CountDownTimer的用法及原理

1.主线程中使用

值得注意的是，CountDownTimer可以在主线程中直接使用。验证一下回调的执行线程。在主线程中执行如下代码

CountDownTimer countDownTimer = new CountDownTimer(1000L * , 1000L) {
            @Override
            public void onTick(long millisUntilFinished) {
                // your logic for tick
                Log.i("TEST", "onTick - " + Thread.currentThread() + ", id - " + Thread.currentThread().getId());
            }

            @Override
            public void onFinish() {
                // your logic for finish

                Log.i("TEST", "onFinish" + Thread.currentThread() + ", id - " + Thread.currentThread().getId());
            }
};
countDownTimer.start();

Log：

- ::13.545   I TEST    : onTick - Thread[main,,main], id -
- ::14.546   I TEST    : onTick - Thread[main,,main], id -
- ::15.548   I TEST    : onTick - Thread[main,,main], id -
- ::16.550   I TEST    : onTick - Thread[main,,main], id -
- ::17.551   I TEST    : onTick - Thread[main,,main], id -
- ::18.553   I TEST    : onTick - Thread[main,,main], id -
- ::19.555   I TEST    : onTick - Thread[main,,main], id -
- ::20.557   I TEST    : onTick - Thread[main,,main], id -
- ::21.560   I TEST    : onTick - Thread[main,,main], id -
- ::22.563   I TEST    : onTick - Thread[main,,main], id -
- ::23.566   I TEST    : onTick - Thread[main,,main], id -
- ::24.569   I TEST    : onTick - Thread[main,,main], id -
- ::25.571   I TEST    : onTick - Thread[main,,main], id -
- ::26.573   I TEST    : onTick - Thread[main,,main], id -
- ::27.575   I TEST    : onTick - Thread[main,,main], id -
- ::28.578   I TEST    : onTick - Thread[main,,main], id -
- ::29.580   I TEST    : onTick - Thread[main,,main], id -
- ::30.581   I TEST    : onTick - Thread[main,,main], id -
- ::31.583   I TEST    : onTick - Thread[main,,main], id -
- ::32.586   I TEST    : onTick - Thread[main,,main], id -
- ::33.588   I TEST    : onTick - Thread[main,,main], id -
- ::34.589   I TEST    : onTick - Thread[main,,main], id -
- ::35.592   I TEST    : onTick - Thread[main,,main], id -
- ::36.595   I TEST    : onTick - Thread[main,,main], id -
- ::37.598   I TEST    : onTick - Thread[main,,main], id -
- ::38.599   I TEST    : onTick - Thread[main,,main], id -
- ::39.601   I TEST    : onTick - Thread[main,,main], id -
- ::40.602   I TEST    : onTick - Thread[main,,main], id -
- ::41.604   I TEST    : onTick - Thread[main,,main], id -
- ::43.546   I TEST    : onFinishThread[main,,main], id - 

可见，回调执行在主线程，这也就意味着可以在回调中直接处理UI，还是很方便的。

2.在工作线程中使用

不能在工作线程中像主线程一样简单使用的原因，是CountDownTimer是基于Handler的，细节将在原理部分说明。主线程已经由安卓框架层启动了消息循环，所以可以直接使用。那么在工作线程中怎样使用呢？可以借助于HanderThread。在onLooperPrepared()中初始化并开启CountDownTimer：

new HandlerThread("TestThread") {
    protected void onLooperPrepared() {
        CountDownTimer countDownTimer = new CountDownTimer(1000L * , 1000L) {
            @Override
            public void onTick(long millisUntilFinished) {
                // your logic for tick
                Log.i("TEST", "onTick - " + Thread.currentThread() + ", id - " + Thread.currentThread().getId());
            }

            @Override
            public void onFinish() {
                // your logic for finish
                Log.i("TEST", "onFinish" + Thread.currentThread() + ", id - " + Thread.currentThread().getId());
            }
        };
        countDownTimer.start();
    }
}.start();

执行log如下：

- ::35.789   I TEST    : onTick - Thread[TestThread,,main], id -
- ::36.790   I TEST    : onTick - Thread[TestThread,,main], id -
- ::37.793   I TEST    : onTick - Thread[TestThread,,main], id -
- ::38.795   I TEST    : onTick - Thread[TestThread,,main], id -
- ::39.797   I TEST    : onTick - Thread[TestThread,,main], id -
- ::40.800   I TEST    : onTick - Thread[TestThread,,main], id -
- ::41.801   I TEST    : onTick - Thread[TestThread,,main], id -
- ::42.803   I TEST    : onTick - Thread[TestThread,,main], id -
- ::43.804   I TEST    : onTick - Thread[TestThread,,main], id -
- ::44.806   I TEST    : onTick - Thread[TestThread,,main], id -
- ::45.809   I TEST    : onTick - Thread[TestThread,,main], id -
- ::46.810   I TEST    : onTick - Thread[TestThread,,main], id -
- ::47.813   I TEST    : onTick - Thread[TestThread,,main], id -
- ::48.814   I TEST    : onTick - Thread[TestThread,,main], id -
- ::49.816   I TEST    : onTick - Thread[TestThread,,main], id -
- ::50.818   I TEST    : onTick - Thread[TestThread,,main], id -
- ::51.820   I TEST    : onTick - Thread[TestThread,,main], id -
- ::52.821   I TEST    : onTick - Thread[TestThread,,main], id -
- ::53.823   I TEST    : onTick - Thread[TestThread,,main], id -
- ::54.825   I TEST    : onTick - Thread[TestThread,,main], id -
- ::55.827   I TEST    : onTick - Thread[TestThread,,main], id -
- ::56.828   I TEST    : onTick - Thread[TestThread,,main], id -
- ::57.829   I TEST    : onTick - Thread[TestThread,,main], id -
- ::58.831   I TEST    : onTick - Thread[TestThread,,main], id -
- ::59.832   I TEST    : onTick - Thread[TestThread,,main], id -
- ::00.835   I TEST    : onTick - Thread[TestThread,,main], id -
- ::01.836   I TEST    : onTick - Thread[TestThread,,main], id -
- ::02.838   I TEST    : onTick - Thread[TestThread,,main], id -
- ::03.840   I TEST    : onTick - Thread[TestThread,,main], id -
- ::05.791   I TEST    : onFinishThread[TestThread,,main], id -