BlockCanary分析实现原理

介绍

BlockCanary 一个android UI检测工具，追踪view耗时操作；
附上地址：
https://github.com/markzhai/AndroidPerformanceMonitor
http://blog.zhaiyifan.cn/2016/01/16/BlockCanaryTransparentPerformanceMonitor/

核心代码

//BlockCanary
    public void start() {
        if (!mMonitorStarted) {
            mMonitorStarted = true;
            Looper.getMainLooper().setMessageLogging(mBlockCanaryCore.monitor);
        }
    }

监听loop处理事件

原来是监听主线程的 loop事件；看下怎样监听的

 public static void loop() {
      //...
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {//1
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
        //...
            try {
                msg.target.dispatchMessage(msg);//2
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
         //..

            if (logging != null) {//3
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }
  //...
        }
    }

原来主线程的消息队列抽取消息 去处理的前后，注释1，注释2 都会执行 logging的日志打印；而logging正是 BlockCanary传入的，实现类是
LooperMonitor;

class LooperMonitor implements Printer {
 @Override
    public void println(String x) {
        if (mStopWhenDebugging && Debug.isDebuggerConnected()) {
            return;
        }
        if (!mPrintingStarted) {//1
            mStartTimestamp = System.currentTimeMillis();
            mStartThreadTimestamp = SystemClock.currentThreadTimeMillis();
            mPrintingStarted = true;
            startDump();
        } else {//2
            final long endTime = System.currentTimeMillis();
            mPrintingStarted = false;
            if (isBlock(endTime)) {//3
                notifyBlockEvent(endTime);
            }
            stopDump();
        }
    }
  
    private boolean isBlock(long endTime) {
        return endTime - mStartTimestamp > mBlockThresholdMillis;
    }
}

注释1 记录当前时间和开始标记，代码执行到注释2的时候，自然是要记录结束时间；
执行注释3 判断handler的消息处理是否 超过阀值，如果超出 就给出ui卡顿提示 以及方法调用（stacktrace）和cpu使用信息，来帮助开发者 即使发现ui卡顿问题；

BlockCanary如何读取stacktrace和cpu使用信息

BlockCanary 有两个采样器，StackSampler和CpuSampler;
handler开始处理消息时，执行startDump；采样器开始工作
handler开始处理结束时，执行stopDumo；采样器结束工作

在handle开始处理信息时，使用Stack采样器获取方法调用信息 使用Cpu采样器获取Cpu使用信息；
在handle处理信息完毕时，采样器结束读取。
这样就获取到了handler处理消息过程中的 方法调用信息和cpu使用信息；

附StackSampler，CpuSampler 部分代码

class StackSampler extends AbstractSampler {
   @Override
    protected void doSample() {
        StringBuilder stringBuilder = new StringBuilder();

        for (StackTraceElement stackTraceElement : mCurrentThread.getStackTrace()) {
            stringBuilder
                    .append(stackTraceElement.toString())
                    .append(BlockInfo.SEPARATOR);
        }

        synchronized (sStackMap) {
            if (sStackMap.size() == mMaxEntryCount && mMaxEntryCount > 0) {
                sStackMap.remove(sStackMap.keySet().iterator().next());
            }
            sStackMap.put(System.currentTimeMillis(), stringBuilder.toString());
        }
    }
}

class CpuSampler extends AbstractSampler {
    @Override
    protected void doSample() {
        BufferedReader cpuReader = null;
        BufferedReader pidReader = null;

        try {
            cpuReader = new BufferedReader(new InputStreamReader(
                    new FileInputStream("/proc/stat")), BUFFER_SIZE);
            String cpuRate = cpuReader.readLine();
            if (cpuRate == null) {
                cpuRate = "";
            }

            if (mPid == 0) {
                mPid = android.os.Process.myPid();
            }
            pidReader = new BufferedReader(new InputStreamReader(
                    new FileInputStream("/proc/" + mPid + "/stat")), BUFFER_SIZE);
            String pidCpuRate = pidReader.readLine();
            if (pidCpuRate == null) {
                pidCpuRate = "";
            }

            parse(cpuRate, pidCpuRate);
        } catch (Throwable throwable) {
            Log.e(TAG, "doSample: ", throwable);
        } finally {
            try {
                if (cpuReader != null) {
                    cpuReader.close();
                }
                if (pidReader != null) {
                    pidReader.close();
                }
            } catch (IOException exception) {
                Log.e(TAG, "doSample: ", exception);
            }
        }
    }
}

StackSampler和CpuSampler都有一个HandlerThread对象，用来在各自的子线程中 间隔性的启动任务；来读取 主线程方法调用和cpu使用信息。