终结篇:Android startActivity原理分析(基于Android 8.1 AOSP)

上:https://www.jianshu.com/p/76a98a341bcd上部分:Android startActivity原理分析(基于Android 8.1 AOSP)
中:https://www.jianshu.com/p/18951d629864中部分:Android startActivity原理分析(基于Android 8.1 AOSP)

继上文两篇后直接挂代码

转交给了另外一个函数去处理后,我们现在往下看

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        if (!mService.mBooting && !mService.mBooted) {
            // Not ready yet!
            return false;
        }

        // Find the next top-most activity to resume in this stack that is not finishing and is
        // focusable. If it is not focusable, we will fall into the case below to resume the
        // 我们的TestActivity已经入栈了,所以next就是我们的TestActivity了
        final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);

        final boolean hasRunningActivity = next != null;

        // TODO: Maybe this entire condition can get removed?
        if (hasRunningActivity && getDisplay() == null) {
            return false;
        }

        mStackSupervisor.cancelInitializingActivities();

        // Remember how we'll process this pause/resume situation, and ensure
        // that the state is reset however we wind up proceeding.
        final boolean userLeaving = mStackSupervisor.mUserLeaving;
        mStackSupervisor.mUserLeaving = false;

        if (!hasRunningActivity) {
            // There are no activities left in the stack, let's look somewhere else.
            return resumeTopActivityInNextFocusableStack(prev, options, "noMoreActivities");
        }

        next.delayedResume = false;

        // 如果我们要跳转的页面已经是resume状态,那么直接通知Wms执行Activity的过渡
        if (mResumedActivity == next && next.state == ActivityState.RESUMED &&
                    mStackSupervisor.allResumedActivitiesComplete()) {
            // Make sure we have executed any pending transitions, since there
            // should be nothing left to do at this point.
            executeAppTransition(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Top activity resumed " + next);
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        final TaskRecord nextTask = next.getTask();
        final TaskRecord prevTask = prev != null ? prev.getTask() : null;
        if (prevTask != null && prevTask.getStack() == this &&
                prevTask.isOverHomeStack() && prev.finishing && prev.frontOfTask) {
            if (DEBUG_STACK)  mStackSupervisor.validateTopActivitiesLocked();
            if (prevTask == nextTask) {
                prevTask.setFrontOfTask();
            } else if (prevTask != topTask()) {
                // This task is going away but it was supposed to return to the home stack.
                // Now the task above it has to return to the home task instead.
                final int taskNdx = mTaskHistory.indexOf(prevTask) + 1;
                mTaskHistory.get(taskNdx).setTaskToReturnTo(HOME_ACTIVITY_TYPE);
            } else if (!isOnHomeDisplay()) {
                return false;
            } else if (!isHomeStack()){
                if (DEBUG_STATES) Slog.d(TAG_STATES,
                        "resumeTopActivityLocked: Launching home next");
                return isOnHomeDisplay() &&
                        mStackSupervisor.resumeHomeStackTask(prev, "prevFinished");
            }
        }

        // 判断一下是否已经进入睡眠状态,如果进入睡眠状态Activity也会进入Pause状态
        if (shouldSleepOrShutDownActivities()
                && mLastPausedActivity == next
                && mStackSupervisor.allPausedActivitiesComplete()) {
            // Make sure we have executed any pending transitions, since there
            // should be nothing left to do at this point.
            executeAppTransition(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Going to sleep and all paused");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        // Make sure that the user who owns this activity is started.  If not,
        // we will just leave it as is because someone should be bringing
        // another user's activities to the top of the stack.
        if (!mService.mUserController.hasStartedUserState(next.userId)) {
            Slog.w(TAG, "Skipping resume of top activity " + next
                    + ": user " + next.userId + " is stopped");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        // The activity may be waiting for stop, but that is no longer
        // appropriate for it.
        mStackSupervisor.mStoppingActivities.remove(next);
        mStackSupervisor.mGoingToSleepActivities.remove(next);
        next.sleeping = false;
        mStackSupervisor.mActivitiesWaitingForVisibleActivity.remove(next);

        if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resuming " + next);

