扒扒Task与Activity启动模式

最近在重新整理Activity的启动模式，顺便也扒了扒任务栈Task，接着又去了解了下Android的概览屏幕，把页面间的跳转、任务栈存放与管理及从任务列表窗口恢复，整体串通的了解了一下。下面有几个基本的问题，你不妨测试一下，看看能掌握多少？

发自灵魂的拷问

• 设置启动模式为singleTask，若栈内已有该实例，是否栈内就一定是复用的，不会创建实例？
• 若Intent设置FLAG_ACTIVITY_NEW_TASK，任何启动模式，如果采取startActivityForResult()启动Activity，onActivityResult()有何变化？
• LauncherActivity->A(standard)->B(singleInstance)按下home键，点击桌面app图标，会发生什么？
• LauncherActivity->A(standard)->B(standard)->C(singleInstance)->A，按下返回键，会发生什么？
• 当调用startActivityForResult启动Activity，那么启动模式会发生什么变化？

下面我就带着大家一块测试和分析一下，本篇博客测试设备Pixel（API19）、小米（API21）和OPPO（API27），主要是把android系统5.0作为一个分水岭来测试，因为官方的文档很多地方未交代清楚，需要考证。 ## ActivityRecord、TaskRecord和ActivityStack 先来扒扒这三者的关系，可以更加方便的让我们去理解启动模式。 `ActivityRecord`可以说是在任务栈中记录或保存Activity信息的实体类，它对应一个Activity，但是同一个Activity可以对应多个ActivityRecord，因为Activity可以多次被启动实例化，由启动模式、taskAffinity和FLAG决定的。

/** 【注意：此处源码的解释】
 * An entry in the history stack, representing an activity.
 */
final class ActivityRecord extends ConfigurationContainer implements AppWindowContainerListener {
  ApplicationInfo appInfo; // information about activity's app
    final int launchedFromPid; // always the pid who started the activity.
    final int launchedFromUid; // always the uid who started the activity.
    final String launchedFromPackage; // always the package who started the activity.
    final int userId;          // Which user is this running for?
    final Intent intent;    // the original intent that generated us
    final ComponentName realActivity;  // the intent component, or target of an alias.
    final String shortComponentName; // the short component name of the intent
    final String resolvedType; // as per original caller;
    final String packageName; // the package implementing intent's component
    final String processName; // process where this component wants to run
    final String taskAffinity; // as per ActivityInfo.taskAffinity
   ......部分省略
}

TaskRecord表示任务栈，记录着Activity开启的先后顺序，特点是先进后出，一系列相同的ActivityRecord保存在一个TaskRecord中，他们的TaskId是相同的，也就是说TaskRecord是由一个或者多个ActivityRecord所组成，简单看下字段

class TaskRecord extends ConfigurationContainer implements TaskWindowContainerListener {
    final int taskId;       // Unique identifier for this task.
    String affinity;        // The affinity name for this task, or null; may change identity.
    String rootAffinity;    // Initial base affinity, or null; does not change from initial root.
    final IVoiceInteractionSession voiceSession;    // Voice interaction session driving task
    final IVoiceInteractor voiceInteractor;         // Associated interactor to provide to app
    Intent intent;          // The original intent that started the task. Note that this value can
                            // be null.
    Intent affinityIntent;  // Intent of affinity-moved activity that started this task.
    int effectiveUid;       // The current effective uid of the identity of this task.
    ComponentName origActivity; // The non-alias activity component of the intent.
    ComponentName realActivity; // The actual activity component that started the task.
    boolean realActivitySuspended; // True if the actual activity component that started the
                                   // task is suspended.
    boolean inRecents;      // Actually in the recents list?
    long lastActiveTime;    // Last time this task was active in the current device session,
    ......部分省略
}

ActivityStack则是用来管理TaskRecord的，里面包含多个TaskRecord，用源码的解释就是一个栈所有活动界面（TaskRecord中）的状态和管理

/**【注意：此处源码的解释】
 * State and management of a single stack of activities.
 */
class ActivityStack<T extends StackWindowController> extends ConfigurationContainer implements StackWindowListener {
 enum ActivityState {
        INITIALIZING,
        RESUMED,
        PAUSING,
        PAUSED,
        STOPPING,
        STOPPED,
        FINISHING,
        DESTROYING,
        DESTROYED
    }
    ......部分省略
 /**
     * The back history of all previous (and possibly still
     * running) activities.  It contains #TaskRecord objects.
     */
    private final ArrayList<TaskRecord> mTaskHistory = new ArrayList<>();
    /**
     * List of running activities, sorted by recent usage.
     * The first entry in the list is the least recently used.
     * It contains HistoryRecord objects.
     */
    final ArrayList<ActivityRecord> mLRUActivities = new ArrayList<>();
    /**
     * 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.
     */
    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.
     */
    ActivityRecord mLastPausedActivity = null;
    ......部分省略
}

简单的举个栗子：从系统桌面开