Android AlarmManager 为什么调用 BroadcastReceiver？

Question

I'm currently working on an app that handles alarms. I have reach the point where I can set the alarms using the AlarmManager and everything seems to work fine, but.. In all the examples that I found, and even in the Android official docs, I have seen people using a BroadcastReceiver for the PendingIntent, and then, calling an activity or whatever they need when the alarm fires. However, I have try to just pass a simple activity to the PendingIntent for the AlarmManager like this:

Intent intent = new Intent (getApplicationContext(), AlarmActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity (this, 0, intent, 0);
AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
alarmManager.set (AlarmManager.RTC_WAKEUP, timeToTrigger.getTimeInMillis(), pendingIntent);

And after testing alarms in several contexts (app in foreground, app not running, etc..) I found that the activity is always been called correctly.

So, my doubt is as simple as: Why do people use a BroadcastReceiver and in the onReceive method call an activity if you can just simple call the activity directly?

Answer 1

Android frowns on this behaviour. You shouldn't interrupt the user if he is doing something else. This is why you don't want to launch an Activity from AlarmManager . Usually, you launch a BroadcastReceiver , which, if it wants to get the user's attention, will post a Notification . The user can then open the app via the Notification whenever he wants to.

Also, often you just want to perform some background process (like fetching data from a server, or updating some statistics), which doesn't require any user-interaction. In this case you would also launch a BroadcastReceiver or Service and not an Activity .

Starting with Android 10 there are more restrictions about background apps launching activities. Therefore, with current versions of Android it has become more difficult to do this. See https://developer.android.com/guide/components/activities/background-starts

So basically, even though it does work, it is not considered "respecting the user" and it probably won't work in the future.