Android--利用Handler消息转发机制实现倒计时（内含防止内存泄露处理）

涉及到的知识点：

1.Handler常用API

2.WeakReference：主线程上下文对象的弱引用

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1.Handler常用API

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代码实现：

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

import java.lang.ref.WeakReference;

public class MainActivity extends AppCompatActivity {

/**
 * 倒计时标记handle code
 */
public  static  final  int COUNTDOWN_TIME_CODE = 100001;
/**
 * 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 mCountdownTimeTextView;


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

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

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

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

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

    }

/*防止内存泄露，弱引用的方式获取上下文
 * 多用于耗时线程中的操作*/
public  static  class CountdownTimeHandler extends Handler{
static  final  int  MIN_COUNT = 0;
final WeakReference<MainActivity> mWeakReference;

        CountdownTimeHandler(MainActivity activity){
            mWeakReference = new WeakReference (activity );
        }

@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//获取MainActivity
            MainActivity activity =mWeakReference.get();

switch (msg.what){
case COUNTDOWN_TIME_CODE:
int value = msg.arg1;
                    activity.mCountdownTimeTextView.setText(String.valueOf(value --));

//循环发的消息控制
if (value >= MIN_COUNT) {
/*这时需要新建Message对象*/
                        Message message = Message.obtain();
                        message.what = COUNTDOWN_TIME_CODE;
                        message.arg1 = value;
                        sendMessageDelayed(message, DELAY_MILLIS);
                    }

break;
            }
        }
    }
}