Android LaunchMode and StartActivityForResult

android4.0+已经占据目前主流android系统版本了，在5.0版本发布后，android的LaunchMode与StartActivityForResult的关系发生了一些改变。

两个Activity，A和B，现在由A页面跳转到B页面，看一下LaunchMode与StartActivityForResult之间的关系：

android5.0之前

android5.0之后

为什么会是这样的呢？

ActivityStackSupervisor类中的startActivityUncheckedLocked方法在5.0中进行了修改。在5.0之前，当启动一个Activity时，系统将首先检查Activity的launchMode，如果为A页面设置为SingleInstance或者B页面设置为singleTask或者singleInstance,则会在LaunchFlags中加入FLAG_ACTIVITY_NEW_TASK标志，而如果含有FLAG_ACTIVITY_NEW_TASK标志的话，onActivityResult将会立即接收到一个cancle的信息。

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;
}

在5.0(含)之后的系统中，对此方法进行了修改：

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;
}

这就解析了为什么5.0(含)之后的系统即便启动的页面设置launchMode为singleTask或singleInstance，onActivityResult依旧可以正常工作。

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