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。


当前回答

同时,我一般不再使用

Bundle savedInstanceState & Co

对于大多数活动来说,生命周期过于复杂且不必要。

谷歌表示,它甚至不可靠。

我的方法是立即保存首选项中的任何更改:

 SharedPreferences p;
 p.edit().put(..).commit()

在某种程度上,SharedPreferences的工作方式与Bundles类似。首先,这些值必须从偏好中读取。

对于复杂数据,您可以使用SQLite而不是首选项。

应用此概念时,活动只会继续使用上次保存的状态,而不管它是在重新启动后的初始打开状态,还是由于后堆栈而重新打开。

其他回答

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)。

要获取存储在onCreate()中的活动状态数据,首先必须通过重写SaveInstanceState(Bundle savedInstanceState)方法将数据保存在savedInstanceStatus中。

当调用activity destroy SaveInstanceState(Bundle savedInstanceState)方法并保存要保存的数据时。当活动重新启动时,在onCreate()中也会得到同样的结果。(savedInstanceState不会为空,因为在活动被破坏之前,您已经在其中保存了一些数据)

onSaveInstanceState(bundle)和onRestoreInstanceState(bundle)方法仅在旋转屏幕(方向更改)时用于数据持久化。在应用程序之间切换时,它们甚至都不好(因为调用了onSaveInstanceState()方法,但不会再次调用onCreate(捆绑包)和onRestoreInstanceState(捆绑)。要获得更多持久性,请使用共享首选项。阅读这篇文章

您需要重写onSaveInstanceState(Bundle savedInstanceState),并将要更改的应用程序状态值写入Bundle参数,如下所示:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
  super.onSaveInstanceState(savedInstanceState);
  // Save UI state changes to the savedInstanceState.
  // This bundle will be passed to onCreate if the process is
  // killed and restarted.
  savedInstanceState.putBoolean("MyBoolean", true);
  savedInstanceState.putDouble("myDouble", 1.9);
  savedInstanceState.putInt("MyInt", 1);
  savedInstanceState.putString("MyString", "Welcome back to Android");
  // etc.
}

Bundle本质上是一种存储NVP(“名称-值对”)映射的方法,它将被传递到onCreate()和onRestoreInstanceState(),然后在那里您可以从如下活动中提取值:

@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
  super.onRestoreInstanceState(savedInstanceState);
  // Restore UI state from the savedInstanceState.
  // This bundle has also been passed to onCreate.
  boolean myBoolean = savedInstanceState.getBoolean("MyBoolean");
  double myDouble = savedInstanceState.getDouble("myDouble");
  int myInt = savedInstanceState.getInt("MyInt");
  String myString = savedInstanceState.getString("MyString");
}

或者从碎片中提取。

@Override
public void onViewStateRestored(@Nullable Bundle savedInstanceState) {
    super.onViewStateRestored(savedInstanceState);
    // Restore UI state from the savedInstanceState.
    // This bundle has also been passed to onCreate.
    boolean myBoolean = savedInstanceState.getBoolean("MyBoolean");
    double myDouble = savedInstanceState.getDouble("myDouble");
    int myInt = savedInstanceState.getInt("MyInt");
    String myString = savedInstanceState.getString("MyString");
}

您通常会使用此技术来存储应用程序的实例值(选择、未保存的文本等)。

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 &amp; 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 &amp; 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