暂停与恢复Activity

> 编写:[kesenhoo](https://github.com/kesenhoo) - 原文:[http://developer.android.com/training/basics/](http://developer.android.com/training/basics/activity-lifecycle/pausing.html)[activity](# "An activity represents a single screen with a user interface.")-lifecycle/pausing.html 在正常使用app时，前端的[activity](# "An activity represents a single screen with a user interface.")有时会被其他可见的组件阻塞(obstructed)，从而导致当前的[activity](# "An activity represents a single screen with a user interface.")进入Pause状态。例如，当打开一个半透明的[activity](# "An activity represents a single screen with a user interface.")时(例如以对话框的形式)，之前的[activity](# "An activity represents a single screen with a user interface.")会被暂停。 只要之前的[activity](# "An activity represents a single screen with a user interface.")仍然被部分可见，这个[activity](# "An activity represents a single screen with a user interface.")就会一直处于Paused状态。 然而，一旦之前的[activity](# "An activity represents a single screen with a user interface.")被完全阻塞并不可见时，则其会进入Stop状态(将在下一小节讨论)。 [activity](# "An activity represents a single screen with a user interface.")一旦进入paused状态，系统就会调用[activity](# "An activity represents a single screen with a user interface.")中的[onPause()](http://developer.android.com/reference/android/app/Activity.html#onPause())方法, 该方法中可以停止不应该在暂停过程中执行的操作，如暂停视频播放；或者保存那些有可能需要长期保存的信息。如果用户从暂停状态回到当前[activity](# "An activity represents a single screen with a user interface.")，系统应该恢复那些数据并执行[onResume()](http://developer.android.com/reference/android/app/Activity.html#onResume())方法。 > **Note:** 当我们的[activity](# "An activity represents a single screen with a user interface.")收到调用onPause()的信号时，那可能意味者[activity](# "An activity represents a single screen with a user interface.")将被暂停一段时间，并且用户很可能回到我们的[activity](# "An activity represents a single screen with a user interface.")。然而，那也是用户要离开我们的activtiy的第一个信号。  **Figure 1.** 当一个半透明的[activity](# "An activity represents a single screen with a user interface.")阻塞[activity](# "An activity represents a single screen with a user interface.")时，系统会调用onPause()方法并且这个[activity](# "An activity represents a single screen with a user interface.")会停留在Paused 状态(1). 如果用户在这个[activity](# "An activity represents a single screen with a user interface.")还是在Paused 状态时回到这个[activity](# "An activity represents a single screen with a user interface.")，系统则会调用它的onResume() (2). ### 暂停[Activity](# "An activity represents a single screen with a user interface.") 当系统调用[activity](# "An activity represents a single screen with a user interface.")中的onPause()，从技术上讲，意味着[activity](# "An activity represents a single screen with a user interface.")仍然处于部分可见的状态.但更多时候意味着用户正在离开这个[activity](# "An activity represents a single screen with a user interface.")，并马上会进入Stopped state. 通常应该在onPause()回调方法里面做以下事情: - 停止动画或者是其他正在运行的操作，那些都会导致CPU的浪费. - 提交在用户离开时期待保存的内容(例如邮件草稿). - 释放系统资源，例如broadcast receivers, sensors (比如GPS), 或者是其他任何会影响到电量的资源。 例如, 如果程序使用[Camera](http://developer.android.com/reference/android/hardware/Camera.html),onPause()会是一个比较好的地方去做那些释放资源的操作。 ~~~ @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; } } ~~~ 通常，**不应该**使用onPause()来保存用户改变的数据 (例如填入表格中的个人信息) 到永久存储(File或者DB)上。仅仅当确认用户期待那些改变能够被自动保存的时候(例如正在撰写邮件草稿)，才把那些数据存到永久存储 。但是，我们应该避免在onPause()时执行CPU-intensive 的工作，例如写数据到DB，因为它会导致切换到下一个[activity](# "An activity represents a single screen with a user interface.")变得缓慢(应该把那些heavy-load的工作放到[onStop()](http://developer.android.com/reference/android/app/Activity.html#onStop())去做)。 如果[activity](# "An activity represents a single screen with a user interface.")实际上是要被Stop，那么我们应该为了切换的顺畅而减少在OnPause()方法里面的工作量。 > **Note:**当[activity](# "An activity represents a single screen with a user interface.")处于暂停状态，[Activity](# "An activity represents a single screen with a user interface.")实例是驻留在内存中的，并且在[activity](# "An activity represents a single screen with a user interface.") 恢复的时候重新调用。我们不需要在恢复到Resumed状态的一系列回调方法中重新初始化组件。 ### 恢复[activity](# "An activity represents a single screen with a user interface.") 当用户从Paused状态恢复[activity](# "An activity represents a single screen with a user interface.")时，系统会调用onResume()方法。 请注意，系统每次调用这个方法时，[activity](# "An activity represents a single screen with a user interface.")都处于前台，包括第一次创建的时候。所以，应该实现onResume()来初始化那些在onPause方法里面释放掉的组件，并执行那些[activity](# "An activity represents a single screen with a user interface.")每次进入Resumed state都需要的初始化动作 (例如开始动画与初始化那些只有在获取用户焦点时才需要的组件) 下面的onResume()的例子是与上面的onPause()例子相对应的。 ~~~ @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 } } ~~~