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
standsingleTopsingleTasksingleInstance
stand√√xx
singleTop√√xx
singleTask√√xx
singleInstancexxxx
android5.0之后
standsingleTopsingleTasksingleInstance
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) {((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) {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) {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) {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。
版权声明：本文为博主原创文章，未经博主允许不得转载。
大海，别为森林的渺小而沮丧，
