啟動與銷毀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/starting.html)[activity](# "An activity represents a single screen with a user interface.")-lifecycle/starting.html 不同于其他編程范式（程序從`main()`方法開始啟動），Android系統根據生命周期的不同階段喚起對應的回調函數來執行代碼。系統存在啟動與銷毀一個[activity](# "An activity represents a single screen with a user interface.")的一套有序的回調函數。本課介紹生命周期中最重要的回調函數，并演示如何處理啟動一個[activity](# "An activity represents a single screen with a user interface.")所涉及到的回調函數。 ### 理解生命周期的回調在一個[activity](# "An activity represents a single screen with a user interface.")的生命周期中，系統會像金字塔模型一樣去調用一系列的生命周期回調函數。[Activity](# "An activity represents a single screen with a user interface.")生命周期的每一個階段就像金字塔中的臺階。當系統創建了一個新的[activity](# "An activity represents a single screen with a user interface.")實例，每一個回調函數會向上一階移動[activity](# "An activity represents a single screen with a user interface.")狀態。處在金字塔頂端意味著當前[activity](# "An activity represents a single screen with a user interface.")處在前臺并處于用戶可與其進行交互的狀態。當用戶退出這個[activity](# "An activity represents a single screen with a user interface.")時，為了回收該[activity](# "An activity represents a single screen with a user interface.")，系統會調用其它方法來向下一階移動[activity](# "An activity represents a single screen with a user interface.")狀態。在某些情況下，[activity](# "An activity represents a single screen with a user interface.")會隱藏在金字塔下等待(例如當用戶切換到其他app),此時[activity](# "An activity represents a single screen with a user interface.")可以重新回到頂端(如果用戶回到這個[activity](# "An activity represents a single screen with a user interface."))并恢復用戶離開時的狀態。 ![basic-lifecycle](https://box.kancloud.cn/2015-07-28_55b7246f9d0d6.png) **Figure 1.** 下面這張圖講解了[activity](# "An activity represents a single screen with a user interface.")的生命周期：_(這個金字塔模型要比之前Dev Guide里面的生命周期圖更加容易理解，更加形象)_ 根據[activity](# "An activity represents a single screen with a user interface.")的復雜度，也許不需要實現所有的生命周期方法。但了解每一個方法的回調時機并在其中填充相應功能，使得確保app能夠像用戶期望的那樣執行是很有必要的。如何實現一個符合用戶期待的app，我們需要注意下面幾點： - 使用app的時候，不會因為有來電通話或者切換到其他app而導致程序crash。 - 用戶沒有激活某個組件時不會消耗寶貴的系統資源。 - 離開app并且一段時間后返回，不會丟失用戶的使用進度。 - 設備發生屏幕旋轉時不會crash或者丟失用戶的使用進度。下面的課程會介紹上圖所示的幾個生命狀態。然而，其中只有三個狀態是靜態的，這三個狀態下[activity](# "An activity represents a single screen with a user interface.")可以存在一段比較長的時間。_(其它幾個狀態會很快就切換掉，停留的時間比較短暫)_ - **Resumed**：該狀態下，[activity](# "An activity represents a single screen with a user interface.")處在前臺，用戶可以與它進行交互。(通常也被理解為"running" 狀態) - **Paused**：該狀態下，[activity](# "An activity represents a single screen with a user interface.")的部分被另外一個[activity](# "An activity represents a single screen with a user interface.")所遮蓋：另外的[activity](# "An activity represents a single screen with a user interface.")來到前臺，但是半透明的，不會覆蓋整個屏幕。被暫停的[activity](# "An activity represents a single screen with a user interface.")不再接受用戶的輸入且不再執行任何代碼。 - **Stopped**：該狀態下, [activity](# "An activity represents a single screen with a user interface.")完全被隱藏，對用戶不可見。可以認為是在后臺。當stopped, [activity](# "An activity represents a single screen with a user interface.")實例與它的所有狀態信息（如成員變量等）都會被保留，但[activity](# "An activity represents a single screen with a user interface.")不能執行任何代碼。其它狀態 (**Created**與**Started**)都是短暫的，系統快速的執行那些回調函數并通過執行下一階段的回調函數移動到下一個狀態。也就是說，在系統調用[onCreate()](http://developer.android.com/reference/android/app/Activity.html#onCreate(android.os.Bundle)), 之后會迅速調用[onStart()](http://developer.android.com/reference/android/app/Activity.html#onStart()), 之后再迅速執行[onResume()](http://developer.android.com/reference/android/app/Activity.html#onResume())。以上就是基本的[activity](# "An activity represents a single screen with a user interface.")生命周期。 ### 指定程序首次啟動的[Activity](# "An activity represents a single screen with a user interface.") 當用戶從主界面點擊程序圖標時，系統會調用app中被聲明為"launcher" (or "main") [activity](# "An activity represents a single screen with a user interface.")中的onCreate()方法。這個[Activity](# "An activity represents a single screen with a user interface.")被用來當作程序的主要進入點。我們可以在[AndroidManifest.xml](http://developer.android.com/guide/topics/manifest/manifest-intro.html)中定義作為主[activity](# "An activity represents a single screen with a user interface.")的[activity](# "An activity represents a single screen with a user interface.")。這個main [activity](# "An activity represents a single screen with a user interface.")必須在manifest使用包括 `MAIN` action 與 `LAUNCHER` category 的[`<intent-filter>`](http://developer.android.com/guide/topics/manifest/intent-filter-element.html)標簽來聲明。