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‘
    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。