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。
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
}
科特林
必须重写onSaveInstanceState和onRestoreInstanceState以存储和检索要持久化的变量
生命周期图
存储变量
public override fun onSaveInstanceState(savedInstanceState: Bundle) {
super.onSaveInstanceState(savedInstanceState)
// prepare variables here
savedInstanceState.putInt("kInt", 10)
savedInstanceState.putBoolean("kBool", true)
savedInstanceState.putDouble("kDouble", 4.5)
savedInstanceState.putString("kString", "Hello Kotlin")
}
检索变量
public override fun onRestoreInstanceState(savedInstanceState: Bundle) {
super.onRestoreInstanceState(savedInstanceState)
val myInt = savedInstanceState.getInt("kInt")
val myBoolean = savedInstanceState.getBoolean("kBool")
val myDouble = savedInstanceState.getDouble("kDouble")
val myString = savedInstanceState.getString("kString")
// use variables here
}
我想我找到了答案。让我简单地说一下我做了什么：
假设我有两个活动，即活动1和活动2，我正在从活动1导航到活动2（我在活动2中做了一些工作），然后通过单击活动1中的按钮再次返回活动1。现在在这个阶段，我想回到活动2，我想看到我的活动2处于上次离开活动2时的相同状态。
对于上述场景，我所做的是在清单中进行了如下更改：
<activity android:name=".activity2"
android:alwaysRetainTaskState="true"
android:launchMode="singleInstance">
</activity>
在活动1的按钮点击事件中，我这样做了：
Intent intent = new Intent();
intent.setFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
intent.setClassName(this,"com.mainscreen.activity2");
startActivity(intent);
在活动2的按钮点击事件中，我这样做了：
Intent intent=new Intent();
intent.setClassName(this,"com.mainscreen.activity1");
startActivity(intent);
现在将要发生的是，我们在activity2中所做的任何更改都不会丢失，并且我们可以以与之前相同的状态查看activity2。
我相信这就是答案，这对我来说很好。如果我错了，请纠正我。
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
}
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）。
