android studio 55线程更新UI handler

第一步activity-main.xml
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        android:id="@+id/mTxtShowTest"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/mBtn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        tools:layout_editor_absoluteX="177dp"
        tools:layout_editor_absoluteY="595dp" /

第二步 MainActivity.java
package com.example.myhandler2;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    TextView mTxtShowTest;
    Button mBtn;
    //1、在主线程中通过匿名内部类创建Handler类对象
    private Handler mhandler2 =  new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            //执行的UI操作
            if (msg.what==1){
                mTxtShowTest.setText("匿名内部类方法");
                            }
        }
    };

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

        mTxtShowTest = (TextView) findViewById(R.id.mTxtShowTest);
        mBtn = (Button) findViewById(R.id.mBtn2);

        mBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        // 2、创建所需消息对象
                        Message msg = Message.obtain();
                        msg.what = 1;

                        //3、发送消息
                        mhandler2.sendMessage(msg);
                    }
                    //4、开启线程
                }).start();
            }
        });
    }
}