        // If we are currently pausing an activity, then don't do anything until that is done.
        if (!mStackSupervisor.allPausedActivitiesComplete()) {
            if (DEBUG_SWITCH || DEBUG_PAUSE || DEBUG_STATES) Slog.v(TAG_PAUSE,
                    "resumeTopActivityLocked: Skip resume: some activity pausing.");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        mStackSupervisor.setLaunchSource(next.info.applicationInfo.uid);

        boolean lastResumedCanPip = false;
        final ActivityStack lastFocusedStack = mStackSupervisor.getLastStack();
        if (lastFocusedStack != null && lastFocusedStack != this) {
            // So, why aren't we using prev here??? See the param comment on the method. prev doesn't
            // represent the last resumed activity. However, the last focus stack does if it isn't null.
            final ActivityRecord lastResumed = lastFocusedStack.mResumedActivity;
            lastResumedCanPip = lastResumed != null && lastResumed.checkEnterPictureInPictureState(
                    "resumeTopActivity", userLeaving /* beforeStopping */);
        }
        // If the flag RESUME_WHILE_PAUSING is set, then continue to schedule the previous activity
        // to be paused, while at the same time resuming the new resume activity only if the
        // previous activity can't go into Pip since we want to give Pip activities a chance to
        // enter Pip before resuming the next activity.
        final boolean resumeWhilePausing = (next.info.flags & FLAG_RESUME_WHILE_PAUSING) != 0
                && !lastResumedCanPip;
        //将发起者置入Pause状态,也就是MainActivity改成onPause状态
        boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        if (pausing && !resumeWhilePausing) {
            if (DEBUG_SWITCH || DEBUG_STATES) Slog.v(TAG_STATES,
                    "resumeTopActivityLocked: Skip resume: need to start pausing");
            // At this point we want to put the upcoming activity's process
            // at the top of the LRU list, since we know we will be needing it
            // very soon and it would be a waste to let it get killed if it
            // happens to be sitting towards the end.
            if (next.app != null && next.app.thread != null) {
                mService.updateLruProcessLocked(next.app, true, null);
            }
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return true;
        } else if (mResumedActivity == next && next.state == ActivityState.RESUMED &&
                mStackSupervisor.allResumedActivitiesComplete()) {
            // It is possible for the activity to be resumed when we paused back stacks above if the
            // next activity doesn't have to wait for pause to complete.
            // So, nothing else to-do except:
            // Make sure we have executed any pending transitions, since there
            // should be nothing left to do at this point.
            executeAppTransition(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Top activity resumed (dontWaitForPause) " + next);
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return true;
        }

        // If the most recent activity was noHistory but was only stopped rather
        // than stopped+finished because the device went to sleep, we need to make
        // sure to finish it as we're making a new activity topmost.
        if (shouldSleepActivities() && mLastNoHistoryActivity != null &&
                !mLastNoHistoryActivity.finishing) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "no-history finish of " + mLastNoHistoryActivity + " on new resume");
            requestFinishActivityLocked(mLastNoHistoryActivity.appToken, Activity.RESULT_CANCELED,
                    null, "resume-no-history", false);
            mLastNoHistoryActivity = null;
        }

        if (prev != null && prev != next) {
            if (!mStackSupervisor.mActivitiesWaitingForVisibleActivity.contains(prev)
                    && next != null && !next.nowVisible) {
                mStackSupervisor.mActivitiesWaitingForVisibleActivity.add(prev);
                if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                        "Resuming top, waiting visible to hide: " + prev);
            } else {
                // The next activity is already visible, so hide the previous
                // activity's windows right now so we can show the new one ASAP.
                // We only do this if the previous is finishing, which should mean
                // it is on top of the one being resumed so hiding it quickly
                // is good.  Otherwise, we want to do the normal route of allowing
                // the resumed activity to be shown so we can decide if the
                // previous should actually be hidden depending on whether the
                // new one is found to be full-screen or not.
                if (prev.finishing) {
                    prev.setVisibility(false);
                    if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                            "Not waiting for visible to hide: " + prev + ", waitingVisible="
                            + mStackSupervisor.mActivitiesWaitingForVisibleActivity.contains(prev)
                            + ", nowVisible=" + next.nowVisible);
                } else {
                    if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                            "Previous already visible but still waiting to hide: " + prev
                            + ", waitingVisible="
                            + mStackSupervisor.mActivitiesWaitingForVisibleActivity.contains(prev)
                            + ", nowVisible=" + next.nowVisible);
                }
            }
        }

