Android Handler 機制（六）：如何檢測性能卡頓

一、Looper檢測卡頓

熟悉Handler機制的話，我們會知道Looper是給線程提供處理消息能力的類，在Android Framework啟動的時候，就會創建一個Main Looper即主線程對應的Looper，Looper中會維護一個MessageQueue，負責接收Handler發送過來的消息，MessageQueue是個消息隊列，它是順序取消息的，只有取完一個任務才會接着取另外一個任務。

查看Looper的源碼后就可以發現我們可以使用Looper的looper方法來進行檢測。因為UI更新界面都是在主線程中進行的，所以在主線程中做耗時操作可能會造成界面卡頓，looper()方法循環去從MessageQueue中去取msg，然后執行，而且是順序執行的，那么前面一個msg還沒處理完，loop()就會等待它處理完了才會再去執行下一個msg，如果前面一個msg處理很慢，那就會造成卡頓了。

二、使用Looper檢測性能原理

Looper的looper方法源碼如下（涉及性能部分的代碼已經加粗）：

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    // Make sure the identity of this thread is that of the local process,
    // and keep track of what that identity token actually is.
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();

    // Allow overriding a threshold with a system prop. e.g.
    // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
    final int thresholdOverride =
            SystemProperties.getInt("log.looper."
                    + Process.myUid() + "."
                    + Thread.currentThread().getName()
                    + ".slow", 0);

    boolean slowDeliveryDetected = false;

    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) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long traceTag = me.mTraceTag;
        long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
        long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
        if (thresholdOverride > 0) {
            slowDispatchThresholdMs = thresholdOverride;
            slowDeliveryThresholdMs = thresholdOverride;
        }
        final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
        final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

        final boolean needStartTime = logSlowDelivery || logSlowDispatch;
        final boolean needEndTime = logSlowDispatch;

        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }

        final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
        final long dispatchEnd;
        try {
            msg.target.dispatchMessage(msg);
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (logSlowDelivery) {
            if (slowDeliveryDetected) {
                if ((dispatchStart - msg.when) <= 10) {
                    Slog.w(TAG, "Drained");
                    slowDeliveryDetected = false;
                }
            } else {
                if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                        msg)) {
                    // Once we write a slow delivery log, suppress until the queue drains.
                    slowDeliveryDetected = true;
                }
            }
        }
        if (logSlowDispatch) { showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg); } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the
        // identity of the thread wasn't corrupted.
        final long newIdent = Binder.clearCallingIdentity();
        if (ident != newIdent) {
            Log.wtf(TAG, "Thread identity changed from 0x"
                    + Long.toHexString(ident) + " to 0x"
                    + Long.toHexString(newIdent) + " while dispatching to "
                    + msg.target.getClass().getName() + " "
                    + msg.callback + " what=" + msg.what);
        }

        msg.recycleUnchecked();
    }
}

首先看到在msg.target.dispatchMessage(msg)方法前面會有一個日志打印：

final Printer logging = me.mLogging;
if (logging != null) {
    logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what);
}

在處理完消息的時候，又會有一個結束的日志打印：

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

看到這里，我們就知道可以通過Looper中的setMessageLogging()方法進行設置。

public void setMessageLogging(@Nullable Printer printer) {
    mLogging = printer;
}

 比較出名的  BlockCanary 檢測性能的框架就是基於此實現的。

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

下圖就是 BlockCanary 框架實現性能監測的邏輯圖：