Handler之三：构造函数源码解析

Handler构造函数，创建实例

概述

Handler的构造分为两类，一类是在UI线程创建实例，一类在非UI线程创建实例。区别是Looper的获得方式。

使用构造方法创建实例

在UI线程创建实例

private Handler mHandler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(Message msg) {
            return false;
        }
    });

在非UI线程创建实例

class LooperThread extends Thread{
        @Override
        public void run() {
        	//1.初始化Looper
            Looper.prepare();
            //2.创建Handler实例
            new Handler(new Handler.Callback() {
                @Override
                public boolean handleMessage(Message msg) {
                    return false;
                }
            });
            //3.Looper循环
            Looper.loop();
        }
    }

源码分析

• 在UI线程创建实例j

public Handler() {
        this(null, false);
    }
    
public Handler(Callback callback) {
        this(callback, false);
    }

public Handler(boolean async) {
        this(null, async);
    }

public Handler(Callback callback, boolean async) {
      	......
    }

1.Callback：方法回调，将Message传回UI线程

 public interface Callback {
        /**
         * @param msg A {@link android.os.Message Message} object
         * @return True if no further handling is desired
         */
        public boolean handleMessage(Message msg);
    }

2.async：true为异步，false为同步

//当async为true时
 msg.setAsynchronous(true);

3.下文是上面所有构造方法中展示的最后一个的全文

public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

		//关联Looper，已在UI线程中创建了Looper
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        //获得消息栈
        mQueue = mLooper.mQueue;
        //传入CallBack，async参数
        mCallback = callback；
        mAsynchronous = async;
    }

• 在非UI线程创建实例

public Handler(Looper looper) {
        this(looper, null, false);
    }

 public Handler(Looper looper, Callback callback) {
        this(looper, callback, false);
    }
    
 public Handler(Looper looper, Callback callback, boolean async) {
 		//传入looper，callback，async参数
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

总结

1.UI线程已经初始化Looper，所以不需要再次初始化，而其他线程并没有初始化，所以需要初始化。