Android Studio 之 LiveData 的配合使用，底层数据变化，自动通知界面

viewModelWithLikeData.getLikeNumber().observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                textView.setText(String.valueOf(integer));
            }
        });

package com.example.livedatatest;

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class ViewModelWithLikeData extends ViewModel {
    private MutableLiveData<Integer> likeNumber;

    //用构造函数，防止访问到的内容是空的，不能这样用，不知道为什么，会出错
    /*
    ViewModelWithLikeData(){
        likeNumber = new MutableLiveData<>();
        likeNumber.setValue(0);
    }
   */

    public MutableLiveData<Integer> getLikeNumber() {
        if(likeNumber==null) {
            likeNumber = new MutableLiveData<>();
            likeNumber.setValue(0);
        }
        return likeNumber;
    }

    public void setLikeNumber(MutableLiveData<Integer> likeNumber) {
        this.likeNumber = likeNumber;
    }

    public void addLikeNumber(int n) {
        likeNumber.setValue(likeNumber.getValue()+n);
    }
}

package com.example.livedatatest;

import android.os.Bundle;
import android.view.View;
import android.widget.ImageButton;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;



public class MainActivity extends AppCompatActivity {

    ViewModelWithLikeData viewModelWithLikeData;
    TextView textView;
    ImageButton imageButton1;
    ImageButton imageButton2;

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

        textView = findViewById(R.id.textView);
        imageButton1 = findViewById(R.id.imageButton);
        imageButton2 = findViewById(R.id.imageButton2 );

        //注意： ViewModelProviders 必须在 build.gradle 中引入如下语句，否则导入不了类
        //implementation ‘androidx.lifecycle:lifecycle-extensions:2.1.0‘
        viewModelWithLikeData = ViewModelProviders.of(this).get(ViewModelWithLikeData.class);

        //设置一个监听器，当这个值有变化时，自动给控件赋值。

        viewModelWithLikeData.getLikeNumber().observe(this, new Observer<Integer>() {
            @Override
            public void onChanged(Integer integer) {
                textView.setText(String.valueOf(integer));
            }
        });

        imageButton1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                viewModelWithLikeData.addLikeNumber(1);  //点赞加1
            }
        });

        imageButton2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                viewModelWithLikeData.addLikeNumber(-1); //踩-1
            }
        });

    }
}