        // Launching this app's activity, make sure the app is no longer
        // considered stopped.
        try {
            AppGlobals.getPackageManager().setPackageStoppedState(
                    next.packageName, false, next.userId); /* TODO: Verify if correct userid */
        } catch (RemoteException e1) {
        } catch (IllegalArgumentException e) {
            Slog.w(TAG, "Failed trying to unstop package "
                    + next.packageName + ": " + e);
        }

        // We are starting up the next activity, so tell the window manager
        // that the previous one will be hidden soon.  This way it can know
        // to ignore it when computing the desired screen orientation.
        //这里准备开启页面
        boolean anim = true;
        if (prev != null) {
            if (prev.finishing) {
                if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION,
                        "Prepare close transition: prev=" + prev);
                if (mNoAnimActivities.contains(prev)) {
                    anim = false;
                    mWindowManager.prepareAppTransition(TRANSIT_NONE, false);
                } else {
                    mWindowManager.prepareAppTransition(prev.getTask() == next.getTask()
                            ? TRANSIT_ACTIVITY_CLOSE
                            : TRANSIT_TASK_CLOSE, false);
                }
                prev.setVisibility(false);
            } else {
                if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION,
                        "Prepare open transition: prev=" + prev);
                if (mNoAnimActivities.contains(next)) {
                    anim = false;
                    mWindowManager.prepareAppTransition(TRANSIT_NONE, false);
                } else {
                    //准备启动
                    mWindowManager.prepareAppTransition(prev.getTask() == next.getTask()
                            ? TRANSIT_ACTIVITY_OPEN
                            : next.mLaunchTaskBehind
                                    ? TRANSIT_TASK_OPEN_BEHIND
                                    : TRANSIT_TASK_OPEN, false);
                }
            }
        } else {
            if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare open transition: no previous");
            if (mNoAnimActivities.contains(next)) {
                anim = false;
                mWindowManager.prepareAppTransition(TRANSIT_NONE, false);
            } else {
                mWindowManager.prepareAppTransition(TRANSIT_ACTIVITY_OPEN, false);
            }
        }

        Bundle resumeAnimOptions = null;
        if (anim) {
            ActivityOptions opts = next.getOptionsForTargetActivityLocked();
            if (opts != null) {
                resumeAnimOptions = opts.toBundle();
            }
            next.applyOptionsLocked();
        } else {
            next.clearOptionsLocked();
        }

        ActivityStack lastStack = mStackSupervisor.getLastStack();
        if (next.app != null && next.app.thread != null) {
            if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resume running: " + next
                    + " stopped=" + next.stopped + " visible=" + next.visible);

            // If the previous activity is translucent, force a visibility update of
            // the next activity, so that it's added to WM's opening app list, and
            // transition animation can be set up properly.
            // For example, pressing Home button with a translucent activity in focus.
            // Launcher is already visible in this case. If we don't add it to opening
            // apps, maybeUpdateTransitToWallpaper() will fail to identify this as a
            // TRANSIT_WALLPAPER_OPEN animation, and run some funny animation.
            final boolean lastActivityTranslucent = lastStack != null
                    && (!lastStack.mFullscreen
                    || (lastStack.mLastPausedActivity != null
                    && !lastStack.mLastPausedActivity.fullscreen));
            // The contained logic must be synchronized, since we are both changing the visibility
            // and updating the {@link Configuration}. {@link ActivityRecord#setVisibility} will
            // ultimately cause the client code to schedule a layout. Since layouts retrieve the
            // current {@link Configuration}, we must ensure that the below code updates it before
            // the layout can occur.
            synchronized(mWindowManager.getWindowManagerLock()) {
                // This activity is now becoming visible.
                if (!next.visible || next.stopped || lastActivityTranslucent) {
                    next.setVisibility(true);
                }

                // schedule launch ticks to collect information about slow apps.
                next.startLaunchTickingLocked();

