**
解決當進程被系統回收後,如何保存數據。
**
使用ViewModel中的SavedStateHandle也可以解決這個問題。
下面代碼中存在的一些小知識:
MutableLiveData:個人的理解就是當數據有變化時,可以通知界面更新數據
- 首先我們創建一個頁面SaveStateActivity
package com.java.jetpackdemo.savestate
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.SavedStateViewModelFactory
import androidx.lifecycle.ViewModelProviders
import com.java.jetpackdemo.R
import com.java.jetpackdemo.databinding.ActivitySaveStateBinding
class SaveStateActivity : AppCompatActivity() {
private lateinit var binding: ActivitySaveStateBinding
private lateinit var viewModel: SaveStateViewModel
companion object {
const val KEY_NUMBER = "KEY_NUMBER"
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
//用databinding來綁定佈局
binding = DataBindingUtil.setContentView(this, R.layout.activity_save_state)
//創建viewmodel
/** 不傳savedstateviewmodel的話創建viewmodel的方式
myViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) **/
viewModel = ViewModelProviders.of(this, SavedStateViewModelFactory(application, this))
.get(SaveStateViewModel::class.java)
//將viewmodel綁定到databinding上
binding.data = viewModel
//databinding監聽activity的生命週期,實時更新數據
binding.lifecycleOwner = this
}
}
- 創建佈局
<?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.java.jetpackdemo.savestate.SaveStateViewModel" />
</data>
<androidx.constraintlayout.widget.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".savestate.SaveStateActivity">
<TextView
android:id="@+id/tvNumber"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{data.number.toString()}"
app:layout_constraintBottom_toTopOf="@+id/guideline3"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<androidx.constraintlayout.widget.Guideline
android:id="@+id/guideline3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.2" />
<Button
android:id="@+id/btnAdd"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="@{()->data.add()}"
android:text="@string/save_state_btn"
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="@+id/guideline3"
app:layout_constraintVertical_bias="0.178" />
</androidx.constraintlayout.widget.ConstraintLayout>
</layout>
- 創建viewmodel
package com.java.jetpackdemo.savestate
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.SavedStateHandle
import androidx.lifecycle.ViewModel
class SaveStateViewModel(handle: SavedStateHandle) : ViewModel() {
private var linkNumber: MutableLiveData<Int> = MutableLiveData()
private var handle = handle
//初始化數據
init {
linkNumber.value = 0
}
//將數據加1
public fun add() {
getNumber().value = getNumber().value?.plus(1)
}
//獲取數據
public fun getNumber(): MutableLiveData<Int> {
if (!handle.contains(SaveStateActivity.KEY_NUMBER)) {
handle.set(SaveStateActivity.KEY_NUMBER, 0)
}
return handle.getLiveData(SaveStateActivity.KEY_NUMBER)
}
}
如何測試呢?可以打開開發者模式,然後打開 【不保留活】 的選項,當我們按下home鍵後,進程就會被系統回收,但是我們重新打開app,數據並沒有丟失。