使用launchMode="singleTask"的onActivityResult函数？

Question

当您尝试为具有launchMode="singleTask"的Activity启动startActivityForResult时，它将不会返回任何值与onActivityResult，而当您设置launchMode="standard"时; 一切正常工作，但是系统要求这个Activity必须是singleTask，有没有什么方法可以解决这个问题？

- Mohammad Ersan

Answer 1

在 startActivityForResult 的文档中写道：

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.

看起来似乎没有办法绕过这个问题。

如果您是被调用 Activity 的开发者，那么您可以让它在一些结果可用时发送广播。然后，调用 Activity 可以侦听此广播。

- Peter Knego

---

那么如何防止在onClick中创建多个实例（例如ListView）？ - Imon

Answer 2

答案：答案显示在ActivityStackSupervisor类的startActivityUncheckedLocked函数中。在Android 5.x之前，启动活动时，首先检查launchMode并将FLAG_ACTIVITY_NEW_TASK添加到launchFlags中，如果launchMode是singleTask或singleInstance。如果活动的launchFlags包含FLAG_ACTIVITY_NEW_TASK，它将立即发送一个取消，让新任务继续正常启动，而不依赖于其发起者。

if (sourceRecord == null) {
    // This activity is not being started from another...  in this
    // case we -always- start a new task.
    if ((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
        Slog.w(TAG, "startActivity called from non-Activity context; forcing " +
                "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
        launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
    }
} else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
    // The original activity who is starting us is running as a single
    // instance...  this new activity it is starting must go on its
    // own task.
    launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
} else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE
        || r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {
    // The activity being started is a single instance...  it always
    // gets launched into its own task.
    launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
}
// ......
if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
    // For whatever reason this activity is being launched into a new
    // task...  yet the caller has requested a result back.  Well, that
    // is pretty messed up, so instead immediately send back a cancel
    // and let the new task continue launched as normal without a
    // dependency on its originator.
    Slog.w(TAG, "Activity is launching as a new task, so cancelling activity result.");
    r.resultTo.task.stack.sendActivityResultLocked(-1,
            r.resultTo, r.resultWho, r.requestCode,
        Activity.RESULT_CANCELED, null);
    r.resultTo = null;
}

但在Android 5.x中，这种情况已经发生了变化，如下所示：

final boolean launchSingleTop = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP;
final boolean launchSingleInstance = r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE;
final boolean launchSingleTask = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK;
int launchFlags = intent.getFlags();
if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 &&
        (launchSingleInstance || launchSingleTask)) {
    // We have a conflict between the Intent and the Activity manifest, manifest wins.
    Slog.i(TAG, "Ignoring FLAG_ACTIVITY_NEW_DOCUMENT, launchMode is " +
            "\"singleInstance\" or \"singleTask\"");
    launchFlags &=
            ~(Intent.FLAG_ACTIVITY_NEW_DOCUMENT | Intent.FLAG_ACTIVITY_MULTIPLE_TASK);
} else {
    switch (r.info.documentLaunchMode) {
        case ActivityInfo.DOCUMENT_LAUNCH_NONE:
            break;
        case ActivityInfo.DOCUMENT_LAUNCH_INTO_EXISTING:
            launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
            break;
        case ActivityInfo.DOCUMENT_LAUNCH_ALWAYS:
            launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
            break;
        case ActivityInfo.DOCUMENT_LAUNCH_NEVER:
            launchFlags &= ~Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
            break;
    }
}
final boolean launchTaskBehind = r.mLaunchTaskBehind
        && !launchSingleTask && !launchSingleInstance
        && (launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0;
if (r.resultTo != null && (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
    // For whatever reason this activity is being launched into a new
    // task...  yet the caller has requested a result back.  Well, that
    // is pretty messed up, so instead immediately send back a cancel
    // and let the new task continue launched as normal without a
    // dependency on its originator.
    Slog.w(TAG, "Activity is launching as a new task, so cancelling activity result.");
    r.resultTo.task.stack.sendActivityResultLocked(-1,
            r.resultTo, r.resultWho, r.requestCode,
            Activity.RESULT_CANCELED, null);
    r.resultTo = null;
}

这就是为什么在 Android 5.x 中，即使你将 launchMode 设置为 singleTask 或 singleInstance，onActivityResult 仍然有效的原因。

- GGCoke

Answer 3

彼得·克内戈（Peter Knego）说：

另外，这个问题在 Android 5.1 中似乎可以解决，但在 Android 4.4.4 中不行。

也就是说，onActivityResult 方法会被触发。

- AndroidGecko

---

是的，我也观察到它在5.0及以上版本中运行，而不是在4.4.4中。 - Mahesh

Answer 4

为了澄清一些答案：

API's < Android 5.x:

startActivityForResult() 结合 singleTask 或 singleInstance 的 launchmodes 不起作用。 onActivityResult() 将立即被调用，结果代码为 Activity.RESULT_CANCELED。

API's >= Android 5.x:

startActivityForResult() 起作用，但是 singleTask 或 singleInstance 的 launchmodes 基本上会被忽略，这意味着该活动将启动到同一任务中，不会创建新的任务。

如果您想验证此内容，请运行 adb shell dumpsys activity activities。我只希望系统至少能向我显示一个警告。

@GGCoke 的代码片段 发布 中也反映了这种变化。

- MattSchmatt

Answer 5

我知道这有点晚了，但是你可以在onNewIntent()方法中实现OnActivityResult的效果，因为这是你的singleTask活动。

- android_griezmann