重新創建Activity · Android Training官方培訓課程中文版

> 編寫:[kesenhoo](https://github.com/kesenhoo) - 原文: [http://developer.android.com/training/basics/](http://developer.android.com/training/basics/activity-lifecycle/recreating.html)[activity](# "An activity represents a single screen with a user interface.")-lifecycle/recreating.html 有幾個場景中，[Activity](# "An activity represents a single screen with a user interface.")是由于正常的程序行為而被Destory的。例如當用戶點擊返回按鈕或者是[Activity](# "An activity represents a single screen with a user interface.")通過調用[finish()](http://developer.android.com/reference/android/app/Activity.html#finish())來發出停止信號。系統也有可能會在[Activity](# "An activity represents a single screen with a user interface.")處于stop狀態且長時間不被使用，或者是在前臺[activity](# "An activity represents a single screen with a user interface.")需要更多系統資源的時關閉后臺進程，以圖獲取更多的內存。當[Activity](# "An activity represents a single screen with a user interface.")是因為用戶點擊Back按鈕或者是[activity](# "An activity represents a single screen with a user interface.")通過調用finish()結束自己時，系統就丟失了對[Activity](# "An activity represents a single screen with a user interface.")實例的引用，因為這一行為意味著不再需要這個[activity](# "An activity represents a single screen with a user interface.")了。然而，如果因為系統資源緊張而導致[Activity](# "An activity represents a single screen with a user interface.")的Destory，系統會在用戶回到這個[Activity](# "An activity represents a single screen with a user interface.")時有這個[Activity](# "An activity represents a single screen with a user interface.")存在過的記錄，系統會使用那些保存的記錄數據（描述了當[Activity](# "An activity represents a single screen with a user interface.")被Destory時的狀態）來重新創建一個新的[Activity](# "An activity represents a single screen with a user interface.")實例。那些被系統用來恢復之前狀態而保存的數據被叫做 "instance state" ，它是一些存放在[Bundle](http://developer.android.com/reference/android/os/Bundle.html)對象中的key-value pairs。(_請注意這里的描述，這對理解onSaveInstanceState執行的時刻很重要_) > **Caution:** 你的[Activity](# "An activity represents a single screen with a user interface.")會在每次旋轉屏幕時被destroyed與recreated。當屏幕改變方向時，系統會Destory與Recreate前臺的[activity](# "An activity represents a single screen with a user interface.")，因為屏幕配置被改變，你的[Activity](# "An activity represents a single screen with a user interface.")可能需要加載另一些替代的資源(例如layout). 默認情況下, 系統使用 Bundle 實例來保存每一個View(視圖)對象中的信息(例如輸入EditText 中的文本內容)。因此，如果[Activity](# "An activity represents a single screen with a user interface.")被destroyed與recreated, 則layout的狀態信息會自動恢復到之前的狀態。然而，[activity](# "An activity represents a single screen with a user interface.")也許存在更多你想要恢復的狀態信息，例如記錄用戶Progress的成員變量(member variables)。 > **Note:** 為了使Android系統能夠恢復[Activity](# "An activity represents a single screen with a user interface.")中的View的狀態，**每個View都必須有一個唯一ID**，由[android:id](http://developer.android.com/reference/android/view/View.html#attr_android:id)定義。為了可以保存額外更多的數據到saved instance state。在[Activity](# "An activity represents a single screen with a user interface.")的生命周期里面存在一個額外的回調函數，你必須重寫這個函數。該回調函數并沒有在前面課程的圖片示例中顯示。這個方法是[onSaveInstanceState()](http://developer.android.com/reference/android/app/Activity.html#onSaveInstanceState(android.os.Bundle)) ，當用戶離開[Activity](# "An activity represents a single screen with a user interface.")時，系統會調用它。當系統調用這個函數時，系統會在[Activity](# "An activity represents a single screen with a user interface.")被異常Destory時傳遞 Bundle 對象，這樣我們就可以增加額外的信息到Bundle中并保存到系統中。若系統在[Activity](# "An activity represents a single screen with a user interface.")被Destory之后想重新創建這個[Activity](# "An activity represents a single screen with a user interface.")實例時，之前的Bundle對象會(系統)被傳遞到你我們[activity](# "An activity represents a single screen with a user interface.")的[onRestoreInstanceState()](http://developer.android.com/reference/android/app/Activity.html#onRestoreInstanceState(android.os.Bundle))方法與 onCreate() 方法中。 ![basic-lifecycle-savestate](https://box.kancloud.cn/2015-07-28_55b7247006a8c.png) **Figure 2.** 當系統開始停止[Activity](# "An activity represents a single screen with a user interface.")時，只有在[Activity](# "An activity represents a single screen with a user interface.")實例會需要重新創建的情況下才會調用到[onSaveInstanceState()](http://developer.android.com/reference/android/app/Activity.html#onSaveInstanceState(android.os.Bundle)) (1) ，在這個方法里面可以指定額外的狀態數據到Bunde中。如果這個[Activity](# "An activity represents a single screen with a user interface.")被destroyed然后這個實例又需要被重新創建時，系統會傳遞在 (1) 中的狀態數據到 onCreate() (2) 與 [onRestoreInstanceState()](http://developer.android.com/reference/android/app/Activity.html#onRestoreInstanceState(android.os.Bundle))(3). _(通常來說，跳轉到其他的[activity](# "An activity represents a single screen with a user interface.")或者是點擊Home都會導致當前的[activity](# "An activity represents a single screen with a user interface.")執行onSaveInstanceState，因為這種情況下的[activity](# "An activity represents a single screen with a user interface.")都是有可能會被destory并且是需要保存狀態以便后續恢復使用的，而從跳轉的[activity](# "An activity represents a single screen with a user interface.")點擊back回到前一個[activity](# "An activity represents a single screen with a user interface.")，那么跳轉前的[activity](# "An activity represents a single screen with a user interface.")是執行退棧的操作，所以這種情況下是不會執行onSaveInstanceState的，因為這個[activity](# "An activity represents a single screen with a user interface.")不可能存在需要重建的操作)_ ### 保存[Activity](# "An activity represents a single screen with a user interface.")狀態當我們的[activity](# "An activity represents a single screen with a user interface.")開始Stop，系統會調用 onSaveInstanceState() ，[Activity](# "An activity represents a single screen with a user interface.")可以用鍵值對的集合來保存狀態信息。這個方法會默認保存[Activity](# "An activity represents a single screen with a user interface.")視圖的狀態信息，如在 EditText 組件中的文本或 ListView 的滑動位置。為了給[Activity](# "An activity represents a single screen with a user interface.")保存額外的狀態信息，你必須實現onSaveInstanceState() 并增加key-value pairs到 Bundle 對象中，例如： ~~~ static final String STATE_SCORE = "playerScore"; static final String STATE_LEVEL = "playerLevel"; ... @Override public void onSaveInstanceState(Bundle savedInstanceState) { // Save the user's current game state savedInstanceState.putInt(STATE_SCORE, mCurrentScore); savedInstanceState.putInt(STATE_LEVEL, mCurrentLevel); // Always call the superclass so it can save the view hierarchy state super.onSaveInstanceState(savedInstanceState); } ~~~ > **Caution**: 必須要調用 onSaveInstanceState() 方法的父類實現，這樣默認的父類實現才能保存視圖狀態的信息。 ### 恢復[Activity](# "An activity represents a single screen with a user interface.")狀態當[Activity](# "An activity represents a single screen with a user interface.")從Destory中重建，我們可以從系統傳遞的[Activity](# "An activity represents a single screen with a user interface.")的Bundle中恢復保存的狀態。 onCreate() 與 onRestoreInstanceState() 回調方法都接收到了同樣的Bundle，里面包含了同樣的實例狀態信息。由于 onCreate() 方法會在第一次創建新的[Activity](# "An activity represents a single screen with a user interface.")實例與重新創建之前被Destory的實例時都被調用，我們必須在嘗試讀取 Bundle 對象前檢測它是否為null。如果它為null，系統則是創建一個新的[Activity](# "An activity represents a single screen with a user interface.")實例，而不是恢復之前被Destory的[Activity](# "An activity represents a single screen with a user interface.")。下面是一個示例：演示在onCreate方法里面恢復一些數據： ~~~ @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // Always call the superclass first // Check whether we're recreating a previously destroyed instance if (savedInstanceState != null) { // Restore value of members from saved state mCurrentScore = savedInstanceState.getInt(STATE_SCORE); mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL); } else { // Probably initialize members with default values for a new instance } ... } ~~~ 我們也可以選擇實現 onRestoreInstanceState() ，而不是在onCreate方法里面恢復數據。 **onRestoreInstanceState()方法會在 onStart() 方法之后執行. 系統僅僅會在存在需要恢復的狀態信息時才會調用 onRestoreInstanceState() ，因此不需要檢查 Bundle 是否為null。** ~~~ public void onRestoreInstanceState(Bundle savedInstanceState) { // Always call the superclass so it can restore the view hierarchy super.onRestoreInstanceState(savedInstanceState); // Restore state members from saved instance mCurrentScore = savedInstanceState.getInt(STATE_SCORE); mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL); } ~~~ > **Caution**: 與上面保存一樣，總是需要調用onRestoreInstanceState()方法的父類實現，這樣默認的父類實現才能保存視圖狀態的信息。更多關于運行時狀態改變引起的recreate我們的[activity](# "An activity represents a single screen with a user interface.")。請參考[Handling Runtime Changes](http://developer.android.com/guide/topics/resources/runtime-changes.html).