Android Jetpack LiveData

向右看齐 2023-06-25 10:28 26阅读 0赞

# 当数据改变时自动刷新界面 #

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### LiveData ###

数据变动时，可加观察者，自动刷新数据，不再需要references(指针)刷新界面，简化Controller

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实现效果：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDU4MDEzOQ_size_16_color_FFFFFF_t_70 3]

### 1.使用矢量图 ###

ImageButton支持矢量图，res目录下，new Vector Asset完成后在drawable下会多个xml文件  
使用Vector必须在build.gradle(Module:app)文件中defaultConfig下设置：

android.defaultConfig.vectorDrawables.useSupportLibrary = true

其次设置ImageButton的ContentDescription属性

### 2.新建ViewModel ###

1.  声明数据类型 private MutableLiveData LikedNumber 它是一个对象类型 所以要保证不为空，通过构造方法或是get前先判断。  
    构造方法

MyViewModelWithLiveData(){ 
            LikedNumber=new MutableLiveData<>();
            LikedNumber.setValue(0);
        }

get前先判断

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

这里采用后者

1.  添加add方法

public void addLikedNumber(int n){ 
            LikedNumber.setValue(LikedNumber.getValue()+n);
        }

自此，ViewModel写完，写activity

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### 各种关联 ###

**重点，以内部匿名类的方式给getLikedNumber()返回的MutableLiveData添加观察，当其变化时，刷新界面**

// getLikedNumber()返回 MutableLiveData<Integer> 观察其变化 就只需写一句set方法了
            myViewModelWithLiveData.getLikedNumber().observe(this, new Observer<Integer>() { 
                @Override
                public void onChanged(Integer integer) { 
                    textView.setText(String.valueOf(integer));
                }
            });

完整代码：

package com.example.livedatatest2;
    
    import androidx.appcompat.app.AppCompatActivity;
    import androidx.lifecycle.Observer;
    import androidx.lifecycle.ViewModelProviders;
    
    import android.os.Bundle;
    import android.view.View;
    import android.widget.ImageButton;
    import android.widget.TextView;
    
    public class MainActivity extends AppCompatActivity { 
        MyViewModelWithLiveData myViewModelWithLiveData;
        ImageButton imageButtonLike,imageButtonDisLike;
        TextView textView;
        @Override
        protected void onCreate(Bundle savedInstanceState) { 
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            textView=findViewById(R.id.textView);
            imageButtonLike=findViewById(R.id.imageButton);
            imageButtonDisLike=findViewById(R.id.imageButton2);
            myViewModelWithLiveData=ViewModelProviders.of(this).get(MyViewModelWithLiveData.class);
            // getLikedNumber()返回 MutableLiveData<Integer> 观察其变化 就只需写一句set方法了
            myViewModelWithLiveData.getLikedNumber().observe(this, new Observer<Integer>() { 
                @Override
                public void onChanged(Integer integer) { 
                    textView.setText(String.valueOf(integer));
                }
            });
            // 单纯让LikedNumber变化
            imageButtonLike.setOnClickListener(new View.OnClickListener() { 
                @Override
                public void onClick(View v) { 
                    myViewModelWithLiveData.addLikedNumber(1);
                }
            });
            imageButtonDisLike.setOnClickListener(new View.OnClickListener() { 
                @Override
                public void onClick(View v) { 
                    myViewModelWithLiveData.addLikedNumber(-1);
                }
            });
        }
    }

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区别就是在ViewModel中使用了LiveData,就可以用observe观察，就只需在观察中写一句set方法。

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