我一直在安卓SDK平台上工作，现在还不清楚如何保存应用程序的状态。因此，考虑到“你好，Android”示例的这个小的重新设计：
package com.android.hello;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class HelloAndroid extends Activity {
private TextView mTextView = null;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mTextView = new TextView(this);
if (savedInstanceState == null) {
mTextView.setText("Welcome to HelloAndroid!");
} else {
mTextView.setText("Welcome back.");
}
setContentView(mTextView);
}
}
我认为这对于最简单的情况来说已经足够了，但无论我如何离开应用程序，它总是以第一条消息来响应。
我确信解决方案就像重写onPause之类的东西一样简单，但我已经在文档中翻了大约30分钟，没有发现任何明显的东西。
Kotlin解决方案：对于onSaveInstanceState中的自定义类保存，您可以将类转换为JSON字符串并使用Gson转换进行恢复，对于单字符串、双精度、Int、Long值保存和恢复，如下所示。以下示例适用于片段和活动：
对于活动：
对于将数据放入saveInstanceState：
override fun onSaveInstanceState(outState: Bundle) {
super.onSaveInstanceState(outState)
//for custom class-----
val gson = Gson()
val json = gson.toJson(your_custom_class)
outState.putString("CUSTOM_CLASS", json)
//for single value------
outState.putString("MyString", stringValue)
outState.putBoolean("MyBoolean", true)
outState.putDouble("myDouble", doubleValue)
outState.putInt("MyInt", intValue)
}
还原数据：
override fun onRestoreInstanceState(savedInstanceState: Bundle) {
super.onRestoreInstanceState(savedInstanceState)
//for custom class restore
val json = savedInstanceState?.getString("CUSTOM_CLASS")
if (!json!!.isEmpty()) {
val gson = Gson()
testBundle = gson.fromJson(json, Session::class.java)
}
//for single value restore
val myBoolean: Boolean = savedInstanceState?.getBoolean("MyBoolean")
val myDouble: Double = savedInstanceState?.getDouble("myDouble")
val myInt: Int = savedInstanceState?.getInt("MyInt")
val myString: String = savedInstanceState?.getString("MyString")
}
您也可以在Activity onCreate上恢复它。
对于碎片：
对于saveInstanceState中的put类：
override fun onSaveInstanceState(outState: Bundle) {
super.onSaveInstanceState(outState)
val gson = Gson()
val json = gson.toJson(customClass)
outState.putString("CUSTOM_CLASS", json)
}
还原数据：
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
//for custom class restore
if (savedInstanceState != null) {
val json = savedInstanceState.getString("CUSTOM_CLASS")
if (!json!!.isEmpty()) {
val gson = Gson()
val customClass: CustomClass = gson.fromJson(json, CustomClass::class.java)
}
}
// for single value restore
val myBoolean: Boolean = savedInstanceState.getBoolean("MyBoolean")
val myDouble: Double = savedInstanceState.getDouble("myDouble")
val myInt: Int = savedInstanceState.getInt("MyInt")
val myString: String = savedInstanceState.getString("MyString")
}
使用Android ViewModel和SavedStateHandle持久化可序列化数据
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ActivityMainBinding binding = ActivityMainBinding.inflate(getLayoutInflater());
binding.setViewModel(new ViewModelProvider(this).get(ViewModel.class));
binding.setLifecycleOwner(this);
setContentView(binding.getRoot());
}
public static class ViewModel extends AndroidViewModel {
//This field SURVIVE the background process reclaim/killing & the configuration change
public final SavedStateHandle savedStateHandle;
//This field NOT SURVIVE the background process reclaim/killing but SURVIVE the configuration change
public final MutableLiveData<String> inputText2 = new MutableLiveData<>();
public ViewModel(@NonNull Application application, SavedStateHandle savedStateHandle) {
super(application);
this.savedStateHandle = savedStateHandle;
}
}
}
在布局文件中
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">
<data>
<variable
name="viewModel"
type="com.xxx.viewmodelsavedstatetest.MainActivity.ViewModel" />
</data>
<LinearLayout xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:autofillHints=""
android:hint="This field SURVIVE the background process reclaim/killing & the configuration change"
android:text='@={(String)viewModel.savedStateHandle.getLiveData("activity_main/inputText", "")}' />
<SeekBar
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:max="100"
android:progress='@={(Integer)viewModel.savedStateHandle.getLiveData("activity_main/progress", 50)}' />
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="This field SURVIVE the background process reclaim/killing & the configuration change"
android:text='@={(String)viewModel.savedStateHandle.getLiveData("activity_main/inputText", "")}' />
<SeekBar
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:max="100"
android:progress='@={(Integer)viewModel.savedStateHandle.getLiveData("activity_main/progress", 50)}' />
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="This field NOT SURVIVE the background process reclaim/killing but SURVIVE the configuration change"
android:text='@={viewModel.inputText2}' />
</LinearLayout>
</layout>
测试：
1. start the test activity
2. press home key to go home
3. adb shell kill <the test activity process>
4. open recent app list and restart the test activity
