问题描述：
同时使用FLAG_AVTIVITY_NEW_TASK和StartActivityForResult时，在新activity启动之前，直接调用了老activity的onActivityResult。
问题原因：
http://blog.csdn.net/dliyuedong/article/details/47172143
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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