Android CountDownTimer use

Original link: http://www.cnblogs.com/butterfly-clover/p/5234052.html

Official usage are as follows:

new CountDownTimer(30000, 1000) {

     public void onTick(long millisUntilFinished) {
         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
     }

     public void onFinish() {
         mTextField.setText("done!");
     }
  }.start();

After creating CountDownTimer instance, it must () function will be turned on by the timer start, in order to ensure CountDownTimer run. CountDownTimer also provides a cancel () method, the timer can be canceled.

In use CountDownTimer, you must implement two methods: onTick () and onFinish (). MillisUntilFinished parameters in onTick (long millisUntilFinished) is the remaining time countdown. At the end of the countdown will call onFinish, after the end of the countdown you need to do can be written in onFinish in.

The CountDownTimer (30000, 1000) 30000, represented by the countdown for 30 seconds, every 1 second 1000 represents onTick method called once.

Detailed method:

call onTick are synchronized to ensure that this does not occur before the call before the call is completed. Synchronization mechanism here is mainly used to: achieve onTick requires a lot of time for more important things than the countdown interval. Constructors public CountDownTimer (long millisInFuture, long countDownInterval ) parameters Number of milliseconds from start to finish millisInFuture start () call and the countdown onFinish () method is called. (Countdown time, in milliseconds) countDownInterval receiving onTick (long) time interval callback. (In milliseconds) Public method 　　public final void cancel () to cancel the countdown (countdown will stop before the 5.0 system can not be called onTick () in) 　　public abstract void onFinish () is called when the countdown is completed 　　public abstract void onTick (long millisUntilFinished ) at regular intervals is called parameter millisUntilFinished countdown time remaining. 　　public synchronized final CountDownTimer start () to start the countdown

在使用过程中发现，在一个activity或者fragment中开启了计时器，如果倒计时没有完成即退出activity或者fragment，此时onTick仍然会继续执行，当执行到mTextField.setText()时，mTextField为null，导致程序crash。因此需要在onTick中进行非空判断。

如果是在Fragment中：

if(getActivity()!=null){
   //todo 
}

如果是在Activity中：

if(!activity.isFinishing()){
    //todo
}

但是我们不希望在fragment或activity退出之后仍然调用onTick，即使已经做了非空判断。
我们希望在退出之后CountDownTimer也随之停止，因此在onTick中，当getActivity(）==null或者activity.isFinishing()==true的时候，可以使用cancel方法取消掉计时器。

But！！！
经过实验发现，cancel在onTick中调用，是无法成功取消计时器的。
调用cancel之后，仍然会每隔固定时间调用onTick方法。
然而，在5.0及以上的系统中，cancel方法就可以起到作用。对比CountDownTimer的源码发现，在5.0中，增加了一个字段

 /**
  * boolean representing if the timer was cancelled
  */
    private boolean mCancelled = false;

通过mCancelled 标识当前计时器是否取消。
然后在handleMessage()中首先对mCancelled进行判断：

// handles counting down
    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {

            synchronized (CountDownTimer.this) {
                if (mCancelled) { //**Attention！！！**
                    return;
                }

                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);
                }
            }
        }
    };

因此，如果想在onTick中调用cancel方法取消计时器，可以自定义一个CountDownTimerUtil,将5.0以上的CountDownTimer源码复制到CountDownTimerUtil即可。

附5.0以上系统的代码：

/*
 * 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;

/**
 * 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;
    
    /**
    * boolean representing if the timer was cancelled
    */
    private boolean mCancelled = false;

    /**
     * @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 synchronized final void cancel() {
        mCancelled = true;
        mHandler.removeMessages(MSG);
    }

    /**
     * Start the countdown.
     */
    public synchronized final CountDownTimer start() {
        mCancelled = false;
        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) {
                if (mCancelled) {
                    return;
                }

                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);
                }
            }
        }
    };
}

4.4系统的代码

/*
 * 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);
                }
            }
        }
    };
}

转载于:https://www.cnblogs.com/butterfly-clover/p/5234052.html

Android CountDownTimer use

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