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")
}
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）。
