Android Jetpack -- ViewModel篇

Posted 2021-03-15 嘉禾旧木

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

 1 apply plugin: ‘com.android.application‘
 2 
 3 android {
 4     compileSdkVersion 29
 5     buildToolsVersion "29.0.2"
 6     defaultConfig {
 7         applicationId "com.example.viewmodelshp"
 8         minSdkVersion 19
 9         targetSdkVersion 29
10         versionCode 1
11         versionName "1.0"
12         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
13     }
14     buildTypes {
15         release {
16             minifyEnabled false
17             proguardFiles getDefaultProguardFile(‘proguard-android-optimize.txt‘), ‘proguard-rules.pro‘
18         }
19     }
20     dataBinding.enabled true
21 }
22 
23 dependencies {
24     implementation fileTree(dir: ‘libs‘, include: [‘*.jar‘])
25     implementation ‘androidx.appcompat:appcompat:1.0.2‘
26     implementation ‘androidx.constraintlayout:constraintlayout:1.1.3‘
27     testImplementation ‘junit:junit:4.12‘
28     androidTestImplementation ‘androidx.test.ext:junit:1.1.0‘
29     androidTestImplementation ‘androidx.test.espresso:espresso-core:3.1.1‘
30     implementation ‘androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0-alpha02‘
31     implementation ‘androidx.lifecycle:lifecycle-extensions:2.1.0‘
32 }

构造MyViewModel.java

 1 package com.example.viewmodelshp;
 2 
 3 import android.app.Application;
 4 import android.content.Context;
 5 import android.content.SharedPreferences;
 6 
 7 import androidx.annotation.NonNull;
 8 import androidx.lifecycle.AndroidViewModel;
 9 import androidx.lifecycle.LiveData;
10 import androidx.lifecycle.SavedStateHandle;
11 
12 public class MyViewModel extends AndroidViewModel {//继承AndroidViewModel类
13     private SavedStateHandle handle;
14     private String key = getApplication().getResources().getString(R.string.data_key);
15     private String shpName = getApplication().getResources().getString(R.string.shp_name);
16 
17     public MyViewModel(@NonNull Application application, SavedStateHandle handle) {
18         super(application);
19         this.handle = handle;
20         if (!handle.contains(key)) {
21             load();
22         }
23     }
24 
25     public LiveData<Integer> getNumber() {
26         return handle.getLiveData(key);
27     }
28 
29     private void load() {
30         SharedPreferences shp = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
31         int x = shp.getInt(key, 0);
32         handle.set(key, x);
33     }
34 
35     private void save() {
36         SharedPreferences shp = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
37         SharedPreferences.Editor editor = shp.edit();
38         editor.putInt(key, getNumber().getValue());
39         editor.apply();
40     }
41 
42     public void add(int x) {
43         handle.set(key, getNumber().getValue() + x);
44         save();
45     }
46 
47 }

MainActivity.java

 1 package com.example.viewmodelshp;
 2 
 3 import android.os.Bundle;
 4 
 5 import androidx.appcompat.app.AppCompatActivity;
 6 import androidx.databinding.DataBindingUtil;
 7 import androidx.lifecycle.SavedStateViewModelFactory;
 8 import androidx.lifecycle.ViewModelProviders;
 9 
10 import com.example.viewmodelshp.databinding.ActivityMainBinding;
11 
12 public class MainActivity extends AppCompatActivity {
13     MyViewModel myViewModel;
14     ActivityMainBinding binding;
15 
16     @Override
17     protected void onCreate(Bundle savedInstanceState) {
18         super.onCreate(savedInstanceState);
19         setContentView(R.layout.activity_main);
20         binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
21         ViewModelProviders.of(this, new SavedStateViewModelFactory(this)).get(MyViewModel.class);
22         binding.setData(myViewModel);
23         binding.setLifecycleOwner(this);
24     }
25 }

布局如下

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <layout xmlns:android="http://schemas.android.com/apk/res/android"
 3     xmlns:app="http://schemas.android.com/apk/res-auto"
 4     xmlns:tools="http://schemas.android.com/tools">
 5 
 6     <data>
 7         <variable
 8             name="data"
 9             type="com.example.viewmodelshp.MyViewModel" />
10     </data>
11 
12     <androidx.constraintlayout.widget.ConstraintLayout
13         android:layout_width="match_parent"
14         android:layout_height="match_parent"
15         tools:context=".MainActivity">
16 
17         <TextView
18             android:layout_width="wrap_content"
19             android:layout_height="wrap_content"
20             android:layout_marginLeft="8dp"
21             android:layout_marginTop="8dp"
22             android:layout_marginRight="8dp"
23             android:layout_marginBottom="8dp"
24             android:text="@{String.valueOf(data.getNumber)}"
25             android:textSize="40sp"
26             app:layout_constraintBottom_toBottomOf="parent"
27             app:layout_constraintLeft_toLeftOf="parent"
28             app:layout_constraintRight_toRightOf="parent"
29             app:layout_constraintTop_toTopOf="parent"
30             app:layout_constraintVertical_bias="0.35"
31             tools:text="100" />
32 
33         <Button
34             android:id="@+id/button"
35             android:layout_width="wrap_content"
36             android:layout_height="wrap_content"
37             android:layout_marginStart="8dp"
38             android:layout_marginTop="8dp"
39             android:layout_marginEnd="8dp"
40             android:layout_marginBottom="8dp"
41             android:onClick="@{()->data.add(1)}"
42             android:text="@string/button_plus"
43             android:textSize="18sp"
44             app:layout_constraintBottom_toBottomOf="parent"
45             app:layout_constraintEnd_toEndOf="parent"
46             app:layout_constraintHorizontal_bias="0.2"
47             app:layout_constraintStart_toStartOf="parent"
48             app:layout_constraintTop_toTopOf="parent"
49             app:layout_constraintVertical_bias="0.5" />
50 
51         <Button
52             android:id="@+id/button2"
53             android:layout_width="wrap_content"
54             android:layout_height="wrap_content"
55             android:layout_marginStart="8dp"
56             android:layout_marginTop="8dp"
57             android:layout_marginEnd="8dp"
58             android:layout_marginBottom="8dp"
59             android:onClick="@{()->data.add(-1)}"
60             android:text="@string/button_mius"
61             android:textSize="18sp"
62             app:layout_constraintBottom_toBottomOf="parent"
63             app:layout_constraintEnd_toEndOf="parent"
64             app:layout_constraintHorizontal_bias="0.8"
65             app:layout_constraintStart_toStartOf="parent"
66             app:layout_constraintTop_toTopOf="parent"
67             app:layout_constraintVertical_bias="0.5" />
68 
69     </androidx.constraintlayout.widget.ConstraintLayout>
70 </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。