1.Android Skill 倒计时功能-CountDownTimer

CountDownTimer（倒数计时器）

  其实在很多时候，我们都需要一个倒计时的功能，这个功能我们自己可以根据java自带的TimerTask

去实现。这里，提到的是一个在基本Android开发书籍中都很少介绍到的一个Android原生自带倒数计

时器 - CountDownTimer 。

接下来，我们通过CountDownTimer的源代码，看观赏一下它的实现原理：

/* * Copyright (C) 2008 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package android.os;import android.util.Log;/** * Schedule a countdown until a time in the future, with * regular notifications on intervals along the way. * * Example of showing a 30 second countdown in a text field: * * <pre class="prettyprint"> * new CountDownTimer(30000, 1000) { * *     public void onTick(long millisUntilFinished) { *         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000); *     } * *     public void onFinish() { *         mTextField.setText("done!"); *     } *  }.start(); * </pre> * * The calls to {@link #onTick(long)} are synchronized to this object so that * one call to {@link #onTick(long)} won't ever occur before the previous * callback is complete.  This is only relevant when the implementation of * {@link #onTick(long)} takes an amount of time to execute that is significant * compared to the countdown interval. */public abstract class CountDownTimer {    /**     * Millis since epoch when alarm should stop.     */    private final long mMillisInFuture;    /**     * The interval in millis that the user receives callbacks     */    private final long mCountdownInterval;    private long mStopTimeInFuture;    /**     * @param millisInFuture The number of millis in the future from the call     *   to {@link #start()} until the countdown is done and {@link #onFinish()}     *   is called.     * @param countDownInterval The interval along the way to receive     *   {@link #onTick(long)} callbacks.     */    public CountDownTimer(long millisInFuture, long countDownInterval) {        mMillisInFuture = millisInFuture;        mCountdownInterval = countDownInterval;    }    /**     * Cancel the countdown.     */    public final void cancel() {        mHandler.removeMessages(MSG);    }    /**     * Start the countdown.     */    public synchronized final CountDownTimer start() {        if (mMillisInFuture <= 0) {            onFinish();            return this;        }        mStopTimeInFuture = SystemClock.elapsedRealtime() + mMillisInFuture;        mHandler.sendMessage(mHandler.obtainMessage(MSG));        return this;    }    /**     * Callback fired on regular interval.     * @param millisUntilFinished The amount of time until finished.     */    public abstract void onTick(long millisUntilFinished);    /**     * Callback fired when the time is up.     */    public abstract void onFinish();    private static final int MSG = 1;    // handles counting down    private Handler mHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            synchronized (CountDownTimer.this) {                final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();                if (millisLeft <= 0) {                    onFinish();                } else if (millisLeft < mCountdownInterval) {                    // no tick, just delay until done                    sendMessageDelayed(obtainMessage(MSG), millisLeft);                } else {                    long lastTickStart = SystemClock.elapsedRealtime();                    onTick(millisLeft);                    // take into account user's onTick taking time to execute                    long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();                    // special case: user's onTick took more than interval to                    // complete, skip to next interval                    while (delay < 0) delay += mCountdownInterval;                    sendMessageDelayed(obtainMessage(MSG), delay);                }            }        }    };}

cancel() ：取消的倒计时。

start() ：开始倒计时。

onTick()：回调执行固定时间间隔。

onFinish() ：倒计时结束时

源代码中，我们可以看出 ：CountDownTimer类的同步start()方法执行后，做了一些简单

的时间判断和计算后(判断总时间、计算剩余时间)，然后发送到mHandler，在mHandler里

同步操作，然后又做了一些逻辑的运算和判断，为了设置onFinish()和onTick()方法的执行点

 然后 如果执行到了onTick的话，继续发送事件到mHandler。 

就是start()->mHandler->mHandler->mHandler... 直到 mHandler中执行了onFinish()。

所以主要的操作，我们都放在onTick()和onFinish()方法中。

那么这里，给一个小小的实现类：

package com.zyy.android_csdn.skill;import android.content.Context;import android.graphics.drawable.Drawable;import android.os.CountDownTimer;import android.widget.Button;/** *  * 倒计时按钮计时器 *  * @author CaMnter *  */public class CountDownButtonTimer extends CountDownTimer {public static final int TIME_COUNT_FUTURE = 60000;public static final int TIME_COUNT_INTERVAL = 1000;// 用于存放 Contextprivate Context mContext;// 用于存放 按钮private Button mButton;// 用于 存放 按钮Textprivate String mOriginalText;// 用于 存放 按钮背景private Drawable mOriginalBackground;// 用于 存放 按钮颜色private int mOriginalTextColor;private Drawable mTickBackground;public CountDownButtonTimer() {super(TIME_COUNT_FUTURE, TIME_COUNT_INTERVAL);}public CountDownButtonTimer(long millisInFuture, long countDownInterval) {super(millisInFuture, countDownInterval);}/** *  * 初始化 Button及其相关内容 *  * @param context * @param button */public void init(Context context, Button button) {this.mContext = context;this.mButton = button;this.mOriginalText = mButton.getText().toString();this.mOriginalBackground = mButton.getBackground();this.mTickBackground = this.mOriginalBackground;this.mOriginalTextColor = mButton.getCurrentTextColor();}public void setTickDrawable(Drawable tickDrawable) {this.mTickBackground = tickDrawable;}/** *  * 计时器结束的时 *  */@Overridepublic void onFinish() {if (mContext != null && mButton != null) {mButton.setText(mOriginalText);mButton.setTextColor(mOriginalTextColor);mButton.setBackground(mOriginalBackground);mButton.setClickable(true);}}/** *  * 倒计时开始时 *  */@Overridepublic void onTick(long millisUntilFinished) {if (mContext != null && mButton != null) {mButton.setClickable(false);mButton.setBackground(mTickBackground);mButton.setTextColor(mContext.getResources().getColor(android.R.color.darker_gray));mButton.setText(millisUntilFinished / 1000 + " 秒后可重新获取验证码");}}}