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Android消息处理机制(Handler、Looper、MessageQueue与Message) ,这一篇文章就够了

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记得看文章三部曲,点赞,评论,转发。

微信搜索【程序员小安】关注还在移动开发领域苟活的大龄程序员,“面试系列”文章将在公众号同步发布。 1.前言


上篇文章介绍了几种hanlder创建方式,其实这种使用方式大家都知道,但是为什么可以这么做,可能很多人不知道,至少不清楚,网上很多文章也是到处粘贴,听别说handler把Message发送到MessageQueue里面去,Looper通过死循环,不断从MessageQueue里面获取Message处理消息,因为Mesage.target就是当前hanlder,所以最后转到handleMessage()方法中去处理,整个流程是这样。其实大概都是对的,之前面试的时候,我也都是这么说,也没有面试官深入问过,这次正好有时间深入源码系统学习下,毕竟还是要知其所以然。

2.使用方法

package com.example.test.myapplication;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.Toast;

public class MainActivity extends Activity {

    private Handler handler1 = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            if (msg.arg1 == 1) {
                Toast.makeText(MainActivity.this, "hanlder1", Toast.LENGTH_SHORT).show();
            }
            super.handleMessage(msg);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new Runnable() {
            @Override
            public void run() {
                Message message = handler1.obtainMessage();
                message.arg1 = 1;
                handler1.sendMessage(message);
            }
        }).start();
    }
}

这种用法是大家最常用的,下面我们就用这个来作为切入点。

3.源码讲解

1)Looper,MessageQueue创建 应用启动的时候会先调用ActivityThread的main方法,main方法会调用Looper.prepareMainLooper();创建Looper对象,Looper对象的构造方法中会创建MessageQueue对象,源码如下所示:

public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        AndroidKeyStoreProvider.install();

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

在第23行可以看到Looper.prepareMainLooper();方法,此方法其实就是UI线程默认为应用创建Looper对象,我们继续看下prepareMainLooper()方法的源码:

public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }
private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

这里通过调用调用prepare()方法,创建了Looper对象,而且sThreadLocal如果没有Looper,则新建Looper进去,如果存在,则抛出异常,而且从判空可以看出一个线程最多只能创建一个Looper对象,验证了很多人说的一个线程一个Looper对象的说法。 上面提到在Looper的构造方法中,会创建MessageQueue对象,我们看一下Looper构造方法:

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

没有错,Looper创建的同时会创建MessageQueue,所以很多人说一个Looper对应一个MessageQueue就是从这里来的。Looper跟MessageQueue都创建好了,接下来看如何发送Message消息。

2)handler.sendMessage(message)发送消息到什么地方,内部怎么处理 要想知道发送到哪里,怎么处理,只有一条路,跟到源码中去看:

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }

前面两个方法就不看了,直接看sendMessageAtTime()方法,sendMessageAtTime()方法中传入两个参数,msg就是我们handler.sendMessage(message)发送的Message对象,而uptimeMillis参数则表示发送消息的时间,它的值等于自系统开机到当前时间的毫秒数再加上延迟时间,如果你调用的不是sendMessageDelayed()方法,则延迟时间就为0。方法最后调用了enqueueMessage()。

 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

上面方法中,会调用queue.enqueueMessage(msg, uptimeMillis),queue其实就是Looper构造函数中初始化的MessageQueue对象,可以看到,handler把消息发送到MessageQueue中了。

3)MessageQueue对象 接下来我们看queue.enqueueMessage(msg, uptimeMillis);的源码:

boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

这段代码有点意思,其实就是入队操作,之前一直以为MessageQueue里面有个列表,Message依次保存在里面,其实并不是。MessageQueue借助Message对象(next)成员变量,实现单向链表,并且用一个mMessages对象表示当前待处理的消息。 我们来分析一下上面代码,第二十行,首先Message p = mMessages;把当前待处理的message赋值给p,接下来是一个很长的判断if (p == null || when == 0 || when < p.when),根据判断可以得知,有以下两种情况会执行if里面的语句:1.应用首次调用sendMessage时,当前待处理的消息为null,则p为空,则会执行;2.当前传入msg的when小于当前待处理msg的when,及当前传入的消息要先于当前待处理mMessages得到处理,所以传入的msg赋值为mMessages,而下一个待处理消息才为原先的待处理消息。

if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } 

当执行过多个sendMessage方法时并且消息的时间(when)比当前待处理的消息的时间(when)大,这个时候就会移动链表,根据时间先后,插入到合适的位置,调用如下代码:

Message prev;
    for (;;) {
    prev = p;
    p = p.next;
    if (p == null || when < p.when) {
    break;
    }
    if (needWake && p.isAsynchronous()) {
    needWake = false;
    }
    }
    msg.next = p; // invariant: p == prev.next
    prev.next = msg;

不停的循环,根据when,及发送时间来先后排序,先发送的排在最前面,用message.next来指定下一个Message对象,这样MessageQueue通过时间把多个Message组成一个单向链表。

4)Looper对象,取当前待处理Message。 有进就有出,如何从MessageQueue中获取当前Message进行处理呢,这就要看Looper.loop()方法了。这里可以明白为什么子线程中初始化Handler最后为什么要显示调用Looper.loop()方法了吧,其实相当于启动获取Message方法的循环,主线程中会默认调用。

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();

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

            msg.target.dispatchMessage(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();
        }
    }

可以看到,第13行依然是一个死循环,不断执行的queue.next()方法,next()方法其实就是做消息出队列操作。获取当前待处理的mMessages对象,然后让下一条消息成为mMessages,其实跟链表操作一样,没有难度。loop()方法的第27行,Message获取到后,则调用msg.target.dispatchMessage(msg)方法,msg是Message对象,target是什么呢?看下Message源码,target其实是发送该Message的Handler对象。 调用Message message = handler1.obtainMessage();的时候,会进行赋值:

public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

        return m;
    }

接下来当然就要看一看Handler中dispatchMessage()方法的源码了,如下所示:

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

一般情况下,按照我们上面的调用方式,msg.callback,mCallback 都为空,则会直接调用handleMessage(msg)方法,这样终于转到我们的handleMessage方法中来了,在里面去做更新UI的操作。当然有人会问,什么时候msg.callback,mCallback不为空呢? msg.callback不为空,其实只要看看源码里面,哪些方法可以赋值就可以了:

public static Message obtain(Handler h, Runnable callback) {
        Message m = obtain();
        m.target = h;
        m.callback = callback;

        return m;
    }

下面是我的调用方法:

package com.example.test.myapplication;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;

public class MainActivity extends Activity {

    private Handler handler1 = new Handler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new Runnable() {
            @Override
            public void run() {
                Message message = Message.obtain(handler1, new Runnable() {
                    @Override
                    public void run() {
                        System.out.println("handler2  Thread========="+Thread.currentThread().getName());
                    }
                });
                message.arg1 = 1;
                handler1.sendMessage(message);
            }
        }).start();
    }
}

从打印信息可以看到,run方法里面其实也是主线程,这种方式其实很多,下面一一介绍。

4.其他:

上面这个callback的调用方式大家有没有很熟悉呢,其实有几种是大家平时经常使用的:

1.Handler的post()方法 2.View的post()方法 3.Activity的runOnUiThread()方法 这些都可以在子线程中转到主线程,从而去更新UI,至于为什么可以呢,相信很多人不知道。 1)Handler的post()源码

public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

万变不离其宗,其实还是调用sendMessageDelayed方法,而且Runnable还被封装成了Message,后面入链表,出链表过程跟前面讲的都一样。

private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

只是最后的时候,msg.callback != null,他会调用handleCallback(msg);

private static void handleCallback(Message message) {
        message.callback.run();
    }

这样,run方法被调用,是不是很简单。

2)View的post()源码 View类中,post方法:

public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }
        // Assume that post will succeed later
        ViewRootImpl.getRunQueue().post(action);
        return true;
    }

竟然还是mHandler.post(action),原理同上了。

3)Activity的runOnUiThread()源码

public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

如果当前的线程不等于UI线程,仍然执行mHandler.post(action)方法,如果就在主线程,当然直接接口回调,调用run方法就可以了。

5.总结:

所以不管何种子线程转到主线程去刷新UI,背后原理其实都是一样的,搞清楚了一个,都清楚了。希望看到这篇文章后,面试官再问子线程更新主线程的问题的时候,你可以是下面的表情: Android消息处理机制(Handler、Looper、MessageQueue与Message) ,这一篇文章就够了


微信搜索【程序员小安】“面试系列(java&andriod)”文章将在公众号同步发布。

转载自:https://juejin.cn/post/7046945567469109255
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