例如： ~~~ <activity android:name=".MainActivity" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> ~~~ > **Note**:當你使用Android SDK工具來創建Android工程時，工程中就包含了一個默認的聲明有這個filter的[activity](# "An activity represents a single screen with a user interface.")類。如果程序中沒有聲明了[MAIN](http://developer.android.com/reference/android/content/Intent.html#ACTION_MAIN) action 或者[LAUNCHER](http://developer.android.com/reference/android/content/Intent.html#CATEGORY_LAUNCHER) category的[activity](# "An activity represents a single screen with a user interface.")，那么在設備的主界面列表里面不會呈現app圖標。 ### 創建一個新的實例大多數app包括多個[activity](# "An activity represents a single screen with a user interface.")，使用戶可以執行不同的動作。不論這個[activity](# "An activity represents a single screen with a user interface.")是當用戶點擊應用圖標創建的main activtiy還是為了響應用戶行為而創建的其他[activity](# "An activity represents a single screen with a user interface.")，系統都會調用新[activity](# "An activity represents a single screen with a user interface.")實例中的onCreate()方法。我們必須實現onCreate()方法來執行程序啟動所需要的基本邏輯。例如可以在onCreate()方法中定義UI以及實例化類成員變量。例如：下面的onCreate()方法演示了為了建立一個[activity](# "An activity represents a single screen with a user interface.")所需要的一些基礎操作。如聲明UI元素，定義成員變量，配置UI等。_(onCreate里面盡量少做事情，避免程序啟動太久都看不到界面)_ ~~~ TextView mTextView; // Member variable for text view in the layout @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // Set the user interface layout for this Activity // The layout file is defined in the project res/layout/main_activity.xml file setContentView(R.layout.main_activity); // Initialize member TextView so we can manipulate it later mTextView = (TextView) findViewById(R.id.text_message); // Make sure we're running on Honeycomb or higher to use ActionBar APIs if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) { // For the main activity, make sure the app icon in the action bar // does not behave as a button ActionBar actionBar = getActionBar(); actionBar.setHomeButtonEnabled(false); } } ~~~ > **Caution**：用[SDK_INT](http://developer.android.com/reference/android/os/Build.VERSION.html#SDK_INT)來避免舊的系統調用了只在Android 2.0（API level 5）或者更新的系統可用的方法（上述if條件中的代碼）。舊的系統調用了這些方法會拋出一個運行時異常。一旦onCreate 操作完成，系統會迅速調用onStart() 與onResume()方法。我們的[activity](# "An activity represents a single screen with a user interface.")不會在Created或者Started狀態停留。技術上來說, [activity](# "An activity represents a single screen with a user interface.")在onStart()被調用后開始被用戶可見，但是 onResume()會迅速被執行使得[activity](# "An activity represents a single screen with a user interface.")停留在Resumed狀態，直到一些因素發生變化才會改變這個狀態。例如接收到一個來電，用戶切換到另外一個[activity](# "An activity represents a single screen with a user interface.")，或者是設備屏幕關閉。在后面的課程中，我們將看到其他方法是如何使用的，onStart() 與 onResume()在用戶從Paused或Stopped狀態中恢復的時候非常有用。 > **Note:** onCreate() 方法包含了一個參數叫做savedInstanceState，這將會在后面的課程 - [重新創建](#)[activity](# "An activity represents a single screen with a user interface.")涉及到。 ![basic_lifecycle-create](https://box.kancloud.cn/2015-07-28_55b7246fb816b.png) **Figure 2.** 上圖顯示了onCreate(), onStart() 和 onResume()是如何執行的。當這三個順序執行的回調函數完成后，[activity](# "An activity represents a single screen with a user interface.")會到達Resumed狀態。 ### 銷毀[Activity](# "An activity represents a single screen with a user interface.") [activity](# "An activity represents a single screen with a user interface.")的第一個生命周期回調函數是 onCreate(),它最后一個回調是[onDestroy()](http://developer.android.com/reference/android/app/Activity.html#onDestroy()).當收到需要將該[activity](# "An activity represents a single screen with a user interface.")徹底移除的信號時，系統會調用這個方法。大多數 app并不需要實現這個方法，因為局部類的references會隨著[activity](# "An activity represents a single screen with a user interface.")的銷毀而銷毀，并且我們的[activity](# "An activity represents a single screen with a user interface.")應該在onPause()與onStop()中執行清除[activity](# "An activity represents a single screen with a user interface.")資源的操作。然而，如果[activity](# "An activity represents a single screen with a user interface.")含有在onCreate調用時創建的后臺線程，或者是其他有可能導致內存泄漏的資源，則應該在OnDestroy()時進行資源清理，殺死后臺線程。 ~~~ @Override public void onDestroy() { super.onDestroy(); // Always call the superclass // Stop method tracing that the activity started during onCreate() android.os.Debug.stopMethodTracing(); } ~~~ > **Note:** 除非程序在onCreate()方法里面就調用了finish()方法，系統通常是在執行了onPause()與onStop() 之后再調用onDestroy() 。在某些情況下，例如我們的[activity](# "An activity represents a single screen with a user interface.")只是做了一個臨時的邏輯跳轉的功能，它只是用來決定跳轉到哪一個[activity](# "An activity represents a single screen with a user interface.")，這樣的話，需要在onCreate里面調用finish方法，這樣系統會直接調用onDestory，跳過生命周期中的其他方法。