Android JetPack组件之LiveData的使用详解

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".LiveDataTestActivity">
    <TextView
        android:id="@+id/tv_text"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginTop="100dp"
        android:gravity="center"
        android:textSize="30sp" />
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="200dp"
        android:gravity="center"
        android:orientation="horizontal">
        <ImageButton
            android:id="@+id/img_button_add"
            android:layout_width="70dp"
            android:layout_height="70dp"
            android:layout_marginRight="20dp"
            app:srcCompat="@drawable/ic_sharp_thumb_up_24" />
        <ImageButton
            android:id="@+id/img_button_subtract"
            android:layout_width="70dp"
            android:layout_height="70dp"
            android:layout_marginLeft="20dp"
            app:srcCompat="@drawable/ic_baseline_thumb_down_24" />
    </LinearLayout>
</LinearLayout>

public class ViewModelWithLiveData extends ViewModel {
    //声明一个变量 MutableLiveData是容器 Integer是变量的类型
    private MutableLiveData<Integer> LikedNumber;
    public MutableLiveData<Integer> getLikedNumber() {
        if (LikedNumber == null) {
            //LikedNumber是对象类型，不是基本数据类型，所以要保证变量不是空的
            LikedNumber = new MutableLiveData<>();
            //初始化为0
            LikedNumber.setValue(0);
        }
        return LikedNumber;
    }
    public void addLikedNumber(int n) {
        LikedNumber.setValue(LikedNumber.getValue() + n);
    }
}

public class LiveDataTestActivity extends AppCompatActivity {
    private ViewModelWithLiveData viewModelWithLiveData;
    private TextView tv_text;
    private ImageButton img_button_add;
    private ImageButton img_button_subtract;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_live_data_test);
        tv_text = findViewById(R.id.tv_text);
        img_button_add = findViewById(R.id.img_button_add);
        img_button_subtract = findViewById(R.id.img_button_subtract);
        viewModelWithLiveData = new ViewModelProvider(this).get(ViewModelWithLiveData.class);
        //给viewModelWithLiveData里面的变量添加一个观察 观察它自己 如果数据发生变化，则呼叫下面的函数
        //observe()的第一个参数：需要具有LifeCycle管理功能的一些对象 Activity就是具有管理LifeCycle功能的对象
        //系统已经帮我们做好了，不用在其他地方取消观察 只需要添加观察就可以了。
        viewModelWithLiveData.getLikedNumber().observe(this, new Observer<Integer>() {
            //当数据发生改变的时候 呼叫这个函数
            @Override
            public void onChanged(Integer integer) {
                tv_text.setText(String.valueOf(integer));
            }
        });
        img_button_add.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                viewModelWithLiveData.addLikedNumber(1);
            }
        });
        img_button_subtract.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                viewModelWithLiveData.addLikedNumber(-1);
            }
        });
    }
}