Android更新UI的几种方法

第一种场景：

在UI线程中更新UI，这种是最简单的，直接更新UI即可。
代码如下

public class MainActivity extends AppCompatActivity {

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                tv_text.setText("111111111111111");
            }
        });
    }
}

第二种场景：

从子线程中更新UI

代码如下

public class MainActivity extends AppCompatActivity {

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        tv_text.setText("111111111111111");
                    }
                });
                thread.start();
            }
        });
    }
}

当点击按钮更新UI的时候就会发现报了异常，异常如下

这个异常证明了子线程不能直接更新UI，解决方案如下

（1）通过Activity中的runOnUIThread方法

public class MainActivity extends AppCompatActivity {

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {

                        runOnUiThread(new Runnable() {
                            @Override
                            public void run() {
                                tv_text.setText("111111111111111");
                            }
                        });

                    }
                });
                thread.start();
            }
        });
    }
}

我们来深入源码

/**
 * Runs the specified action on the UI thread. If the current thread is the UI
 * thread, then the action is executed immediately. If the current thread is
 * not the UI thread, the action is posted to the event queue of the UI thread.
 *
 * @param action the action to run on the UI thread
 */
public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

源码的意思是说， 如果当前线程不是UI线程， 那么执行

mHandler.post(action);

否则直接执行run。

这个结论直接告诉了我们，Handler的post方法也能做到从子线程更新UI。

（2）通过Handler的post方法

public class MainActivity extends AppCompatActivity {

    private Handler handler = new Handler();

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        handler.post(new Runnable() {
                            @Override
                            public void run() {
                                tv_text.setText("111111111111111");
                            }
                        });
                    }
                });
                thread.start();
            }
        });
    }
}

我在UI线程中new了一个Handler对象，在子线程中用这个对象来调用post方法。

我们来深入源码

/**
 * Causes the Runnable r to be added to the message queue.
 * The runnable will be run on the thread to which this handler is 
 * attached. 
 *  
 * @param r The Runnable that will be executed.
 * 
 * @return Returns true if the Runnable was successfully placed in to the 
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */
public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

在Handler对象中，有一个post方法，分析源码得知， 这个方法将形参r封装到一个消息里面， 再利用sendMessageDelayed方法将消息发送（添加）到消息队列。（注：理解这句话需要对Handler机制有一定的了解）

我们得出结论，通过handler发送消息也能实现子线程更新UI。

（3）通过handler发送消息来实现子线程更新UI

public class MainActivity extends AppCompatActivity {

    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what){
                case 1:
                    tv_text.setText("111111111111111");
                    break;
            }
        }
    };

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {

                        Message message = Message.obtain();
                        message.what = 1;
                        handler.sendMessage(message);

                    }
                });
                thread.start();
            }
        });
    }
}

（4）通过view的post方法实现

public class MainActivity extends AppCompatActivity {

    private Button bt_click_me;
    private TextView tv_text;

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

        bt_click_me = findViewById(R.id.bt_click_me);
        tv_text = findViewById(R.id.tv_text);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {

                        bt_click_me.post(new Runnable() {
                            @Override
                            public void run() {
                                tv_text.setText("111111111111111");
                            }
                        });

                    }
                });
                thread.start();
            }
        });
    }
}

我们来深入源码

/**
 * <p>Causes the Runnable to be added to the message queue.
 * The runnable will be run on the user interface thread.</p>
 *
 * @param action The Runnable that will be executed.
 *
 * @return Returns true if the Runnable was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 *
 * @see #postDelayed
 * @see #removeCallbacks
 */
public boolean post(Runnable action) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.post(action);
    }

    // Postpone the runnable until we know on which thread it needs to run.
    // Assume that the runnable will be successfully placed after attach.
    getRunQueue().post(action);
    return true;
}

其实最终也调用了mHandler.post(action)方法。

第二种场景总结：

（1）Android从子线程更新UI就是通过Handler来实现的，官方发明Handler主要就是给我们更新UI用的。

其实吧， 一些脑洞大开的猿类动物偏不按照常理出牌：

（1）在子线程中他偏偏不用Handler更新UI？

public class MainActivity extends AppCompatActivity {

    private TextView tv_text;

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

        tv_text = findViewById(R.id.tv_text);
        tv_text.setText("1111111");
        Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        for (int i=0;i<500000;i++){
                            Log.e("aa", String.valueOf(i));//耗时操作
                            if(i==499999){
                                tv_text.setText("22222222");
                            }
                        }
                    }
                });
        thread.start();
    }
}

这个例子是从onCreate方法中的子线程更新UI， 其中有耗时操作

上述的例子依然报错， 那么怎么才能不让他报错呢，往下看

public class MainActivity extends AppCompatActivity {

    private TextView tv_text;

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

        tv_text = findViewById(R.id.tv_text);
        tv_text.setText("1111111");
        Thread thread = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        tv_text.setText("22222222");
                    }
                });
        thread.start();
    }
}

当我去除耗时操作时，就不会报这个错误了，那么为什么呢？

我们来翻看源码

在ViewRootImpl类中找到了这个方法，这个方法就是之所以报错的根本

void checkThread() {
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
                "Only the original thread that created a view hierarchy can touch its views.");
    }
}

而ViewRootImpl对象是在执行到onResume才创建时的，所以得出结论，onCreate中的子线程如果不是耗时操作，基本都是可以更新UI的，但不能保证。因为一个是UI线程，一个是子线程，我们谁也不知道哪个线程更快一些。

（2）把消息从UI线程发送到子线程？

public class MainActivity extends AppCompatActivity {

    private Button bt_click_me;
    private Handler handler;

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

        bt_click_me = findViewById(R.id.bt_click_me);

        bt_click_me.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Message message = Message.obtain();
                message.what = 1;
                handler.sendMessage(message);
            }
        });

        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                Looper looper = Looper.myLooper();
                handler = new Handler(looper){
                    @Override
                    public void handleMessage(Message msg) {
                        super.handleMessage(msg);
                        Log.d("aa", "11111");
                    }
                };
                Looper.loop();
            }
        });
        thread.start();
    }
}

UI线程本身就有Looper，但是子线程是没有Looper的，所以必须新建Looper来轮询Looper中的消息队列。