Handler实现倒计时并优化内存

我们先看一个简单的例子

---

MainActivity.java

package com.studio.countdowntime;

import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

public static final int COUNTDOWN_TIME_CODE = 10001;
public static final int DELAY_MILLIS = 1000;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

//得到控件
final TextView countdownTimeTextView = (TextView) findViewById(R.id.countdownTimeTextView);

        Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(msg != null && msg.what == COUNTDOWN_TIME_CODE){
                    countdownTimeTextView.setText(String.valueOf(9));
                }
            }
        };

        Message message = Message.obtain();
        message.what = COUNTDOWN_TIME_CODE;

        handler.sendMessageDelayed(message, DELAY_MILLIS);
    }
}

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我们这么写程序运行结果是没有问题的，但是我们会发现handler上有一个警告：

大意是这个handler应该是静态的，否则会造成内存泄露。原因是我们看到if语句中的countdownTimeTextView是一个在Mainactivity里面定义的控件对象，假如if语句还没有执行到，而当前的Activity已经被销毁了，这个时候发的消息还有可能过来，过来之后还持有这个控件的引用，还需要处理if语句。这个时候handler还持有Activity，但是该Acitivity垃圾回收的时候就无法回收，这个时候就有可能造成内存的泄露

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Java 中强应用和弱引用

package com.studio.countdowntime;

import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

import java.lang.ref.WeakReference;

public class MainActivity extends AppCompatActivity {

/**
 * 倒计时标记handler code
 */
public static final int COUNTDOWN_TIME_CODE = 10001;
/**
 * 倒计时间隔
 */
public static final int DELAY_MILLIS = 1000;
/**
 * 倒计时最大值
 */
public static final int MAX_COUNT = 10;

private TextView countdownTimeTextView;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

//得到控件
        countdownTimeTextView = (TextView) findViewById(R.id.countdownTimeTextView);

//创建了一个handler
        CountdownTileHandler handler = new CountdownTileHandler(this);

//新建了一个message
        Message message = Message.obtain();
        message.what = COUNTDOWN_TIME_CODE;
        message.arg1 = MAX_COUNT;

//第一次发送message
        handler.sendMessageDelayed(message, DELAY_MILLIS);
    }

public static class CountdownTileHandler extends Handler {
public static final int MIN_COUNT = 0;

final WeakReference<MainActivity> mWeakReference;

        CountdownTileHandler(MainActivity activity) {
            mWeakReference = new WeakReference<>(activity);
        }

@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);

            MainActivity mainActivity = mWeakReference.get();

switch (msg.what) {
case COUNTDOWN_TIME_CODE:

int value = msg.arg1;

                    mainActivity.countdownTimeTextView.setText(String.valueOf(value--));

//循环发消息控制
if (value >= MIN_COUNT) {
                        Message message = Message.obtain();
                        message.what = COUNTDOWN_TIME_CODE;
                        message.arg1 = value;
                        sendMessageDelayed(message, DELAY_MILLIS);
                    }
break;
            }
        }
    }
}