Handler(消息机制)

Demo演示

//通过Handler事件倒计时的一个操作，并判断状态

public class MainActivity extends AppCompatActivity {
private TextView mTextView;
    private Button mButton,mButton2;
    private final static int STATE_START = 100;
    private final static int STATE_RUN = 101;
    private final static int STATE_END = 102;
    //创建Handler对象
    private Handler mHandler = new Handler() {
        public void handleMessage(Message msg){
            switch (msg.what){
                case STATE_START:
                    mTextView.setText("倒计时开始");
                    break;
                case  STATE_RUN:
                    mTextView.setText(String.valueOf(msg.arg1));
                    break;
                case STATE_END:
                    mTextView.setText("倒计时结束");
                    break;
            }
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        mButton = (Button)findViewById(R.id.button);
             mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyThread myThread = new MyThread(10);
                myThread.start();
            }
        });
    }
    class MyThread extends Thread{
        int startnum;
        MyThread(int num){
            startnum = num ;

        }

        @Override
        public void run() {
            super.run();
            Message startMsg = mHandler.obtainMessage();
            startMsg.what=STATE_START;
            mHandler.sendMessage(startMsg);
            while (startnum >=0){
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Message runMsg = mHandler.obtainMessage();
                runMsg.what =STATE_RUN;
                runMsg.arg1 = startnum;
                mHandler.sendMessage(runMsg);
                startnum--;

            }
            Message endMsg = mHandler.obtainMessage();
            endMsg.what = STATE_END;
            mHandler.sendMessage(endMsg);
        }
    }
}

//演示post()  postAtTime()  postdelayed()

public class NextActivity extends AppCompatActivity implements View.OnClickListener {
    private TextView mTextView;
    private Button button_post, button_postattime, button_postdelayed;
    private Handler mHandler = new Handler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_next);
        mTextView = (TextView) findViewById(R.id.textview_next);
        button_post = (Button) findViewById(R.id.button_next_post);
        button_postattime = (Button) findViewById(R.id.button_next_postattime);
        button_postdelayed = (Button) findViewById(R.id.button_next_postdelayed);
        button_postdelayed.setOnClickListener(this);
        button_post.setOnClickListener(this);
        button_postattime.setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.button_next_post:
                mHandler.post(new Runnable() {
                    @Override
                    public void run() {
                        mTextView.setText("立即执行");
                    }
                });
                break;
            case R.id.button_next_postattime:
                mHandler.postAtTime(new Runnable() {
                    @Override
                    public void run() {
                        mTextView.setText("指定时间执行");
                    }
                },android.os.SystemClock.uptimeMillis() +2000);    //PostAtTime的时间使用的是uptimeMillis（）不是currentTimeMillis（）
                break;
            case R.id.button_next_postdelayed:
                mHandler.postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        mTextView.setText("延迟时间执行");
                    }
                }, 2000);
                break;
        }
    }
}