1、startactivityForResult 这个页面跳转方法大家都知道，这个方法相对于startactivity的好处在于他可以回收B页面带回来的返回值。大家都知道想要收到回调一定要实现一个方法
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
switch（requestCode）
{//这个值是我们从A页面跳转时候输入的请求码
switch（resultCode）{//这个值是从B页面通过setResult（resultCode）返回的结果码，其中data这个参数便是从B页面返回来给我的值，值是保存在Intent对象中的
}
}
}但是一般这个回调是在B页面通过Setresult 之后才会执行。但是我之前遇到了。每次通过startactivityForResult（）这个方法之后，这个回调就已经执行了。B页面也能正常跳转，但是把B销毁后，并没有执行这个回调了。返回的resultCode也默认是0，这一点令我很奇怪。找了很久才发现原来B页面在MainFest中注册的时候其中lunchMode 用的是singleTask。这种加载模式会导致startactivityForResult（） 这个方法失效。此时我们看一下这个方法的源码：
/**
* 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.***][1]**
***
*
* <p>As a special case, if you call startActivityForResult() with a requestCode
* >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your
* activity, then your window will not be displayed until a result is
* returned back from the started activity. This is to avoid visible
* flickering when redirecting to another activity.
*
* <p>This method throws {@link android.content.ActivityNotFoundException}
* if there was no Activity found to run the given Intent.
*
* @param intent The intent to start.
* @param requestCode If >= 0, this code will be returned in
* onActivityResult() when the activity exits.
* @param options Additional options for how the Activity should be started.
* See {@link android.content.Context#startActivity(Intent, Bundle)
* Context.startActivity(Intent, Bundle)} for more details.
*
* @throws android.content.ActivityNotFoundException
*
* @see #startActivity
*/
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);
}
}
}其中加粗那段意思是如果你加载模式设置为singleTask 那么你的任务将不会收到返回值，并且将会立即执行收到一个退出的返回码也就是0。所以以后用的时候一定要注意了。