使用handler倒计时

package com.example.jikangwang.myapplication;

import android.content.Intent;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import butterknife.BindView;
import butterknife.ButterKnife;

public class MainActivity extends AppCompatActivity {

    @BindView(R.id.button)
    Button button;

    @BindView(R.id.first_textview)
    TextView textView;

    Handler mHandler;

    volatile boolean flag=true;
    Object object=new Object();
    MThread mThread;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ButterKnife.bind(this);


        textView.setText(String.valueOf(1));

        mHandler=new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                textView.setText(String.valueOf(msg.what));
            }
        };


        mThread=new MThread();
        mThread.start();

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(!mThread.isAlive()){
                    mThread=new MThread();
                    mThread.start();
                    return;
                }
                if(flag==true){
                    flag=!flag;
                }
                else{
                    synchronized (object){
                        flag=!flag;
                        object.notify();
                    }
                }

            }
        });
    }


    public class MThread extends Thread{
        @Override
        public void run() {
            System.out.println("enter thread");
            for(int i=20;i>=0;i--){
                if(flag==false){
                    synchronized (object) {
                        try {
                            object.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
                System.out.println(i);
                mHandler.sendEmptyMessage(i);
                try {
                    sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

        }
    }


}