Android Jetpack -- ViewModel篇（二）

支持SharedPreference等使用到Application的相关

因为 SharedPreference 需要使用到 Application 来获取到，所以要想配合ViewModel还需要传入Application作为参数，当然，Jetpack已经为我们准备好了
AndroidViewModel：感知应用上下文的ViewModel，它继承自ViewModel。

下面以一个非常简单的实例来说明

因为使用dataBinding以及SharedPreference需要在build.grade中添加依赖

         apply plugin: 'com.android.application'
         
         android {
             compileSdkVersion 29
             buildToolsVersion "29.0.2"
             defaultConfig {
                 applicationId "com.example.viewmodelshp"
                 minSdkVersion 19
                 targetSdkVersion 29
                  versionCode 1
                  versionName "1.0"
                  testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
              }
              buildTypes {
                  release {
                      minifyEnabled false
                      proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
                  }
              }
              dataBinding.enabled true
          }
         
          dependencies {
              implementation fileTree(dir: 'libs', include: ['*.jar'])
              implementation 'androidx.appcompat:appcompat:1.0.2'
              implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
              testImplementation 'junit:junit:4.12'
              androidTestImplementation 'androidx.test.ext:junit:1.1.0'
              androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
              implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0-alpha02'//想要使用最新的依赖可以去官网查询
              implementation 'androidx.lifecycle:lifecycle-extensions:2.1.0'
          }

构造MyViewModel.java

        package com.example.viewmodelshp;
        
        import android.app.Application;
        import android.content.Context;
        import android.content.SharedPreferences;
        
        import androidx.annotation.NonNull;
        import androidx.lifecycle.AndroidViewModel;
        import androidx.lifecycle.LiveData;
         import androidx.lifecycle.SavedStateHandle;
        
         public class MyViewModel extends AndroidViewModel {//继承AndroidViewModel类
             private SavedStateHandle handle;
             private String key = getApplication().getResources().getString(R.string.data_key);
             private String shpName = getApplication().getResources().getString(R.string.shp_name);
        
             public MyViewModel(@NonNull Application application, SavedStateHandle handle) {
                 super(application);
                 this.handle = handle;
                 if (!handle.contains(key)) {
                       load();
                   }
             }
        
             public LiveData<Integer> getNumber() {
                 return handle.getLiveData(key);
             }
        
             private void load() {
                 SharedPreferences shp = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
                 int x = shp.getInt(key, 0);
                 handle.set(key, x);
             }
        
             private void save() {
                 SharedPreferences shp = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
                 SharedPreferences.Editor editor = shp.edit();
                 editor.putInt(key, getNumber().getValue());
                 editor.apply();
             }
        
             public void add(int x) {
                 handle.set(key, getNumber().getValue() + x);
                 save();
             }
        
         }

MainActivity.java

 package com.example.viewmodelshp;
 
 import android.os.Bundle;
 
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.databinding.DataBindingUtil;
 import androidx.lifecycle.SavedStateViewModelFactory;
 import androidx.lifecycle.ViewModelProviders;
 
 import com.example.viewmodelshp.databinding.ActivityMainBinding;
 
 public class MainActivity extends AppCompatActivity {
     MyViewModel myViewModel;
     ActivityMainBinding binding;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
         ViewModelProviders.of(this, new SavedStateViewModelFactory(this)).get(MyViewModel.class);
         binding.setData(myViewModel);
         binding.setLifecycleOwner(this);
     }
 }

布局如下

 <?xml version="1.0" encoding="utf-8"?>
 <layout 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">
 
     <data>
         <variable
             name="data"
             type="com.example.viewmodelshp.MyViewModel" />
     </data>
 
     <androidx.constraintlayout.widget.ConstraintLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         tools:context=".MainActivity">
 
         <TextView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginLeft="8dp"
             android:layout_marginTop="8dp"
             android:layout_marginRight="8dp"
             android:layout_marginBottom="8dp"
             android:text="@{String.valueOf(data.getNumber)}"
             android:textSize="40sp"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintLeft_toLeftOf="parent"
             app:layout_constraintRight_toRightOf="parent"
             app:layout_constraintTop_toTopOf="parent"
             app:layout_constraintVertical_bias="0.35"
             tools:text="100" />
 
         <Button
             android:id="@+id/button"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginStart="8dp"
             android:layout_marginTop="8dp"
             android:layout_marginEnd="8dp"
             android:layout_marginBottom="8dp"
             android:onClick="@{()->data.add(1)}"
             android:text="@string/button_plus"
             android:textSize="18sp"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintHorizontal_bias="0.2"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toTopOf="parent"
             app:layout_constraintVertical_bias="0.5" />
 
         <Button
             android:id="@+id/button2"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginStart="8dp"
             android:layout_marginTop="8dp"
             android:layout_marginEnd="8dp"
             android:layout_marginBottom="8dp"
             android:onClick="@{()->data.add(-1)}"
             android:text="@string/button_mius"
             android:textSize="18sp"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintHorizontal_bias="0.8"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toTopOf="parent"
             app:layout_constraintVertical_bias="0.5" />
 
     </androidx.constraintlayout.widget.ConstraintLayout>
 </layout>

在这个例子里通过获取SharedPreference来存储数据，再配合LiveData，SaveStataHandler实现跨越整个App的生命周期同步。