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

其他回答

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")
    }

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()用于瞬时数据(在onCreate()/onRestoreInstanceState(()中还原),onPause()用于持久数据(在on Resume()中恢复)。从Android技术资源:

如果活动正在停止,Android会调用onSaveInstanceState(),并且可能会在恢复之前被终止!这意味着它应该存储在重新启动“活动”时重新初始化为相同条件所需的任何状态。它与onCreate()方法相对应,实际上传递给onCreate(()的savedInstanceState Bundle与您在onSaveInstanceState()方法中构造的outState Bundle相同。onPause()和onResume()也是互补的方法。onPause()总是在“活动”结束时调用,即使我们启动了它(例如,使用finish()调用)。我们将使用它将当前注释保存回数据库。好的做法是释放所有可以在onPause()期间释放的资源,以便在处于被动状态时占用更少的资源。

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);
  }
}

创建活动时,将调用其onCreate()方法。

   @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

savedInstanceState是Bundle类的一个对象,它第一次为空,但在重新创建时包含值。要保存“活动”的状态,必须重写onSaveInstanceState()。

   @Override
    protected void onSaveInstanceState(Bundle outState) {
      outState.putString("key","Welcome Back")
        super.onSaveInstanceState(outState);       //save state
    }

将值放在“outState”Bundle对象中,如outState.putString(“key”,“Welcome Back”),并通过调用super保存。当活动将被销毁时,它的状态将保存在Bundle对象中,并且可以在onCreate()或onRestoreInstanceState()中重新创建后恢复。在onCreate()和onRestoreInstanceState()中接收的捆绑包相同。

   @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

          //restore activity's state
         if(savedInstanceState!=null){
          String reStoredString=savedInstanceState.getString("key");
            }
    }

or

  //restores activity's saved state
 @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
      String restoredMessage=savedInstanceState.getString("key");
    }