Handler工作原理

標籤：emc   迴圈   called   further   als   訊息機制   contain   lock   lis   

Handler是訊息機制的面板類,我們通過Handler.send或者post訊息然後在handleMessage()中去處理訊息.發送訊息

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

sendMessage操作都只是對Message做了一個入隊的操作.Looper無限迴圈從MessageQueue中取出Message然後調用Handler的dispatchMessage來處理訊息處理訊息

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

msg的callback是一個Runnable,handlecallback(msg)就是調用了Runnable的run方法.callable介面第五行的mCallback是一個介面,定義如下

    /**     * Callback interface you can use when instantiating a Handler to avoid     * having to implement your own subclass of Handler.     */    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);    }

可以通過Handler handler = new Handler(new CallBack(){})來使用,作用就是避免建立Handler的子類. 

Handler工作原理