThatHasToBeStarted :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if ((getIntent().getFlags() & Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT) != 0) {
        finish();
        return;
    }
    // Code for this creation
}

Answer 5

If you don't need the second activity anymore, it is good practise to finish it, then you should do this on the second activity after the operation is ended:

startActivity(new Intent(this, FirstActivity.class).setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP));
finish();

Answer 6

您可以考虑使用android:launchMode="singleTop"而不是android:launchMode="singleInstance" 关于差异的好文章

Answer 7

I would suggest you use Intent.FLAG_ACTIVITY_CLEAR_TOP as it will remove all activities which start on top of your targeted activity.

eg :

Intent intent = new Intent(sourceActivity, Target activity);

intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

startActivity(intent)