Handler中sendMessageAtTime 和sendMessageDelayed的区别

sendMessageAtTime 使用的uptimeMillis依赖的是系统以开机时间的绝对时间；

而sendMessageDelayed使用的delayMillis依赖的是系统以开机时间的相对时间。

啥意思呢：就是说delayMillis使用的时候要加一个SystemClock.uptimeMillis(),

也就是sendMessageAtTime等于snedMessageDelayed的情况为uptimeMillis == delayMillis - SystemClock.uptimeMillis()[这是一个相对时间].

SystemClock.uptimeMillis()是获取系统从开机启动到现在的时间,期间不包括休眠的时间,这里获得到的时间是一个相对的时间,而不是通过获取当前的时间(绝对时间)。

而之所以使用这种方式来计算时间，而不是获得当前currenttime来计算，在于handler会受到阻塞，挂起状态，睡眠等，这些时候是不应该执行的；如果使用绝对时间的话，就会抢占资源来执行当前handler的内容，显然这是不应该出现的情况，所以要避免。

查看源码如下：

    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) uptimeMillis.
     * The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public boolean sendMessageAtTime(Message msg, long uptimeMillis)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;
            sent = queue.enqueueMessage(msg, uptimeMillis);
        }
        else {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }