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。
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。
2020年，我们有一些变化：
如果您希望“活动”在进程终止并再次启动后恢复其状态，则可能需要使用“保存状态”功能。以前，需要重写Activity中的两个方法：onSaveInstanceState和onRestoreInstanceState。您还可以在onCreate方法中访问恢复的状态。类似地，在Fragment中，您有onSaveInstanceState方法可用（恢复的状态在onCreate、onCreateView和onActivityCreated方法中可用）。
从AndroidX SavedState 1.0.0开始，它是AndroidX Activity和AndroidX Fragment的依赖项，您可以访问SavedStateRegistry。您可以从Activity/Fragment获取SavedStateRegistry，然后注册SavedStateProvider：
class MyActivity : AppCompatActivity() {
companion object {
private const val MY_SAVED_STATE_KEY = "MY_SAVED_STATE_KEY "
private const val SOME_VALUE_KEY = "SOME_VALUE_KEY "
}
private lateinit var someValue: String
private val savedStateProvider = SavedStateRegistry.SavedStateProvider {
Bundle().apply {
putString(SOME_VALUE_KEY, someValue)
}
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
savedStateRegistry.registerSavedStateProvider(MY_SAVED_STATE_KEY, savedStateProvider)
someValue = savedStateRegistry.consumeRestoredStateForKey(MY_SAVED_STATE_KEY)?.getString(SOME_VALUE_KEY) ?: ""
}
}
如您所见，SavedStateRegistry强制您对数据使用键。这可以防止您的数据被附加到同一Activity/Fragment的另一个SavedStateProvider损坏。此外，您还可以将SavedStateProvider提取到另一个类中，以便通过使用所需的抽象来处理数据，从而在应用程序中实现干净的保存状态行为。
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")
}
