背景

最近老是看到各种面试中的lanchMode和Intent Flag, 大多数分析停留在表面，并且有些还自相矛盾。lz 最近
做一个需求需要用Intent flag，查阅了下相关资料，并从源码论证了原因。添加Intent.FLAG_ACTIVITY_CLEAR_TOP我们来看看系统是怎么做的？别问我怎么找到源码的，我不告诉你是通过androidxref查找的。

整体流程

private int startActivityUnchecked 整体的逻辑就在这个函数中了，逻辑也比较清楚，

setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession, voiceInteractor);
初始化lauchMode和Intent Flags
computeLaunchingTaskFlags();
computeSourceStack();
mReusedActivity = getReusableIntentActivity(); 查找可复用的activity
特殊Flag处理如本文的 FLAG_ACTIVITY_CLEAR_TOP

 private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
1025            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
1026            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {
1027
1028        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
1029                voiceInteractor);
1030
1031        computeLaunchingTaskFlags();
1032
1033        computeSourceStack();
1034
1035        mIntent.setFlags(mLaunchFlags);
1036
1037        mReusedActivity = getReusableIntentActivity();
1038
1039        final int preferredLaunchStackId =
1040                (mOptions != null) ? mOptions.getLaunchStackId() : INVALID_STACK_ID;
1041
1042        if (mReusedActivity != null) {
                       ......
1066            if ((mLaunchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
1067                    || mLaunchSingleInstance || mLaunchSingleTask) {
1068                // In this situation we want to remove all activities from the task up to the one
1069                // being started. In most cases this means we are resetting the task to its initial
1070                // state.
1071                final ActivityRecord top = mReusedActivity.task.performClearTaskForReuseLocked(
1072                        mStartActivity, mLaunchFlags);
1073                if (top != null) {
1074                    if (top.frontOfTask) {
1075                        // Activity aliases may mean we use different intents for the top activity,
1076                        // so make sure the task now has the identity of the new intent.
1077                        top.task.setIntent(mStartActivity);
1078                    }
1079                    ActivityStack.logStartActivity(AM_NEW_INTENT, mStartActivity, top.task);
1080                    top.deliverNewIntentLocked(mCallingUid, mStartActivity.intent,
1081                            mStartActivity.launchedFromPackage);
1082                }
1083            }
1084
 .......  //中间为嘛省略呢？ 与我们主题讨论的Intent.FLAG_ACTIVITY_CLEAR_TOP 相关度不大，其他的flag处理
1250    }

直接到第4步