background

Recently, I've been seeing lanchMode and Intent Flag in various interviews. Most of the analysis stays on the surface, and some of them are self-contradictory. lz recently
To make a requirement, we need to use Intent flag, consult the relevant information below, and prove the reason from the source code. Add Intent.FLAG_ACTIVITY_CLEAR_TOP. Let's see how the system works. Don't ask me how to find the source code. I won't tell you it was found through Android xref.

Whole process

The whole logic of private start activity unchecked is in this function, and the logic is clear.

1. setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession, voiceInteractor);
   Initialize lauchMode and Intent Flags

2. computeLaunchingTaskFlags();

3. computeSourceStack();

4. MReusedActivity = getReusable IntentActivity (); Find reusable activities

5. Special Flag Processing such as FLAG_ACTIVITY_CLEAR_TOP in this article

 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
 .......  //Why omit in the middle? Intent.FLAG_ACTIVITY_CLEAR_TOP, which is not related to our topic discussion, has little correlation with other flag processing.
1250    }

Go straight to Step 4

image.png

The annotations are very detailed, let alone deal with lanchMode and flag

Step 5 Processing FLAG_ACTIVITY_CLEAR_TOP

image.png

The main task is to find the corresponding activity record, traverse from back to front, simulate the stack operation, and then find the target activity, and remove the activity finish es on the top of the activity stack. Finally, deal with Activity. finish the activity if there is no intent flag for singletop.