ViewModel与SharedPreferences
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ViewModel与SharedPreferences的关系
在学习了ViewModel之后,我们知道了其功能是最大限度的保存系统的数据状态(系统配置改变,后台无声杀死程序的情况下都能保存数据),然而在系统主动退出或者是系统重新启动时,由于ViewModel也被重置,数据不再保留造成数据丢失。正因如此,引入SharedPreferences来进行数据的永久保存,ViewModel的数据也得到恢复,界面上的数据得到恢复。
使用SharedPreferences需要使用Context来指向应用程序
- 添加依赖
android {
...
dataBinding.enabled true
}
dependencies {
...
implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:1.0.0'
}
Sync
2. 要想使创建的MyViewModel类访问全局,可以使用Application或者Context(两者是继承关系)再在controller中实现传递
package com.example.viewmodelshp;
import android.app.Application;
import android.content.Context;
import androidx.lifecycle.ViewModel;
public class MyViewModel extends ViewModel {
//Application application;//或者用Context传递参数
Context context;
public void fun(){
context.getSharedPreferences();//实现对系统的数据进行访问
}
}
package com.example.viewmodelshp;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
public class MainActivity extends AppCompatActivity {
MyViewModel myViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//myViewModel.application = getApplication();//传递过去所需参数,或者用Context
myViewModel.context = getApplicationContext();
}
}
- 也可以使用简便方法:利用AndroidViewModel
将ViewModel的继承类由ViewModel改变为AndroidViewModel,其为ViewModel的一个子类,好处在于内部有直接对Context访问的工具
对于缺省的参数根据代码提示创建即可:
并在MyViewModel中带入第二个参数savedStateHandle
public MyViewModel(@NonNull Application application, SavedStateHandle handle) {
super(application);
this.handle = handle;
if (!handle.contains(key)) {
load();
}
}
完善代码以便于完成数据的更新操作:
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 {
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() { //声明为共有类用以xml中的使用
return handle.getLiveData(key);
}
private void load() {
SharedPreferences sharedPreferences = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
int x = sharedPreferences.getInt(key, 0);
handle.set(key, x);//读取
}
void save() {
SharedPreferences sharedPreferences = getApplication().getSharedPreferences(shpName, Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putInt(key, getNumber().getValue() == null ? 0 : getNumber().getValue());//warning中说没判空,但是逻辑中我们已经会显示为0。消除警告方法:
editor.apply();
}
public void add(int x) {
handle.set(key, getNumber().getValue() == null ? 0 : getNumber().getValue() + x);
//保存法一,但是保存开销大占用内存极高
//save();
}
}
MainActivity中添加代码:
package com.example.viewmodelshp;
import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.SavedStateViewModelFactory;
import androidx.lifecycle.ViewModelProvider;
import android.os.Bundle;
import com.example.viewmodelshp.databinding.ActivityMainBinding;
public class MainActivity extends AppCompatActivity {
MyViewModel myViewModel;
ActivityMainBinding binding;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
myViewModel = new ViewModelProvider(this, new SavedStateViewModelFactory(getApplication(), this)).get(MyViewModel.class);
binding.setData(myViewModel);
binding.setLifecycleOwner(this);
}
//法二,在onPause()中保存
@Override
protected void onPause() {
super.onPause();
myViewModel.save();
}
}
更改xml格式为databinding layout
<?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:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{String.valueOf(data.getNumber())}"
android:textSize="36sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.285"
tools:text="100" />
<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_plus"
android:onClick="@{()->data.add(1)}"
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="0.6" />
<Button
android:id="@+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_minus"
android:onClick="@{()->data.add(-1)}"
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="0.6" />
</androidx.constraintlayout.widget.ConstraintLayout>
</layout>
运行后能对数据进行变动,后台杀死与重启设备依然能够保存: