一、FPS評測應用流暢度不準确
說到應用的流暢度，都會想到FPS，系統擷取FPS的原理是：手機螢幕顯示的内容是通過Android系統的SurfaceFLinger類，把目前系統裡所有程序需要顯示的資訊合成一幀，然後送出到螢幕上進行顯示，FPS就是1秒内SurfaceFLinger送出到螢幕的幀數。用FPS來評測一個應用是否真的卡頓存在兩個問題。
- 有的時候FPS很低，APP看起來卻很流暢；
-
APP停止操作之後，FPS還是在一直變化，這種情況是否會影響到FPS的準确度？
有的時候FPS很低，APP看起來卻很流暢，是因為目前界面在1秒内隻需要10幀的顯示需求，當然不會卡頓，此時FPS隻要高于10就可以了，如果螢幕根本沒有繪制需求，那FPS的值就是0。
Android性能優化第（四）篇—Android渲染機制說過，Android系統每隔16ms發出VSYNC信号，觸發對UI的渲染，16ms沒完成繪制就會卡頓。VSync機制就像是一台轉速固定的發動機(60轉/s)。每一轉會帶動着去做一些UI相關的事情，但不是每一轉都會有工作去做(就像有時在空擋，有時在D檔)。有時候因為各種阻力某一圈工作量比較重超過了16.6ms，那麼這台發動機這秒内就不是60轉了，當然也有可能被其他因素影響，比如給油不足(主線程裡幹的活太多)等等，就會出現轉速降低的狀況。我們把這個轉速叫做流暢度。當流暢度越小的時候說明目前程式越卡頓。
二、Choreographer幀率檢測原理
我們有時候會看到這樣的log，系統幫助我們列印出了跳幀數。
02-07 19:47:04.333 17601-17604/zhangwan.wj.com.choreographertest D/dalvikvm: GC_CONCURRENT freed 143K, 3% free 9105K/9384K, paused 2ms+0ms, total 6ms
02-07 19:47:04.337 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 60 frames! The application may be doing too much work on its main thread.
02-07 19:47:11.685 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 85 frames! The application may be doing too much work on its main thread.
02-07 19:47:12.545 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 37 frames! The application may be doing too much work on its main thread.
02-07 19:47:14.893 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 37 frames! The application may be doing too much work on its main thread.
02-07 19:47:23.049 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 36 frames! The application may be doing too much work on its main thread.
02-07 19:47:23.929 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 37 frames! The application may be doing too much work on its main thread.
02-07 19:47:24.961 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 61 frames! The application may be doing too much work on its main thread.
02-07 19:47:25.817 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 36 frames! The application may be doing too much work on its main thread.
02-07 19:47:26.433 17601-17601/zhangwan.wj.com.choreographertest I/Choreographer: Skipped 36 frames! The application may be doing too much work on its main thread.
這個log就出自于Choreographer中（英[ˌkɒrɪ’ɒɡrəfə®] 美[ˌkɒrɪ’ɒɡrəfə®]）。
oid doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
if (!mFrameScheduled) {
return; // no work to do
}
if (DEBUG_JANK && mDebugPrintNextFrameTimeDelta) {
mDebugPrintNextFrameTimeDelta = false;
Log.d(TAG, "Frame time delta: "
+ ((frameTimeNanos - mLastFrameTimeNanos) * 0.000001f) + " ms");
}
long intendedFrameTimeNanos = frameTimeNanos;
startNanos = System.nanoTime();
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
+ "which is more than the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Skipping " + skippedFrames + " frames and setting frame "
+ "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
}
frameTimeNanos = startNanos - lastFrameOffset;
}
}
}
其中SKIPPED_FRAME_WARNING_LIMIT是Choreographer的成員變量。
// Set a limit to warn about skipped frames.
// Skipped frames imply jank.
private static final int SKIPPED_FRAME_WARNING_LIMIT =SystemProperties.getInt( "debug.choreographer.skipwarning", 30);
也就是當跳幀數大于設定的SKIPPED_FRAME_WARNING_LIMIT 值時會在目前程序輸出這個log。由于 SKIPPED_FRAME_WARNING_LIMIT 的值預設為 30，是以上面的log并不是經常看到，如果我們用反射的方法把SKIPPED_FRAME_WARNING_LIMIT的值設定成1，這樣可以保證隻要有丢幀，就會有上面的log輸出來。
static {
try {
Field field = Choreographer.class.getDeclaredField("SKIPPED_FRAME_WARNING_LIMIT");
field.setAccessible(true);
field.set(Choreographer.class,1);
} catch (Throwable e) {
e.printStackTrace();
}
}
注意，這個方案是 API 16 以上才支援。Choreographer就是一個消息處理器，根據vsync 信号 來計算frame，而計算frame的方式就是處理三種回調，包括事件回調、動畫回調、繪制回調。這三種事件在消息輸入、加入動畫、準備繪圖layout 等動作時均會發給Choreographer。一句話，我們隻要捕獲這個log提取出skippedFrames 就可以知道界面是否卡頓。
三、如何檢測
采用上面的方式就可以在App内部觀測目前App的流暢度了。并且在丢幀的地方列印，就可以知道丢幀的大概原因，大概位置，定位代碼問題。
在Choreographer中有個回調接口，FrameCallback。
public interface FrameCallback {
//當新的一幀被繪制的時候被調用。
public void doFrame(long frameTimeNanos);
}
根據上面的代碼，重寫doFrame方法，是以照葫蘆畫瓢，自定義FrameCallback。我們可以在每一幀被渲染的時候記錄下它開始渲染的時間，這樣在下一幀被處理時，判斷上一幀在渲染過程中是否出現掉幀。
public class SMFrameCallback implements Choreographer.FrameCallback {
public static SMFrameCallback sInstance;
private String TAG="SMFrameCallback";
public static final float deviceRefreshRateMs=16.6f;
public static long lastFrameTimeNanos=0;//納秒為機關
public static long currentFrameTimeNanos=0;
public void start() {
Choreographer.getInstance().postFrameCallback(SMFrameCallback.getInstance());
}
public static SMFrameCallback getInstance() {
if (sInstance == null) {
sInstance = new SMFrameCallback();
}
return sInstance;
}
@Override
public void doFrame(long frameTimeNanos) {
if(lastFrameTimeNanos==0){
lastFrameTimeNanos=frameTimeNanos;
Choreographer.getInstance().postFrameCallback(this);
return;
}
currentFrameTimeNanos=frameTimeNanos;
float value=(currentFrameTimeNanos-lastFrameTimeNanos)/1000000.0f;
final int skipFrameCount = skipFrameCount(lastFrameTimeNanos, currentFrameTimeNanos, deviceRefreshRateMs);
Log.e(TAG,"兩次繪制時間間隔value="+value+" frameTimeNanos="+frameTimeNanos+" currentFrameTimeNanos="+currentFrameTimeNanos+" skipFrameCount="+skipFrameCount+"");
lastFrameTimeNanos=currentFrameTimeNanos;
Choreographer.getInstance().postFrameCallback(this);
}
/**
*
*計算跳過多少幀
* @param start
* @param end
* @param devicefreshRate
* @return
*/
private int skipFrameCount(long start,long end,float devicefreshRate){
int count =0;
long diffNs=end-start;
long diffMs = Math.round(diffNs / 1000000.0f);
long dev=Math.round(devicefreshRate);
if(diffMs>dev){
long skipCount=diffMs/dev;
count=(int)skipCount;
}
return count;
}
}
在需要檢測的Activity中調用 SMFrameCallback.getInstance().start()即可。
一般情況下，我們會寫在我們的BaseActivity或者Activitylifecyclecallbacks中去調用。
自定義MyActivityLifeCycle實作Application.ActivityLifecycleCallbacks。
public class MyActivityLifeCycle implements Application.ActivityLifecycleCallbacks {
private Handler mHandler = new Handler(Looper.getMainLooper());
private boolean mPaused = true;
private Runnable mCheckForegroundRunnable;
private boolean mForeground = false;
private static MyActivityLifeCycle sInstance;
//目前Activity的弱引用
private WeakReference<Activity> mActivityReference;
protected final String TAG = "#MyActivityLifeCycle";
public static final int ACTIVITY_ON_RESUME = 0;
public static final int ACTIVITY_ON_PAUSE = 1;
private MyActivityLifeCycle() {
}
public static synchronized MyActivityLifeCycle getInstance() {
if (sInstance == null) {
sInstance = new MyActivityLifeCycle();
}
return sInstance;
}
public Activity getCurrentActivity() {
if (mActivityReference != null) {
return mActivityReference.get();
}
return null;
}
public boolean isForeground() {
return mForeground;
}
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
