Why does starting an activity from a widget cause

问题:

When I start an activitiy from a widget I want the back button to go to the home screen but instead it goes to the app's main activity. After toying around I found that if I somehow close the main app activity, this problem doesn't occur. Strange.

I found a solution here that said to call finish(); in my main activity's onPause(). Obviously this is the wrong solution e.g. reorientation of the screen causes an onPause() so the will activity will die whenever the phone is rotated.

This is how I start my activity:

@Override
public void onReceive(Context context, Intent intent) {
    [...]
        //new Emergency().emDialog(context).show();
        Intent myIntent = new Intent(context, EmergencyActivity.class);

        // FLAG_ACTIVITY_NEW_TASK is needed because we're not in an activity
        // already, without it we crash.
        myIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        context.startActivity(myIntent);

You can see the rest of the code at http://code.google.com/p/emergencybutton/source/browse

edit: I tried running the activity differently, but still it doesn't work correctly:

Intent myIntent = new Intent();
myIntent.setClassName("com.emergency.button", "com.emergency.button.EmergencyActivity");

回答1:

Although I've never used widgets, I believe that when you click the widget you are resuming an existing task. Hence, when you are in that task, you will return to the latest activity in that task (instead of Home).

See this link and choose the proper launch mode for your widget http://developer.android.com/guide/topics/fundamentals.html#lmodes

回答2:

Ok, so I'm not exactly sure what happened here but android:launchMode="singleInstance" in the activity in AndroidManifest.xml fixed it somehow.

    <activity android:name=".EmergencyActivity"
              android:launchMode="singleInstance"

@Octavian - I should have clarified that I start the activity from an onReceive in an AppWidgetProvider. I'm at the home screen, starting an activity titled B, but somehow both A and B are in the activity stack instead of just B.

http://developer.android.com/guide/topics/manifest/activity-element.html#lmode

回答3:

The behavior is not strange it is just the way Android works. The activity stack just keeps track of the all the activities. Now when you start an activity A which starts another activity B then your stack looks like (B, A). If you press the back key then activity B is going to get popped off the stack and A is going to be brought to foreground again.

The right solution is to just call finish() right after firing off the Intent.

回答4:

Sometimes it's not possible to use launchMode singleInstance in application for some reasons. In this case, you should start your activity and clear activity stack. You can archive this using flags. There is an example:

intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);