LiveData

ViewModel

• 添加依赖

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

• activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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">

    <Button
        android:id="@+id/btn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/btn1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.501"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.327" />

    <Button
        android:id="@+id/btn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/btn2"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.501"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.499" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        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="0.151" />

    <ImageButton
        android:id="@+id/imageButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.154"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.4"
        app:srcCompat="@android:drawable/btn_star_big_off" />

    <ImageButton
        android:id="@+id/imageButton2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.853"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.4"
        app:srcCompat="@android:drawable/btn_star_big_on" />

</LinearLayout>

• MyViewModel.java

package com.example.myviewmodel;

import androidx.lifecycle.ViewModel;

public class MyViewModel extends ViewModel {
    public int count;
}

• MainActivity.java

package com.example.myviewmodel;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelProviders;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private Button btn1, btn2;
    private TextView tv;

    MyViewModel myViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);

        btn1 = findViewById(R.id.btn1);
        btn2 = findViewById(R.id.btn2);
        tv = findViewById(R.id.textView);

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                myViewModel.count++;
                tv.setText(String.valueOf(myViewModel.count));
            }
        });

        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                myViewModel.count += 2;
                tv.setText(String.valueOf(myViewModel.count));
            }
        });

    }
}

保存状态

使viewmodel存活到系统配置改变，后台进程被杀死。但系统重新启动后，viewmodel也会重新创建，数据就没了。

• 打开虚拟机开发者模式，在开发者选项中app一栏，打开dont keep activities，可以模拟viewmodel被杀死。点击home按键，再返回app，数据还在。

• 添加依赖

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:2.2.0'

• activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>
        <variable
            name="data"
            type="com.example.myviewmodelrestore.MyViewModel" />
    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <TextView
            android:id="@+id/tv"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(data.getNumber)}"/>

        <Button
            android:id="@+id/btn"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Button"
            android:onClick="@{()->data.add()}"/>

    </LinearLayout>
</layout>

• MyViewModel.java

package com.example.myviewmodelrestore;

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.SavedStateHandle;
import androidx.lifecycle.ViewModel;

public class MyViewModel extends ViewModel {
    private SavedStateHandle handle;
    public MyViewModel(SavedStateHandle handle){
        if (!handle.contains(MainActivity.KEY_NUMBER)){
            handle.set(MainActivity.KEY_NUMBER, 0); // 设置一个初始值
        }
        this.handle = handle;
    }

    public MutableLiveData<Integer> getNumber() {
        return handle.getLiveData(MainActivity.KEY_NUMBER);
    }

    public void add(){
        handle.set(MainActivity.KEY_NUMBER, (int) handle.get(MainActivity.KEY_NUMBER) + 1);
    }
}

• MainActivity.java

package com.example.myviewmodelrestore;

import androidx.annotation.Keep;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.SavedStateViewModelFactory;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProviders;

import android.os.Bundle;

import com.example.myviewmodelrestore.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

    MyViewModel myViewModel;
    ActivityMainBinding binding;
    final static String KEY_NUMBER = "my_number";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        myViewModel = new ViewModelProvider(this).get(MyViewModel.class);

        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }
}

ViewModel访问全局资源

• 添加依赖

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:2.2.0'

• 添加dataBinding

dataBinding.enabled true

• activity_main.xml

<?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.myviewmodelsp.MyViewModel" />

    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

        <TextView
            android:id="@+id/tv"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(data.getNumber())}"
            android:textSize="48sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            tools:text="100" />

        <Button
            android:id="@+id/btn_add"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/btn_add"
            android:onClick="@{()->data.add(1)}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.264"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.582" />

        <Button
            android:id="@+id/btn_minus"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/btn_minus"
            android:onClick="@{()->data.add(-1)}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.735"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.582" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

• MyViewModel.java

package com.example.myviewmodelsp;

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;
import androidx.lifecycle.ViewModel;

public class MyViewModel extends AndroidViewModel { // 继承AndroidViewModel,可以直接使用getApplication()

    SavedStateHandle handle;
    String key = getApplication().getResources().getString(R.string.data_key);
    String spName = getApplication().getResources().getString(R.string.sp_name);

    public MyViewModel(@NonNull Application application, SavedStateHandle handle) {
        super(application);

        this.handle = handle;
        // 如果handle里没有数据，就从sharedPreferences中取出
        if (!handle.contains(key)){
            load();
        }
    }

    public LiveData<Integer> getNumber(){
        return handle.getLiveData(key);
    }

    private void load(){
        // 从sharedPreferences中取出数据存到savedStateHandle中
        SharedPreferences sp = getApplication().getSharedPreferences(spName, Context.MODE_PRIVATE);
        int x = sp.getInt(key, 0);
        handle.set(key, x);
    }

    public void save(){
        SharedPreferences sp = getApplication().getSharedPreferences(spName, Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = sp.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.myviewmodelsp;

import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.lifecycle.SavedStateViewModelFactory;
import androidx.lifecycle.ViewModelProviders;

import android.os.Bundle;

import com.example.myviewmodelsp.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).get(MyViewModel.class);
        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }

    @Override
    protected void onPause() {
        super.onPause();
        myViewModel.save();
    }
}