什么是Handler

public class MainActivity extends AppCompatActivity {

    private TextView text;

    private static final int UPDATE_TEXT = 1;  //整型常量用于表示更新TextView这个动作

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.id.activity_main);
        text = (TextView) findViewById(R.id.text); 
        Button chaneText = (Button) findViewById(R.id.change_text);
        change_text.setOnClickListener(this);
    }

    //新增Handler对象，并重写父类的handlerMessage()方法，在这里对具体的Message进行处理
    private Handler handler = new Handler(){  
        public void handleMessage(Message msg){  //收到消息,在HandleMessage()方法中对消息进行处理
            switch (msg.what){
                case UPDATE_TEXT:
                // 在这里进行UI操作,处于主线程中
                    text.setText("Nice to meet you");
                    break;
                default:
                    break;
            }
        }
    };

    @Override
    public void onClick(View v){
        switch(v.getId()){
            case R.id.change_text:
                new Thread(new Runnable(){
                    @Override
                    public void run(){
                        Message message = new Message();
                        message.what = UPDATE_TEXT;    //将waht字段的值指定为UPDATE_TEXT
                        handler.sendMessage(message);    //调用Handler的sendMessage()方法发送消息
                    }
                }).start();
                break;
            default:
                break;
        }
    }
}

public class ThreadActivity extends AppCompatActivity implements View.OnClickListener {
    private ActivityThreadBinding mBinding = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBinding = DataBindingUtil.setContentView(this, R.layout.activity_thread);
        // 设置点击事件
        mBinding.clickBtn.setOnClickListener(this);
        mBinding.resetBtn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            // 响应load按钮
            case R.id.clickBtn:
                // 开启一个线程
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        // 在Runnable中进行网络读取操作，返回bitmap
                        final Bitmap bitmap = loadPicFromInternet();
                        // 在子线程中实例化Handler同样是可以的，只要在构造函数的参数中传入主线程的Looper即可
                        Handler handler = new Handler(Looper.getMainLooper());
                        // 通过Handler的post Runnable到UI线程的MessageQueue中去即可
                        handler.post(new Runnable() {
                            @Override
                            public void run() {
                                // 在MessageQueue出队该Runnable时进行的操作
                                mBinding.photo.setImageBitmap(bitmap);
                            }
                        });
                    }
                }).start();
                break;
            case R.id.resetBtn:
                mBinding.photo.setImageBitmap(BitmapFactory.decodeResource(getResources(), R.mipmap.default_pic));
                break;
        }
    }

    /***
     * HttpUrlConnection加载图片，不多说
     * @return
     */
    public Bitmap loadPicFromInternet() {
        Bitmap bitmap = null;
        int respondCode = 0;
        InputStream is = null;
        try {
            URL url = new URL("https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=1421494343,3838991329&fm=23&gp=0.jpg");
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("GET");
            connection.setConnectTimeout(10 * 1000);
            connection.setReadTimeout(5 * 1000);
            connection.connect();
            respondCode = connection.getResponseCode();
            if (respondCode == 200) {
                is = connection.getInputStream();
                bitmap = BitmapFactory.decodeStream(is);
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
            Toast.makeText(getApplicationContext(), "访问失败", Toast.LENGTH_SHORT).show();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return bitmap;
    }
}