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

	stand	singleTop	singleTask	singleInstance
stand	√	√	x	x
singleTop	√	√	x	x
singleTask	√	√	x	x
singleInstance	x	x	x	x

android5.0之后

	stand	singleTop	singleTask	singleInstance
stand	√	√	√	√
singleTop	√	√	√	√
singleTask	√	√	√	√
singleInstance	√	√	√	√

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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android5.0之前

android5.0之后

为什么会是这样的呢？

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