Question

我们知道，startActivity方法将被调用startActivityForResult方法；还有一些如下代码：

    /**
 * 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);
}

我会尽力找到处理requestCode的源代码。但是我什么也没找到。有人可以复制源代码并告诉我仅在requestCode> = 0OnActivityResult（）被调用时如何处理吗？

Answer 1

如您所包含的代码片段所示，startActivityForResult的两个参数版本调用了startActivityForResult的三个参数版本。三参数版本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 {
        //...
    }
}

if (requestCode >= 0) {}块中的注释描述了当请求代码为>= 0与< 0时行为如何变化。

如果我们查看the source code for mInstrumentation.execStartActivity，则会发现以下内容：

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;
                    }

    // ...
}

如果return requestCode >= 0 ? am.getResult() : null;为负，则行null返回ActivityResult requestCode。回到该答案顶部的startActivityForResult代码，我们看到mMainThread.sendActivityResult被调用，当且仅当ActivityResult返回的execStartActivity是非{{1 }}。最终导致调用null的是sendActivityResult。因此，这里的逻辑路径是：

onActivityResult以负数startActivityForResult调用
called requestCode以负数mInstrumentation.execStartActivity调用
↳requestCode返回mInstrumentation.execStartActivity
called null不被称为
called mMainThread.sendActivityResult没有被叫

为什么仅在requestCode> = 0时才调用onActivityResult（）？

1 个答案: