Android-Message更新UI

运行截图：

布局文件： 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:gravity="center"
        android:text="TextView"
        android:textSize="25sp" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="vertical">

        <Button
            android:id="@+id/btn1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="子线程直接更新UI会崩溃" />

        <Button
            android:id="@+id/btn2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="主线程通过异步消息更新UI" />

    </LinearLayout>
</LinearLayout>

主类：

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private static final int UPDATE_TEXT = 1;

    private TextView textView;
    private Button button1;
    private Button button2;

    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case UPDATE_TEXT:
                    textView.setText("更新后的TextView");
                    break;
            }
        }
    };

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

        init();
    }

    private void init() {
        textView = findViewById(R.id.textView);
        button1 = findViewById(R.id.btn1);
        button2 = findViewById(R.id.btn2);
        button1.setOnClickListener(this);
        button2.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn1:
//                子线程直接更新UI程序会崩溃
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        textView.setText("更新后的TextView");
                    }
                }).start();
                break;
            case R.id.btn2:
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        Message message = new Message();
                        message.what = UPDATE_TEXT;
                        handler.sendMessage(message);
                    }
                }).start();
                break;
        }
    }
}