mActivityReference = new WeakReference<>(activity);
}
@Override
public void onActivityStarted(Activity activity) {
}
@Override
public void onActivityResumed(Activity activity) {
String activityName = activity.getClass().getName();
notifyActivityChanged(activityName, ACTIVITY_ON_RESUME);
mPaused = false;
if (Build.VERSION.SDK_INT > Build.VERSION_CODES.JELLY_BEAN) {
FrameSkipMonitor.getInstance().setActivityName(activityName);
FrameSkipMonitor.getInstance().OnActivityResume();
if (!mForeground) {
FrameSkipMonitor.getInstance().start();
}
}
mForeground = true;
if (mCheckForegroundRunnable != null) {
mHandler.removeCallbacks(mCheckForegroundRunnable);
}
mActivityReference = new WeakReference<Activity>(activity);
}
@Override
public void onActivityPaused(Activity activity) {
notifyActivityChanged(activity.getClass().getName(), ACTIVITY_ON_PAUSE);
mPaused = true;
if (mCheckForegroundRunnable != null) {
mHandler.removeCallbacks(mCheckForegroundRunnable);
}
if (Build.VERSION.SDK_INT > Build.VERSION_CODES.JELLY_BEAN)
FrameSkipMonitor.getInstance().OnActivityPause();
mHandler.postDelayed(mCheckForegroundRunnable = new Runnable() {
@Override
public void run() {
if (mPaused && mForeground) {
if (Build.VERSION.SDK_INT > Build.VERSION_CODES.JELLY_BEAN) {
FrameSkipMonitor.getInstance().report();
}
mForeground = false;
}
}
}, 1000);
}
}
然後在自定義的Application中調用。
public class MyApplication extends Application {
...
@Override
public void onCreate() {
super.onCreate();
registerActivityLifecycleCallbacks(MyActivityLifeCycle.getInstance());
}
···
}
除了上述的Choreographer幀率檢測外，還有loop()列印日志等方法來對幀率進行統計監測。這裡就不一一舉例了。
正常情況下輸出的日志是：
02-07 20:18:52.605 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996166386820 currentFrameTimeNanos=6996166386820 skipFrameCount=0
02-07 20:18:52.621 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996183053486 currentFrameTimeNanos=6996183053486 skipFrameCount=0
02-07 20:18:52.637 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996199720152 currentFrameTimeNanos=6996199720152 skipFrameCount=0
02-07 20:18:52.657 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996216386818 currentFrameTimeNanos=6996216386818 skipFrameCount=0
02-07 20:18:52.673 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996233053484 currentFrameTimeNanos=6996233053484 skipFrameCount=0
02-07 20:18:52.689 6683-6683/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=6996249720150 currentFrameTimeNanos=6996249720150 skipFrameCount=0
有跳幀的時候輸出的日志是
02-07 20:21:53.909 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=7177466379568 currentFrameTimeNanos=7177466379568 skipFrameCount=0
02-07 20:21:53.925 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=7177483046234 currentFrameTimeNanos=7177483046234 skipFrameCount=0
02-07 20:21:54.133 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=200.0 frameTimeNanos=7177683046226 currentFrameTimeNanos=7177683046226 skipFrameCount=11
02-07 20:21:54.745 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=616.6666 frameTimeNanos=7178299712868 currentFrameTimeNanos=7178299712868 skipFrameCount=36
02-07 20:21:54.757 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=7178316379534 currentFrameTimeNanos=7178316379534 skipFrameCount=0
02-07 20:21:54.773 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=7178333046200 currentFrameTimeNanos=7178333046200 skipFrameCount=0
02-07 20:21:54.789 9530-9530/zhangwan.wj.com.choreographertest E/SMFrameCallback: 兩次繪制時間間隔value=16.666666 frameTimeNanos=7178349712866 currentFrameTimeNanos=7178349712866 skipFrameCount=0
看到兩次繪制的時間間隔相差616.6666毫秒，跳過了36幀，這個卡頓使用者是能夠明顯感覺的。
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