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

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")
    }

其他回答

虽然公认的答案是正确的,但有一种更快、更容易的方法可以使用名为Icepick的库在Android上保存“活动”状态。Icepick是一个注释处理器,它负责为您保存和恢复状态时使用的所有样板代码。

对Icepick:

class MainActivity extends Activity {
  @State String username; // These will be automatically saved and restored
  @State String password;
  @State int age;

  @Override public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Icepick.restoreInstanceState(this, savedInstanceState);
  }

  @Override public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    Icepick.saveInstanceState(this, outState);
  }
}

与执行以下操作相同:

class MainActivity extends Activity {
  String username;
  String password;
  int age;

  @Override
  public void onSaveInstanceState(Bundle savedInstanceState) {
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putString("MyString", username);
    savedInstanceState.putString("MyPassword", password);
    savedInstanceState.putInt("MyAge", age); 
    /* remember you would need to actually initialize these variables before putting it in the
    Bundle */
  }

  @Override
  public void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    username = savedInstanceState.getString("MyString");
    password = savedInstanceState.getString("MyPassword");
    age = savedInstanceState.getInt("MyAge");
  }
}

Icepick可以处理任何使用Bundle保存其状态的对象。

Kotlin代码:

保存:

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState.apply {
        putInt("intKey", 1)
        putString("stringKey", "String Value")
        putParcelable("parcelableKey", parcelableObject)
    })
}

然后在onCreate()或onRestoreInstanceState()中

    val restoredInt = savedInstanceState?.getInt("intKey") ?: 1 //default int
    val restoredString = savedInstanceState?.getString("stringKey") ?: "default string"
    val restoredParcelable = savedInstanceState?.getParcelable<ParcelableClass>("parcelableKey") ?: ParcelableClass() //default parcelable

如果不想有Optionals,请添加默认值

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

解决这个问题的简单快捷方法是使用IcePick

首先,在app/build.gradle中设置库

repositories {
  maven {url "https://clojars.org/repo/"}
}
dependencies {
  compile 'frankiesardo:icepick:3.2.0'
  provided 'frankiesardo:icepick-processor:3.2.0'
}

现在,让我们看看下面的示例,如何在“活动”中保存状态

public class ExampleActivity extends Activity {
  @State String username; // This will be automatically saved and restored

  @Override public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Icepick.restoreInstanceState(this, savedInstanceState);
  }

  @Override public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    Icepick.saveInstanceState(this, outState);
  }
}

它适用于Activities、Fragments或任何需要在Bundle上序列化其状态的对象(例如,迫击炮的ViewPresenter)

Icepick还可以为自定义视图生成实例状态代码:

class CustomView extends View {
  @State int selectedPosition; // This will be automatically saved and restored

  @Override public Parcelable onSaveInstanceState() {
    return Icepick.saveInstanceState(this, super.onSaveInstanceState());
  }

  @Override public void onRestoreInstanceState(Parcelable state) {
    super.onRestoreInstanceState(Icepick.restoreInstanceState(this, state));
  }

  // You can put the calls to Icepick into a BaseCustomView and inherit from it
  // All Views extending this CustomView automatically have state saved/restored
}