Android 中的 Handler 同步分割栏实现方法

handler异步实现控制message的处理的，异步线程可以在handler中设置异步卡子，设置好了以后，当前handler的同步message都不再执行，直到异步线程将卡子去掉。

一、postsyncbarrier 设置同步卡

方法是隐藏的需要反射获取 api版本<23方法位于looper中，api版本>23位于
messagequeue中 作用是为handler设置卡子，过滤掉同步消息，同步消息等待执行

二、removesyncbarrier 移除同步卡

方法是隐藏的需要反射获取 api版本<23方法位于looper中，api版本>23位于
messagequeue中 作用是为handler移除卡子，放开同步消息过滤，同步消息继续执行。

package qin.xue.imageviewscaletype.activity;

import android.app.activity;
import android.os.bundle;
import android.os.handler;
import android.os.handlerthread;
import android.os.looper;
import android.os.messagequeue;
import android.util.log;

import java.lang.reflect.method;

import qin.xue.imageviewscaletype.r;

/**
 * created by qinxue on 2018/5/21.
 */

public class syncbarrieractivity extends activity {
    private static final string tag = "syncbarrieractivity";

    private handler handler;
    private handler workhanlder;
    private handlerthread workhandlerthread;
    private method methodpostsyncbarrier;
    private looper looper;
    private int token;
    private int count = 0;
    private long start = 0;
    private long end = 0;

    @override
    protected void oncreate(bundle savedinstancestate) {
        super.oncreate(savedinstancestate);
        setcontentview(r.layout.sync_barrier_activity_layout);
        workhandlerthread = new handlerthread("work_thread");
        workhandlerthread.start();
        looper = workhandlerthread.getlooper();
        workhanlder = new handler(looper);
        handler = new handler();
        workhanlder.post(runnable);

        handler.post(new runnable() {
            @override
            public void run() {
                start = system.currenttimemillis();
                log.i(tag, "设置同步卡 时间：" + start);
                postsyncbarrier();        //设置同步卡
            }
        });

        handler.postdelayed(new runnable() {
            @override
            public void run() {
                end = system.currenttimemillis();
                log.i(tag, "移除同步卡 时间：" + end + "耗时：" + (end - start));
                removesyncbarrier();    //5秒后移除同步卡
            }
        }, 5000);
    }

    //postsyncbarrier 和 removesyncbarrier 方法是否在messagequeue类中（api23及以上）
    private static boolean methodinqueue() {
        return android.os.build.version.sdk_int >= android.os.build.version_codes.m;
    }


    private runnable runnable = new runnable() {
        @override
        public void run() {
            count++;
            log.i(tag, "count: " + count + "time: " + system.currenttimemillis()); //同步打印数据
            workhanlder.postdelayed(runnable, 100);
        }
    };

    /**
     * 移除卡子
     */
    private void removesyncbarrier() {
        if (android.os.build.version.sdk_int >= android.os.build.version_codes.m) {
            removesyncbarrier(looper.getqueue());
        } else {
            removesyncbarrier(looper);
        }
    }

    private void removesyncbarrier(object obj) {
        try {
            method method = messagequeue.class.getmethod("removesyncbarrier", int.class);
            method.invoke(obj, token);
        } catch (exception e) {
            e.printstacktrace();
        }
    }

    /**
     * 添加卡子
     */
    private void postsyncbarrier() {
        if (methodinqueue()) {
            postsyncbarrier(looper.getqueue());
        } else {
            postsyncbarrier(looper);
        }
    }

    private void postsyncbarrier(object obj) {
        try {
            methodpostsyncbarrier = obj.getclass().getmethod("postsyncbarrier");
            token = (int) methodpostsyncbarrier.invoke(obj);
        } catch (exception e) {
            e.printstacktrace();
        }
    }

}

三、结果展示

05-21 17:44:22.926 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 1time: 1526895862926
05-21 17:44:22.947 12715-12715/qin.xue.imageviewscaletype i/syncbarrieractivity: 设置同步卡 时间：1526895862947
05-21 17:44:27.931 12715-12715/qin.xue.imageviewscaletype i/syncbarrieractivity: 移除同步卡 时间：1526895867930耗时：4983
05-21 17:44:27.932 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 2time: 1526895867932
05-21 17:44:28.033 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 3time: 1526895868033
05-21 17:44:28.134 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 4time: 1526895868134
05-21 17:44:28.236 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 5time: 1526895868235
05-21 17:44:28.336 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 6time: 1526895868336
05-21 17:44:28.437 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 7time: 1526895868437
05-21 17:44:28.538 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 8time: 1526895868538
05-21 17:44:28.639 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 9time: 1526895868639
05-21 17:44:28.739 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 10time: 1526895868739
05-21 17:44:28.841 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 11time: 1526895868841
05-21 17:44:28.941 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 12time: 1526895868941
05-21 17:44:29.043 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 13time: 1526895869043
05-21 17:44:29.144 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 14time: 1526895869144
05-21 17:44:29.244 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 15time: 1526895869244
05-21 17:44:29.345 12715-12732/qin.xue.imageviewscaletype i/syncbarrieractivity: count: 16time: 1526895869345

四、代码解释

代码中首先加了使用主线程设置异步线程workhandler中的异步卡子，导致workhandler中的message不在执行，仅保留待执行。 然后在5s之后在主线程移除了异步线程workhandler中的卡子，workhandler中的message继续执行下去。

源码分析">五、源码分析

我这里是6.0的源码

postsyncbarrier

 public int postsyncbarrier() {
        return postsyncbarrier(systemclock.uptimemillis());
    }

    private int postsyncbarrier(long when) {
        // enqueue a new sync barrier token.
        // we don't need to wake the queue because the purpose of a barrier is to stall it.
        synchronized (this) {
            final int token = mnextbarriertoken++;
            final message msg = message.obtain();
            msg.markinuse();
            msg.when = when;
            msg.arg1 = token;

            message prev = null;
            message p = mmessages;
            if (when != 0) {
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
            }
            if (prev != null) { // invariant: p == prev.next
                msg.next = p;
                prev.next = msg;
            } else {
                msg.next = p;
                mmessages = msg;
            }
            return token;
        }
    }

-这里可以看到这个卡子msg，新建了一个msg并根据时间插入到消息队列中，而且这个msg没有设置target

removesyncbarrier

    public void removesyncbarrier(int token) {
        // remove a sync barrier token from the queue.
        // if the queue is no longer stalled by a barrier then wake it.
        synchronized (this) {
            message prev = null;
            message p = mmessages;
            while (p != null && (p.target != null || p.arg1 != token)) {
                prev = p;
                p = p.next;
            }
            if (p == null) {
                throw new illegalstateexception("the specified message queue synchronization "
                        + " barrier token has not been posted or has already been removed.");
            }
            final boolean needwake;
            if (prev != null) {
                prev.next = p.next;
                needwake = false;
            } else {
                mmessages = p.next;
                needwake = mmessages == null || mmessages.target != null;
            }
            p.recycleunchecked();

            // if the loop is quitting then it is already awake.
            // we can assume mptr != 0 when mquitting is false.
            if (needwake && !mquitting) {
                nativewake(mptr);
            }
        }
    }

-（p.target != null || p.arg1 != token）这里如果arg1 == token或者target为null，那么这是一个卡子。需要移除掉 ，代码如下。 prev.next = p.next;

next()

在messagequeue中还有这个方法，获取消息队列中的下一个message

      if (msg != null && msg.target == null) {
                    // stalled by a barrier.  find the next asynchronous message in the queue.
                    do {
                        prevmsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isasynchronous());
                }

摘取部分代码，循环便利整个消息队列，如果target为null，那么直接利用isasynchronous()找到下一个异步的message，跳过同步消息，设置执行时间。
大致就是这样。

handler中还有阻塞机制

-如果发先下一条消息执行时间过长，会阻塞掉线程looper循环的。在c++层，下次再研究下。