                ActivityRecord lastResumedActivity =
                        lastStack == null ? null :lastStack.mResumedActivity;
                ActivityState lastState = next.state;

                mService.updateCpuStats();

                if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to RESUMED: " + next
                        + " (in existing)");

                setResumedActivityLocked(next, "resumeTopActivityInnerLocked");
                //更新进程信息
                mService.updateLruProcessLocked(next.app, true, null);
                updateLRUListLocked(next);
                mService.updateOomAdjLocked();

                // Have the window manager re-evaluate the orientation of
                // the screen based on the new activity order.
                boolean notUpdated = true;

                if (mStackSupervisor.isFocusedStack(this)) {

                    // We have special rotation behavior when Keyguard is locked. Make sure all
                    // activity visibilities are set correctly as well as the transition is updated
                    // if needed to get the correct rotation behavior.
                    // TODO: Remove this once visibilities are set correctly immediately when
                    // starting an activity.
                    if (mStackSupervisor.mKeyguardController.isKeyguardLocked()) {
                        mStackSupervisor.ensureActivitiesVisibleLocked(null /* starting */,
                                0 /* configChanges */, false /* preserveWindows */);
                    }
                    final Configuration config = mWindowManager.updateOrientationFromAppTokens(
                            mStackSupervisor.getDisplayOverrideConfiguration(mDisplayId),
                            next.mayFreezeScreenLocked(next.app) ? next.appToken : null,
                                    mDisplayId);
                    if (config != null) {
                        next.frozenBeforeDestroy = true;
                    }
                    notUpdated = !mService.updateDisplayOverrideConfigurationLocked(config, next,
                            false /* deferResume */, mDisplayId);
                }

                if (notUpdated) {
                    // The configuration update wasn't able to keep the existing
                    // instance of the activity, and instead started a new one.
                    // We should be all done, but let's just make sure our activity
                    // is still at the top and schedule another run if something
                    // weird happened.
                    ActivityRecord nextNext = topRunningActivityLocked();
                    if (DEBUG_SWITCH || DEBUG_STATES) Slog.i(TAG_STATES,
                            "Activity config changed during resume: " + next
                                    + ", new next: " + nextNext);
                    if (nextNext != next) {
                        // Do over!
                        mStackSupervisor.scheduleResumeTopActivities();
                    }
                    if (!next.visible || next.stopped) {
                        next.setVisibility(true);
                    }
                    next.completeResumeLocked();
                    if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                    return true;
                }

                try {
                    // 判断是否需要触发onActivityResult函数
                    ArrayList<ResultInfo> a = next.results;
                    if (a != null) {
                        final int N = a.size();
                        if (!next.finishing && N > 0) {
                            if (DEBUG_RESULTS) Slog.v(TAG_RESULTS,
                                    "Delivering results to " + next + ": " + a);
                            next.app.thread.scheduleSendResult(next.appToken, a);
                        }
                    }

                    //是否需要触发onNewIntent函数
                    if (next.newIntents != null) {
                        next.app.thread.scheduleNewIntent(
                                next.newIntents, next.appToken, false /* andPause */);
                    }

                    // Well the app will no longer be stopped.
                    // Clear app token stopped state in window manager if needed.
                    next.notifyAppResumed(next.stopped);

                    EventLog.writeEvent(EventLogTags.AM_RESUME_ACTIVITY, next.userId,
                            System.identityHashCode(next), next.getTask().taskId,
                            next.shortComponentName);

                    next.sleeping = false;
                    mService.showUnsupportedZoomDialogIfNeededLocked(next);
                    mService.showAskCompatModeDialogLocked(next);
                    next.app.pendingUiClean = true;
                    next.app.forceProcessStateUpTo(mService.mTopProcessState);
                    next.clearOptionsLocked();
                    //最后通过ActivityThread的scheduleResumeActivity函数进行启动
                    next.app.thread.scheduleResumeActivity(next.appToken, next.app.repProcState,
                            mService.isNextTransitionForward(), resumeAnimOptions);

                    if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Resumed "
                            + next);
                } catch (Exception e) {
                    // Whoops, need to restart this activity!
                    if (DEBUG_STATES) Slog.v(TAG_STATES, "Resume failed; resetting state to "
                            + lastState + ": " + next);
                    next.state = lastState;
                    if (lastStack != null) {
                        lastStack.mResumedActivity = lastResumedActivity;
                    }
                    Slog.i(TAG, "Restarting because process died: " + next);
                    if (!next.hasBeenLaunched) {
                        next.hasBeenLaunched = true;
                    } else  if (SHOW_APP_STARTING_PREVIEW && lastStack != null &&
                            mStackSupervisor.isFrontStackOnDisplay(lastStack)) {
                        next.showStartingWindow(null /* prev */, false /* newTask */,
                                false /* taskSwitch */);
                    }
                    mStackSupervisor.startSpecificActivityLocked(next, true, false);
                    if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                    return true;
                }
            }

            // From this point on, if something goes wrong there is no way
            // to recover the activity.
            try {
                next.completeResumeLocked();
            } catch (Exception e) {
                // If any exception gets thrown, toss away this
                // activity and try the next one.
                Slog.w(TAG, "Exception thrown during resume of " + next, e);
                requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED, null,
                        "resume-exception", true);
                if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                return true;
            }
        } else {
            // Whoops, need to restart this activity!
            if (!next.hasBeenLaunched) {
                next.hasBeenLaunched = true;
            } else {
                if (SHOW_APP_STARTING_PREVIEW) {
                    next.showStartingWindow(null /* prev */, false /* newTask */,
                            false /* taskSwich */);
                }
                if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Restarting: " + next);
            }
            if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Restarting " + next);
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }

        if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
        return true;
    }

这个函数就是对目标页的很多状态判断,还有pause我们的启动者,然后通知AT(ActivityThread)准备启动,我们继续查看AT内的启动逻辑

@Override
        public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
                ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
                CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
                int procState, Bundle state, PersistableBundle persistentState,
                List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
                boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

            updateProcessState(procState, false);

            ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.referrer = referrer;
            r.voiceInteractor = voiceInteractor;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;
            r.persistentState = persistentState;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profilerInfo = profilerInfo;

            r.overrideConfig = overrideConfig;
            updatePendingConfiguration(curConfig);

            sendMessage(H.LAUNCH_ACTIVITY, r);
        }

这个函数内创建了一个ActivityClientRecord对象,然后通过handler发送命令LAUNCH_ACTIVITY,我们看一下命令所对应的函数是什么吧

case LAUNCH_ACTIVITY: {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

    r.packageInfo = getPackageInfoNoCheck(
            r.activityInfo.applicationInfo, r.compatInfo);
    handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}

这个case下调用了handleLaunchActivity函数,我们继续跟进分析

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        if (!ThreadedRenderer.sRendererDisabled) {
            GraphicsEnvironment.earlyInitEGL();
        }
        WindowManagerGlobal.initialize();

        //通过反射创建Activity的对象
        //并且触发attach、onCreate、onStart生命周期方法
        //此时我们也会自己掉用setContentView来组装DecorView
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            Bundle oldState = r.state;
            //执行onResume生命周期方法,还会通知ViewRootImpl绘制页面
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

            if (!r.activity.mFinished && r.startsNotResumed) {
                // The activity manager actually wants this one to start out paused, because it
                // needs to be visible but isn't in the foreground. We accomplish this by going
                // through the normal startup (because activities expect to go through onResume()
                // the first time they run, before their window is displayed), and then pausing it.
                // However, in this case we do -not- need to do the full pause cycle (of freezing
                // and such) because the activity manager assumes it can just retain the current
                // state it has.
                performPauseActivityIfNeeded(r, reason);

                // We need to keep around the original state, in case we need to be created again.
                // But we only do this for pre-Honeycomb apps, which always save their state when
                // pausing, so we can not have them save their state when restarting from a paused
                // state. For HC and later, we want to (and can) let the state be saved as the
                // normal part of stopping the activity.
                if (r.isPreHoneycomb()) {
                    r.state = oldState;
                }
            }
        } else {
            // If there was an error, for any reason, tell the activity manager to stop us.
            try {
                ActivityManager.getService()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

到此startActivity的完整流程都全部呈现给大家了,里边的校验是真的多。涉及到的技术面也很广,偶尔读读源码还是可以收获很多的,上面的每一个判断,都是一个知识点,有些在开发中可以用到,有些判断是给系统app留的。

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