Android之startActivityForResult问题

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近期在写项目时，在使用startActivityForResult时遇到一些问题，虽然以前也有遇到过，由于时间比较久了，就忘记了。这次开始写博客，又恰逢遇到问题重现，就此写下短篇的记录短文。

先就现在比较常用的FragmengActivity的startActivityForResult源码进行解读：

先查看FragmengActivity的startActivityForResult源码

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/**
 * Modifies the standard behavior to allow results to be delivered to fragments.
 * This imposes a restriction that requestCode be <= 0xffff.
 */
@Override
public void startActivityForResult(Intent intent, int requestCode) {
    // If this was started from a Fragment we've already checked the upper 16 bits were not in
    // use, and then repurposed them for the Fragment's index.
    if (!mStartedActivityFromFragment) {
        if (requestCode != -1) {
            checkForValidRequestCode(requestCode);
        }
    }
    super.startActivityForResult(intent, requestCode);
}

通过mStartedActivityFromFragment判断我们的startActivityForResult方法是否从Fragment发起的。如果不是，则需要对requestCode进行检验，至于这里为何要对requestCode检验，我们后面再解释。我们跟着方法进入FragmengtActivity中的父类方法中

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public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            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);
            }
        }
    }

具体这里就不往下分析了，继续深入涉及ActivityManager以及ActivityThread的流程。有兴趣可自行查看。

这里说下我们经常遇到的一些问题，在一些APP上，我们需要用到startActivityForResult，来获取某个界面的数据。

现在Android大部分Activity开发基本都是使用FragmentActivity，内部嵌套多个Fragment。这时使用会遇到一些问题，如：

1、 在当前Fragment的onActivityResult的方法未调用，导致无法完成数据获取。

2、 在当前Activity中获取到的requestCode与发起的requestCode不一致。

等。

说下问题1：

当我们在Activity中或者Fragment中通过调用getActivity().startActivityForResult启动下一个activity时，当在activity setResult并调用finish时，回到调用的Activity我们去看下FragmentActivity的源码

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/**
    * Dispatch incoming result to the correct fragment.
    */
   @Override
   protected void onActivityResult(int requestCode, int resultCode, Intent data) {
       mFragments.noteStateNotSaved();
       int requestIndex = requestCode>>16;
       if (requestIndex != 0) {
           requestIndex--;

           String who = mPendingFragmentActivityResults.get(requestIndex);
           mPendingFragmentActivityResults.remove(requestIndex);
           if (who == null) {
               Log.w(TAG, "Activity result delivered for unknown Fragment.");
               return;
           }
           Fragment targetFragment = mFragments.findFragmentByWho(who);
           if (targetFragment == null) {
               Log.w(TAG, "Activity result no fragment exists for who: " + who);
           } else {
               targetFragment.onActivityResult(requestCode & 0xffff, resultCode, data);
           }
           return;
       }

       ActivityCompat.PermissionCompatDelegate delegate =
               ActivityCompat.getPermissionCompatDelegate();
       if (delegate != null && delegate.onActivityResult(this, requestCode, resultCode, data)) {
           // Delegate has handled the activity result
           return;
       }

       super.onActivityResult(requestCode, resultCode, data);
   }

在此方法中可以看到进入方法后，会对requestCode进行处理，使用过的都知道requestCode的值是有限制的。

此处对requestCode进行右移操作，最终获取的requestCode在正常范围内都是趋于零，导致不会进入条件内，也不会进入Fragment的onActivityResult，若此时需要对fragment的进行回调，则需要自己在代码自行添加。

问题2，进过排查，发现在通过调用fragment的startActivityForResult，在FragmentActivity处获取到的requestCode与发出的requestCode不一致，并且有点随机数，不过该随机数都很大。具体还没详细查阅源码。而且会进入Fragment的onActivityResult,并不会出现问题1的情况。