（1）发起`startActivityForResult`调用

这种情况会直接调用到Fragment的startActivityForResult方法

//Fragment.class
public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
if (mHost == null) {
throw new IllegalStateException(“Fragment " + this + " not attached to Activity”);
}
mHost.onStartActivityFromFragment(this /fragment/, intent, requestCode, options);
}

上面的mHost对应的就是Fragment的父FragmentActivity，所以会调用到父FragmentActivity的startActivityFromFragment方法

//FragmentActivity.class
public void startActivityFromFragment(Fragment fragment, Intent intent,
int requestCode, @Nullable Bundle options) {
mStartedActivityFromFragment = true;
try {
//一般requestCode都不会为-1，所以不会走if里面
if (requestCode == -1) {
ActivityCompat.startActivityForResult(this, intent, -1, options);
return;
}
//这里检查requestCode是否越界了，不能超过2^16
checkForValidRequestCode(requestCode);
//根据这个requestIndex可以获取到对应Fragment的唯一标识mWho
int requestIndex = allocateRequestIndex(fragment);
//发起startActivityForResult调用，这里requestIndex和requestCode关联起来
ActivityCompat.startActivityForResult(
this, intent, ((requestIndex + 1) << 16) + (requestCode & 0xffff), options);
} finally {
mStartedActivityFromFragment = false;
}
}

每一个Fragment在内部都有一个唯一的标识字段who，在FragmentActivity中把所有调用startActivityFromFragment方法的fragment的requestCode和who通过key-value的方式保存在mPendingFragmentActivityResults变量中

private int allocateRequestIndex(Fragment fragment) {
…

int requestIndex = mNextCandidateRequestIndex;

//将requestIndex和fragment的mWho保存起来
mPendingFragmentActivityResults.put(requestIndex, fragment.mWho);
mNextCandidateRequestIndex =
(mNextCandidateRequestIndex + 1) % MAX_NUM_PENDING_FRAGMENT_ACTIVITY_RESULTS;

return requestIndex;
}

这里allocateRequestIndex方法就把requestIndex和Fragment的mWho变量关联起来了

在上面的startActivityFromFragment方法中调用ActivityCompat的startActivityForResult方法发起启动Activity的时候又把requestIndex和requestCode关联起来了

这样后面回调onActivityResult方法时就可以根据requestCode获取对应的Fragment，以便调用Fragment的onActivityResult方法

最后看一下ActivityCompat的startActivityForResult方法

public static void startActivityForResult(@NonNull Activity activity, @NonNull Intent intent,
int requestCode, @Nullable Bundle options) {
if (Build.VERSION.SDK_INT >= 16) {
activity.startActivityForResult(intent, requestCode, options);
} else {
activity.startActivityForResult(intent, requestCode);
}
}

（2）`onActivityResult`方法回调

通过断点调试的方法，我们会发现最先被回调的就是父Activity的onActivityResult，也就是我们的FragmentActivity的onActivityResult

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
mFragments.noteStateNotSaved();
int requestIndex = requestCode>>16;
//requestIndex = 0就表示没有Fragment发起过startActivityForResult调用
if (requestIndex != 0) {
requestIndex–;

//根据requestIndex获取Fragment的who变量
String who = mPendingFragmentActivityResults.get(requestIndex);
mPendingFragmentActivityResults.remove(requestIndex);
if (who == null) {
Log.w(TAG, “Activity result delivered for unknown Fragment.”);
return;
}

//然后根据who变量获取目标Fragment
Fragment targetFragment = mFragments.findFragmentByWho(who);
if (targetFragment == null) {
Log.w(TAG, "Activity result no fragment exists for who: " + who);
} else {
//最后调用Fragment的onActivityResult
targetFragment.onActivityResult(requestCode & 0xffff, resultCode, data);
}
return;
}

…