android - 为什么 onActivityResult() 只在 requestCode>=0 时调用？

Question

我们可以知道，startActivity方法会调用startActivityForResult方法；还有一些代码如下:

    /**
 * Same as calling {@link #startActivityForResult(Intent, int, Bundle)}
 * with no options.
 *
 * @param intent The intent to start.
 * @param requestCode If >= 0, this code will be returned in
 *                    onActivityResult() when the activity exits.
 *
 * @throws android.content.ActivityNotFoundException
 *
 * @see #startActivity
 */
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
    startActivityForResult(intent, requestCode, null);
}

我尽力找到处理requestCode的源代码。但我什么也没找到。有人可以复制源代码并告诉我当只调用 requestCode>=0 时如何处理 OnActivityResult() 吗？

Answer 1

如您包含的代码 fragment 所示，startActivityForResult 的双参数版本调用 startActivityForResult 的三参数版本.三参数版looks like this :

public void startActivityForResult(@RequiresPermission 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;
        }

        //...
    } else {
        //...
    }
}

评论在if (requestCode >= 0) {} block 描述了当请求代码为 >= 0 时行为如何变化。对比< 0 .

如果我们看the source code for mInstrumentation.execStartActivity ，我们发现以下内容:

public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, Activity target,
        Intent intent, int requestCode, Bundle options) {

    //...

                    if (am.isBlocking()) {
                        return requestCode >= 0 ? am.getResult() : null;
                    }

    // ...
}

行return requestCode >= 0 ? am.getResult() : null;返回 null ActivityResult如果requestCode是负的。跳回到 startActivityForResult在此答案顶部的代码，我们看到 mMainThread.sendActivityResult当且仅当 ActivityResult 被调用由 execStartActivity 返回是非 null .是sendActivityResult最终导致调用 onActivityResult .所以这里的逻辑路径是:

startActivityForResult被称为负数 requestCode
↳ mInstrumentation.execStartActivity被称为负数 requestCode
↳ mInstrumentation.execStartActivity返回 null
↳ mMainThread.sendActivityResult未被调用
↳ onActivityResult不叫