ViewModel 简单使用详解手机开发

viewmodel：以注重生命周期的方式管理界面相关的数据,为Activity/Fragment 获得以及保留必要信息

通过下面例子引出viewModel，界面如下，界面中三个控件，最上面是个分数，下面两个按钮分别对分数做+1和+3的操作

MainActivity

/** 
 * 普通mvc写法实现 
 */ 
public class MainActivity extends AppCompatActivity {
    
 
    private TextView textView; 
    private Button button1; 
    private Button button2; 
    private ScoreModel scoreModel; 
 
    @Override 
    protected void onCreate(Bundle savedInstanceState) {
    
        super.onCreate(null); 
        setContentView(R.layout.activity_main); 
        textView = findViewById(R.id.textView); 
        button1 = findViewById(R.id.button1); 
        button2 = findViewById(R.id.button2); 
        scoreModel = new ScoreModel(); 
        textView.setText(String.valueOf(scoreModel.getScore())); 
 
        button1.setOnClickListener(new View.OnClickListener() {
    
            @Override 
            public void onClick(View v) {
    
                scoreModel.addScore(1); 
                textView.setText(String.valueOf(scoreModel.getScore())); 
            } 
        }); 
 
        button2.setOnClickListener(new View.OnClickListener() {
    
            @Override 
            public void onClick(View v) {
    
                scoreModel.addScore(2); 
                textView.setText(String.valueOf(scoreModel.getScore())); 
            } 
        }); 
    } 
 
} 
 
 

分数ScoreModel 实体类

/** 
 * 普通实体类 
 */ 
public class ScoreModel {
    
    private int score = 0; 
 
    public int getScore() {
    
        return score; 
    } 
 
    public void addScore(int n) {
    
        this.score += n; 
    } 
} 

布局文件activity_main.xml

<?xml version="1.0" encoding="utf-8"?> 
<androidx.constraintlayout.widget.ConstraintLayout 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" 
    tools:context=".MainActivity"> 
 
    <TextView 
        android:id="@+id/textView" 
        android:layout_width="wrap_content" 
        android:layout_height="32dp" 
        android:text="0" 
        android:textSize="18sp" 
        app:layout_constraintBottom_toBottomOf="parent" 
        app:layout_constraintEnd_toEndOf="parent" 
        app:layout_constraintStart_toStartOf="parent" 
        app:layout_constraintTop_toTopOf="parent" 
        app:layout_constraintVertical_bias="0.21" /> 
 
    <Button 
        android:id="@+id/button2" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:text="+2" 
        app:layout_constraintBottom_toBottomOf="parent" 
        app:layout_constraintEnd_toEndOf="parent" 
        app:layout_constraintHorizontal_bias="0.498" 
        app:layout_constraintStart_toStartOf="parent" 
        app:layout_constraintTop_toTopOf="parent" 
        app:layout_constraintVertical_bias="0.461" /> 
 
    <Button 
        android:id="@+id/button1" 
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content" 
        android:text="+1" 
        app:layout_constraintBottom_toBottomOf="parent" 
        app:layout_constraintEnd_toEndOf="parent" 
        app:layout_constraintHorizontal_bias="0.498" 
        app:layout_constraintStart_toStartOf="parent" 
        app:layout_constraintTop_toTopOf="parent" 
        app:layout_constraintVertical_bias="0.32" /> 
</androidx.constraintlayout.widget.ConstraintLayout> 

运行效果如下

app切换到后台，当内存吃紧、手机为了省电杀掉进程、用户手动手改手机配置（如手机语言）或者app设置，再打开app，这时候之前计算的分数就变成0了；效果如下

之前做界面数据缓存都是在onSaveInstanceState方法中保存，并在onRestoreInstanceState方法或者onCreate方法中进行对bundle判断，有值则回显；有了viewmodel后，就简单多了

新增MyMainModel类继承ViewModel替换刚才的ScoreModel ，代码如下

/** 
 * ViewModel测试类 
 */ 
public class MainViewModel1 extends ViewModel {
    
    private int score = 0; 
 
    public int getScore() {
    
        return score; 
    } 
 
    public void addScore(int n) {
    
        this.score += n; 
    } 
} 
 
 

添加依赖

implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0' 

改后的MainActivity，其他不变

/** 
 * ViewModel测试类 
 */ 
public class MainActivity1 extends AppCompatActivity {
    
 
    private TextView textView; 
    private Button button1; 
    private Button button2; 
    private MainViewModel1 viewModel; 
 
    @Override 
    protected void onCreate(Bundle savedInstanceState) {
    
        super.onCreate(null); 
        setContentView(R.layout.activity_main); 
        textView = findViewById(R.id.textView); 
        button1 = findViewById(R.id.button1); 
        button2 = findViewById(R.id.button2); 
        viewModel = ViewModelProviders.of(this).get(MainViewModel1.class);//新版写法可能不同 
        textView.setText(String.valueOf(viewModel.getScore())); 
 
        button1.setOnClickListener(new View.OnClickListener() {
    
            @Override 
            public void onClick(View v) {
    
                viewModel.addScore(1); 
                textView.setText(String.valueOf(viewModel.getScore())); 
            } 
        }); 
 
        button2.setOnClickListener(new View.OnClickListener() {
    
            @Override 
            public void onClick(View v) {
    
                viewModel.addScore(2); 
                textView.setText(String.valueOf(viewModel.getScore())); 
            } 
        }); 
    } 
 
} 
 
 

运行效果和之前一样，现在尝试下切换系统语言，发现界面数据不会丢失了，因为viewodel帮我们做了处理

仅此记录，如有问题欢迎指出，谢谢