前言
我相信大家肯定遇到过Activity启动慢的BUG，在优化问题之前，我们都会自我测试一下Activity启动耗时是不是和测试给的数值一样，在我们没有高速相机的情况下，我们如何获得Activity启动时间。
我一般都用这个指令：
adb logcat -b all | grep am_activity_launch_time
注意结果最后的数字297，代表Activity启动的时长297ms。
am_activity_launch_time: [0,44198042,com.kobe.jankblock/.MainActivity,297]
一、am_activity_launch_time计算的截止时间
一般对于一个Activity显示到屏幕上，有三个重要时间点，请问am_activity_launch_time计算的截止时间是以下哪个事件完成之后的。
onCreate
onResume
doTraversal-View.onDraw
二、Demo时间
写个demo试试看。
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
Thread.sleep(100);//休眠100ms
}catch (Exception e) {
}
}
@Override
protected void onResume() {
super.onResume();
try {
Thread.sleep(200);//休眠200ms
}catch (Exception e) {
}
}
}
public class MyTextView extends TextView {
public MyTextView(Context context) {
super(context);
}
public MyTextView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
public MyTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
public MyTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
try {
Thread.sleep(400);//休眠100ms
}catch (Exception e) {
}
}
}
上述代码非常简单，我分别在三个阶段block了100ms，200ms，400ms
这几个数值选取还是很有讲究的，我们可以通过一次测试就可以得到结论。
测试结果：1008 - 297 = 711 约等于 700ms。
am_activity_launch_time: [0,44198042,com.kobe.jankblock/.MainActivity,1008]
答案：am_activity_launch_time是以第一次doTraversal-View.onDraw完成为截止时间的，也就是Activity的第一帧绘制完成的时间（当然这句话不准确，因为真正的绘制还需要通过RenderThread去绘制）。
三、源码分析
我们就看看这个am_activity_launch_time是怎么计算的。
3.1 logAppDisplayed
/frameworks/base/services/core/java/com/android/server/wm/ActivityMetricsLogger.java
private void logAppDisplayed(WindowingModeTransitionInfoSnapshot info) {
if (info.type != TYPE_TRANSITION_WARM_LAUNCH && info.type != TYPE_TRANSITION_COLD_LAUNCH) {
return;
}
EventLog.writeEvent(AM_ACTIVITY_LAUNCH_TIME,
info.userId, info.activityRecordIdHashCode, info.launchedActivityShortComponentName,
info.windowsDrawnDelayMs);//注意这个windowsDrawnDelayMs的值
StringBuilder sb = mStringBuilder;
sb.setLength(0);
sb.append("Displayed ");
sb.append(info.launchedActivityShortComponentName);
sb.append(": ");
TimeUtils.formatDuration(info.windowsDrawnDelayMs, sb);
Log.i(TAG, sb.toString());
}
我们需要找到windowsDrawnDelayMs赋值的地方
3.2 notifyWindowsDrawn
/frameworks/base/services/core/java/com/android/server/wm/ActivityMetricsLogger.java
/**
* Notifies the tracker that all windows of the app have been drawn.
*/
WindowingModeTransitionInfoSnapshot notifyWindowsDrawn(@WindowingMode int windowingMode,
long timestamp) {
if (DEBUG_METRICS) Slog.i(TAG, "notifyWindowsDrawn windowingMode=" + windowingMode);
final WindowingModeTransitionInfo info = mWindowingModeTransitionInfo.get(windowingMode);
if (info == null || info.loggedWindowsDrawn) {
return null;
}
info.windowsDrawnDelayMs = calculateDelay(timestamp);//调用3.2.1
info.loggedWindowsDrawn = true;
final WindowingModeTransitionInfoSnapshot infoSnapshot =
new WindowingModeTransitionInfoSnapshot(info);
if (allWindowsDrawn() && mLoggedTransitionStarting) {
reset(false /* abort */, info, "notifyWindowsDrawn - all windows drawn");
}
return infoSnapshot;
}
//3.2.1
private int calculateDelay(long timestamp) {
// Shouldn't take more than 25 days to launch an app, so int is fine here.
