暂停和恢复一个Activity

Pausing and Resuming an Activity

PreviousNext

This lesson teaches you to

Pause Your Activity
Resume Your Activity

You should also read

Activities

Try it out

DOWNLOAD THE DEMO

ActivityLifecycle.zip

During normal app use, the foreground activity is sometimes obstructed by other visual components that cause the activity to pause. For example, when a semi-transparent activity opens (such
as one in the style of a dialog), the previous activity pauses. As long as the activity is still partially visible but currently not the activity in focus, it remains paused.
However, once the activity is fully-obstructed and not visible, itstops (which is discussed in the next lesson).
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.



Figure 1. When a semi-transparent activity obscures your activity, the system calls 

onPause()

 and
the activity waits in the Paused state (1). If the user returns to the activity while it's still paused, the system calls 

onResume()

 (2).

Pause Your Activity

When the system calls 

onPause()

 for
your activity, it technically means your activity is still partially visible, but most often is an indication that the user is leaving the activity and it will soon enter the Stopped state. You should usually use the 

onPause()

 callback
to:
Stop animations or other ongoing actions that could consume CPU.
Commit unsaved changes, but only if users expect such changes to be permanently saved when they leave (such as a draft email).
Release system resources, such as broadcast receivers, handles to sensors (like GPS), or any resources that may affect battery life while your activity is paused and the user does not need them.
For example, if your application uses the 

Camera

,
the 

onPause()

 method is a good place to release it.

@Override
public void onPause() {
    super.onPause();  // Always call the superclass method first

    // Release the Camera because we don't need it when paused
    // and other activities might need to use it.
    if (mCamera != null) {
        mCamera.release();
        mCamera = null;
    }
}

Generally, you should not use 

onPause()

 to
store user changes (such as personal information entered into a form) to permanent storage. The only time you should persist user changes to permanent storage within

onPause()

 is
when you're certain users expect the changes to be auto-saved (such as when drafting an email). However, you should avoid performing CPU-intensive work during 

onPause()

,
such as writing to a database, because it can slow the visible transition to the next activity (you should instead perform heavy-load shutdown operations during 

onStop()

).
You should keep the amount of operations done in the 

onPause()

 method
relatively simple in order to allow for a speedy transition to the user's next destination if your activity is actually being stopped.

Note: When your activity is paused, the 

Activity

 instance is kept resident
in memory and is recalled when the activity resumes. You don’t need to re-initialize components that were created during any of the callback methods leading up to the Resumed state.

Resume Your Activity

When the user resumes your activity from the Paused state, the system calls the 

onResume()

 method.
Be aware that the system calls this method every time your activity comes into the foreground, including when it's created for the first time. As such, you should implement 

onResume()

 to
initialize components that you release during 

onPause()

 and perform any other
initializations that must occur each time the activity enters the Resumed state (such as begin animations and initialize components only used while the activity has user focus).
The following example of 

onResume()

 is
the counterpart to the 

onPause()

 example above, so it initializes the camera that's
released when the activity pauses.

@Override
public void onResume() {
    super.onResume();  // Always call the superclass method first

    // Get the Camera instance as the activity achieves full user focus
    if (mCamera == null) {
        initializeCamera(); // Local method to handle camera init
    }
}