ActivityStack 簡介，activitystack簡介

ActivityStack 簡介，activitystack簡介

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http://blog.csdn.net/yujun411522/article/details/46659413
本文出自:【yujun411522的部落格】

在ActivityManagerService 的啟動過程中出現了ActivityStack的這麼一個類，這個類的作用是什麼呢？

我們知道在Android中由Activity來負責展示和互動，而假如現在有這樣的一個啟動順序Activity1->Activity2->Activity3，那麼現在顯示的是Activity3，我們在按住返回鍵的時候時回到Activity2，再按一次返回鍵，返回到Activity1。這種互動邏輯符合我們使用的習慣，這種後進先出的資料結構就是棧，android中就是使用stack來管理Activity的。系統總是顯示處於棧頂的Activity，每當啟動一個新的Activity時就會入棧，原來處於棧頂的Activity被新的Activity取代，這時系統就會顯示新的Activity。同樣使用者返回時，處於棧頂的Activity就會出棧，原棧頂後面的Activity處於棧頂位置，系統就會顯示它。一直到該ActivityStack中所有的Activity都不再存在，這時返回到Home介面。
下面看看AndroidStack中有哪些重要的成員變數和方法 1 ActivityState一個Activity可能處於的狀態:    

    enum ActivityState {        INITIALIZING,//正在初始化        RESUMED,//獲得焦點        PAUSING,//正在暫停                                                 PAUSED,//已經暫停        STOPPING,//正在停止        STOPPED,//已經停止        FINISHING,//正在完成        DESTROYING,//正在摧毀        DESTROYED//已經摧毀    }

狀態的變化可以參看Activity的生命週期的變化圖



2 ArrayList<ActivityRecord>儲存一系列的處於不同功能的Activity集合，這裡使用的是ActivityRecord，這個類它用來記錄每個Activity的運行資訊，主要有以下幾個Arraylist     

 /**     * The back history of all previous (and possibly still running) activities.  It contains HistoryRecord objects.     */     //所有被返回的Activity    final ArrayList<ActivityRecord> mHistory = new ArrayList<ActivityRecord>();        /**     * List of running activities, sorted by recent usage. The first entry in the list is the least recently used.     * It contains HistoryRecord objects.     */     // 正在啟動並執行Activity，按照LRU來排序，第一個是最近最少使用的    final ArrayList<ActivityRecord> mLRUActivities = new ArrayList<ActivityRecord>();    /**     * List of activities that are waiting for a new activity to become visible before completing whatever operation they are     * supposed to do.     */     //正在等待一個新的Activity變成Visible時的Activity集合    final ArrayList<ActivityRecord> mWaitingVisibleActivities = new ArrayList<ActivityRecord>();    /**     * List of activities that are ready to be stopped, but waiting for the next activity to settle down before doing so.  It contains     * HistoryRecord objects.     */     //正準備被Stop，但是在等待另一個Activity完全stop時的Activity集合    final ArrayList<ActivityRecord> mStoppingActivities  = new ArrayList<ActivityRecord>();    /**     * List of activities that are in the process of going to sleep.     */     // app進程中將要處於sleep的Activity集合    final ArrayList<ActivityRecord> mGoingToSleepActivities= new ArrayList<ActivityRecord>();    /**     * Animations that for the current transition have requested not to be considered for the transition animation.     */     // 需不要遷移動畫的Activity集合    final ArrayList<ActivityRecord> mNoAnimActivities  = new ArrayList<ActivityRecord>();    /**     * List of activities that are ready to be finished, but waiting for the previous activity to settle down before doing so.  It contains     * HistoryRecord objects.     */     // 正要別finished的，但是等待上一個Activityfinished完畢的Activity集合    final ArrayList<ActivityRecord> mFinishingActivities   = new ArrayList<ActivityRecord>();

3 特殊的Activity還有一些記錄特殊狀態的Activity，包括:

 /**     * When we are in the process of pausing an activity, before starting the next one, this variable holds the activity that is currently being paused.     */     //當要暫訂一個Activity時且沒有啟動一個新的Activity之前，用這個變數來儲存正在處於暫停Activity    ActivityRecord mPausingActivity = null;    /**     * This is the last activity that we put into the paused state.  This is used to determine if we need to do an activity transition while sleeping,     * when we normally hold the top activity paused.     */     //最近一個被暫停Activity    ActivityRecord mLastPausedActivity = null;    /**     * Current activity that is resumed, or null if there is none.     */    //正在處於resume狀態的Activity    ActivityRecord mResumedActivity = null;        /**     * This is the last activity that has been started.  It is only used to identify when multiple activities are started at once so that the user     * can be warned they may not be in the activity they think they are.     */     //最近被啟動的Activity。    ActivityRecord mLastStartedActivity = null;

這些變數就是用來記錄和管理Activity的資料結構，ActivityStack中還有很多方法
這些都是用來調度Activity的，後面在啟動Activity中會涉及到這些方法的調用。