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。
现在,在视图模型中使用两种方法是有意义的。如果要将第一个保存为保存的实例:您可以像这样在视图模型中添加状态参数https://developer.android.com/topic/libraries/architecture/viewmodel-savedstate#java
或者您可以在视图模型中保存变量或对象,在这种情况下,视图模型将保存生命周期,直到活动被破坏。
public class HelloAndroidViewModel extends ViewModel {
public Booelan firstInit = false;
public HelloAndroidViewModel() {
firstInit = false;
}
...
}
public class HelloAndroid extends Activity {
private TextView mTextView = null;
HelloAndroidViewModel viewModel = ViewModelProviders.of(this).get(HelloAndroidViewModel.class);
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mTextView = new TextView(this);
//Because even if the state is deleted, the data in the viewmodel will be kept because the activity does not destroy
if(!viewModel.firstInit){
viewModel.firstInit = true
mTextView.setText("Welcome to HelloAndroid!");
}else{
mTextView.setText("Welcome back.");
}
setContentView(mTextView);
}
}
请注意,根据Activity上的文档,对持久数据使用onSaveInstanceState和onRestoreInstanceState是不安全的。
文档说明(在“活动生命周期”部分):
请注意,保存改为onPause()中的持久数据的onSaveInstanceState(捆绑包)因为后者不是生命周期回调,因此不会在描述的每种情况下调用在其文档中。
换句话说,将持久数据的保存/恢复代码放在onPause()和onResume()中!
为了进一步澄清,这里有onSaveInstanceState()文档:
在活动被终止之前调用此方法,以便在未来的某个时候,它可以恢复其状态。对于例如,如果活动B在活动A之前启动点活动A被杀死以回收资源,活动A将具有通过此选项保存用户界面的当前状态方法,以便当用户返回活动A时用户界面可以通过onCreate(Bundle)或onRestoreInstanceState(捆绑包)。
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
我的问题是,我只在应用程序生命周期内需要持久性(即一次执行,包括在同一应用程序内启动其他子活动和旋转设备等)。我尝试了以上答案的各种组合,但在所有情况下都没有得到我想要的答案。最后,对我有用的是在onCreate期间获取savedInstanceState的引用:
mySavedInstanceState=savedInstanceState;
并在需要时使用它来获取变量的内容,如下所示:
if (mySavedInstanceState !=null) {
boolean myVariable = mySavedInstanceState.getBoolean("MyVariable");
}
如上所述,我使用了onSaveInstanceState和onRestoreInstanceState,但我想我也可以或替代地使用我的方法在变量更改时保存变量(例如,使用putBoolean)