return (int) (timestamp - mCurrentTransitionStartTime);
}
info.windowsDrawnDelayMs = timestamp - mCurrentTransitionStartTime，所以我们需要找到调用notifyWindowsDrawn的地方。
3.3 onWindowsDrawn
/frameworks/base/services/core/java/com/android/server/wm/ActivityRecord.java
/** Called when the windows associated app window container are drawn. */
public void onWindowsDrawn(boolean drawn, long timestamp) {
synchronized (mAtmService.mGlobalLock) {
mDrawn = drawn;
if (!drawn) {
return;
}
final WindowingModeTransitionInfoSnapshot info = mStackSupervisor
.getActivityMetricsLogger().notifyWindowsDrawn(getWindowingMode(), timestamp);
final int windowsDrawnDelayMs = info != null ? info.windowsDrawnDelayMs : INVALID_DELAY;
final @LaunchState int launchState = info != null ? info.getLaunchState() : -1;
mStackSupervisor.reportActivityLaunchedLocked(false /* timeout */, this,
windowsDrawnDelayMs, launchState);
mStackSupervisor.stopWaitingForActivityVisible(this);
finishLaunchTickingLocked();
if (task != null) {
task.hasBeenVisible = true;
}
}
}
我们只要找对到onWindowsDrawn被调用的地方就可以了，我就不继续逆向分析，直接给出调用关系。
3.4 调用关系
3.4.1 App绘制完成如何通知WindowManagerService
有兴趣的朋友可以去看reportDrawFinished是什么时候被回调的，我就不深入讲了，不想研究的，只要记住第一帧绘制结束就会调用reportDrawFinished。
最后WindowSurfacePlacer中post(mPerformSurfacePlacement)
异步执行mPerformSurfacePlacement.run()
3.4.2 mPerformSurfacePlacement.run()如何调用到3.3 onWindowsDrawn
因为整个代码调用关系比较长，我就不画时序图了，直接给出调用堆栈。
V kobe : java.lang.Exception: kobe
V kobe : at com.android.server.wm.ActivityRecord.onWindowsDrawn(ActivityRecord.java:2458)
V kobe : at com.android.server.wm.AppWindowToken.updateReportedVisibilityLocked(AppWindowToken.java:432)
V kobe : at com.android.server.wm.AppWindowToken.onFirstWindowDrawn(AppWindowToken.java:394)
V kobe : at com.android.server.wm.WindowState.performShowLocked(WindowState.java:3955)
V kobe : at com.android.server.wm.WindowStateAnimator.commitFinishDrawingLocked(WindowStateAnimator.java:349)
V kobe : at com.android.server.wm.DisplayContent.lambda$new$8$DisplayContent(DisplayContent.java:859)
V kobe : at com.android.server.wm.-$$Lambda$DisplayContent$qxt4izS31fb0LF2uo_OF9DMa7gc.accept(Unknown Source:4)
V kobe : at com.android.server.wm.WindowContainer$ForAllWindowsConsumerWrapper.apply(WindowContainer.java:1178)
V kobe : at com.android.server.wm.WindowContainer$ForAllWindowsConsumerWrapper.apply(WindowContainer.java:1168)
V kobe : at com.android.server.wm.WindowState.applyInOrderWithImeWindows(WindowState.java:4168)
V kobe : at com.android.server.wm.WindowState.forAllWindows(WindowState.java:4067)
V kobe : at com.android.server.wm.WindowContainer.forAllWindows(WindowContainer.java:877)
V kobe : at com.android.server.wm.AppWindowToken.forAllWindowsUnchecked(AppWindowToken.java:2061)
V kobe : at com.android.server.wm.AppWindowToken.forAllWindows(AppWindowToken.java:2051)
V kobe : at com.android.server.wm.WindowContainer.forAllWindows(WindowContainer.java:877)
V kobe : at com.android.server.wm.WindowContainer.forAllWindows(WindowContainer.java:877)
V kobe : at com.android.server.wm.WindowContainer.forAllWindows(WindowContainer.java:877)
V kobe : at com.android.server.wm.DisplayContent$TaskStackContainers.forAllWindows(DisplayContent.java:4447)
V kobe : at com.android.server.wm.DisplayContent.forAllWindows(DisplayContent.java:2224)
V kobe : at com.android.server.wm.WindowContainer.forAllWindows(WindowContainer.java:894)
V kobe : at com.android.server.wm.DisplayContent.applySurfaceChangesTransaction(DisplayContent.java:3838)
V kobe : at com.android.server.wm.RootWindowContainer.applySurfaceChangesTransaction(RootWindowContainer.java:864)
V kobe : at com.android.server.wm.RootWindowContainer.performSurfacePlacementNoTrace(RootWindowContainer.java:641)
V kobe : at com.android.server.wm.RootWindowContainer.performSurfacePlacement(RootWindowContainer.java:598)
V kobe : at com.android.server.wm.WindowSurfacePlacer.performSurfacePlacementLoop(WindowSurfacePlacer.java:159)
V kobe : at com.android.server.wm.WindowSurfacePlacer.performSurfacePlacement(WindowSurfacePlacer.java:105)
V kobe : at com.android.server.wm.WindowSurfacePlacer.performSurfacePlacement(WindowSurfacePlacer.java:95)
V kobe : at com.android.server.wm.WindowSurfacePlacer.lambda$new$0$WindowSurfacePlacer(WindowSurfacePlacer.java:62)
V kobe : at com.android.server.wm.-$$Lambda$WindowSurfacePlacer$4Hbamt-LFcbu8AoZBoOZN_LveKQ.run(Unknown Source:2)
V kobe : at android.os.Handler.handleCallback(Handler.java:883)
V kobe : at android.os.Handler.dispatchMessage(Handler.java:100)
V kobe : at android.os.Looper.loop(Looper.java:214)
V kobe : at android.os.HandlerThread.run(HandlerThread.java:67)
V kobe : at com.android.server.ServiceThread.run(ServiceThread.java:44)
四、总结
am_activity_launch_time是截止到activity的第一帧绘制完成的时间，整个时间包含onCreate，onResume，doTraversal，一般情况下am_activity_launch_time还是很客观地展现出一个activity启动时间。
但是有时候测试标准不一样，测试标准可能要求是整个界面完全显示出来，尤其是需要异步网络请求数据，后面几帧展现更多UI，对于这个情况，Activity启动时间就要远远大于am_activity_launch_time。
但是通过am_activity_launch_time来衡量一个Activity启动时间还是很科学的。
五、尾巴
我主要是跟踪了这个timestamp是怎么来的，你们自己可以跟一下mCurrentTransitionStartTime
private int calculateDelay(long timestamp) {
// Shouldn't take more than 25 days to launch an app, so int is fine here.
return (int) (timestamp - mCurrentTransitionStartTime);
}
如何跟踪Android的源码，最要的是记住关键的跨进程Binder调用点，跨线程的Handler异步调用点。我上面跟踪代码中就有一处跨进程Binder调用点和跨线程的Handler异步调用点，欢迎留言回答。