startActivity和startActivityForResult以及ActivityManager框架

最近研究启动模式，顺便查看了一下startActivity和startActivityForResult的源码：

最后发现startActivity最后也是调用

public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) ；这个方法的。

我们来看看这个方法（sdk23）：

 * Launch an activity for which you would like a result when it finished. * When this activity exits, your * onActivityResult() method will be called with the given requestCode. * Using a negative requestCode is the same as calling * {@link #startActivity} (the activity is not launched as a sub-activity). * * <p>Note that this method should only be used with Intent protocols * that are defined to return a result.  In other protocols (such as * {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may * not get the result when you expect.  For example, if the activity you * are launching uses the singleTask launch mode, it will not run in your * task and thus you will immediately receive a cancel result. *public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {    if (mParent == null) {        Instrumentation.ActivityResult ar =            mInstrumentation.execStartActivity(                this, mMainThread.getApplicationThread(), mToken, this,                intent, requestCode, options);        if (ar != null) {            mMainThread.sendActivityResult(                mToken, mEmbeddedID, requestCode, ar.getResultCode(),                ar.getResultData());        }        if (requestCode >= 0) {            // If this start is requesting a result, we can avoid making            // the activity visible until the result is received.  Setting            // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the            // activity hidden during this time, to avoid flickering.            // This can only be done when a result is requested because            // that guarantees we will get information back when the            // activity is finished, no matter what happens to it.            mStartedActivity = true;        }        cancelInputsAndStartExitTransition(options);        // TODO Consider clearing/flushing other event sources and events for child windows.    } else {        if (options != null) {            mParent.startActivityFromChild(this, intent, requestCode, options);        } else {            // Note we want to go through this method for compatibility with            // existing applications that may have overridden it.            mParent.startActivityFromChild(this, intent, requestCode);        }    }}
分析：从它的注释中我们可以知道两点：

1）这个方法的请求码参数分为>=0和<0两种情况，startActivity方法中调用这个方法时使用的就是-1，如果直接调用这个方法并且

请求码传值为负数就相当于是调用startActivity，不会有onActivityResult方法的回调。前者启动的activity是当前activity的

子activity，后者启动的activity则不是。

2）使用这个方法时传的Intent的协议需要是属于等待返回一个结果的，如果是其他不需要返回结果的，

比如：@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may not get the result when you expect。

当然，这里有个例子：比如使用这个方法准备启动的activity的启动模式是singleTask，你将会直接接受到一个表示取消的返回结果。

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