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
我的同事写了一篇文章，解释了Android设备上的应用程序状态，包括活动生命周期和状态信息的解释，如何存储状态信息，以及保存到状态Bundle和SharedPreferences。看看这里。
本文涵盖三种方法：
使用实例状态包存储应用程序生存期（即临时）的本地变量/UI控制数据
[Code sample – Store state in state bundle]
@Override
public void onSaveInstanceState(Bundle savedInstanceState)
{
// Store UI state to the savedInstanceState.
// This bundle will be passed to onCreate on next call. EditText txtName = (EditText)findViewById(R.id.txtName);
String strName = txtName.getText().toString();
EditText txtEmail = (EditText)findViewById(R.id.txtEmail);
String strEmail = txtEmail.getText().toString();
CheckBox chkTandC = (CheckBox)findViewById(R.id.chkTandC);
boolean blnTandC = chkTandC.isChecked();
savedInstanceState.putString(“Name”, strName);
savedInstanceState.putString(“Email”, strEmail);
savedInstanceState.putBoolean(“TandC”, blnTandC);
super.onSaveInstanceState(savedInstanceState);
}
使用共享首选项在应用程序实例之间（即永久）存储本地变量/UI控制数据
[Code sample – store state in SharedPreferences]
@Override
protected void onPause()
{
super.onPause();
// Store values between instances here
SharedPreferences preferences = getPreferences(MODE_PRIVATE);
SharedPreferences.Editor editor = preferences.edit(); // Put the values from the UI
EditText txtName = (EditText)findViewById(R.id.txtName);
String strName = txtName.getText().toString();
EditText txtEmail = (EditText)findViewById(R.id.txtEmail);
String strEmail = txtEmail.getText().toString();
CheckBox chkTandC = (CheckBox)findViewById(R.id.chkTandC);
boolean blnTandC = chkTandC.isChecked();
editor.putString(“Name”, strName); // value to store
editor.putString(“Email”, strEmail); // value to store
editor.putBoolean(“TandC”, blnTandC); // value to store
// Commit to storage
editor.commit();
}
使用保留的非配置实例在应用程序生存期内的活动之间保持对象实例在内存中的活动状态
[Code sample – store object instance]
private cMyClassType moInstanceOfAClass; // Store the instance of an object
@Override
public Object onRetainNonConfigurationInstance()
{
if (moInstanceOfAClass != null) // Check that the object exists
return(moInstanceOfAClass);
return super.onRetainNonConfigurationInstance();
}
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")
}
就我而言，拯救国家充其量是一个拙劣的动作。如果需要保存持久数据，只需使用SQLite数据库即可。Android让SOOO变得简单。
类似于：
import java.util.Date;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class dataHelper {
private static final String DATABASE_NAME = "autoMate.db";
private static final int DATABASE_VERSION = 1;
private Context context;
private SQLiteDatabase db;
private OpenHelper oh ;
public dataHelper(Context context) {
this.context = context;
this.oh = new OpenHelper(this.context);
this.db = oh.getWritableDatabase();
}
public void close() {
db.close();
oh.close();
db = null;
oh = null;
SQLiteDatabase.releaseMemory();
}
public void setCode(String codeName, Object codeValue, String codeDataType) {
Cursor codeRow = db.rawQuery("SELECT * FROM code WHERE codeName = '"+ codeName + "'", null);
String cv = "" ;
if (codeDataType.toLowerCase().trim().equals("long") == true){
cv = String.valueOf(codeValue);
}
else if (codeDataType.toLowerCase().trim().equals("int") == true)
{
cv = String.valueOf(codeValue);
}
else if (codeDataType.toLowerCase().trim().equals("date") == true)
{
cv = String.valueOf(((Date)codeValue).getTime());
}
else if (codeDataType.toLowerCase().trim().equals("boolean") == true)
{
String.valueOf(codeValue);
}
else
{
cv = String.valueOf(codeValue);
}
if(codeRow.getCount() > 0) //exists-- update
{
db.execSQL("update code set codeValue = '" + cv +
"' where codeName = '" + codeName + "'");
}
else // does not exist, insert
{
db.execSQL("INSERT INTO code (codeName, codeValue, codeDataType) VALUES(" +
"'" + codeName + "'," +
"'" + cv + "'," +
"'" + codeDataType + "')" );
}
}
public Object getCode(String codeName, Object defaultValue){
//Check to see if it already exists
String codeValue = "";
String codeDataType = "";
boolean found = false;
Cursor codeRow = db.rawQuery("SELECT * FROM code WHERE codeName = '"+ codeName + "'", null);
if (codeRow.moveToFirst())
{
codeValue = codeRow.getString(codeRow.getColumnIndex("codeValue"));
codeDataType = codeRow.getString(codeRow.getColumnIndex("codeDataType"));
found = true;
}
if (found == false)
{
return defaultValue;
}
else if (codeDataType.toLowerCase().trim().equals("long") == true)
{
if (codeValue.equals("") == true)
{
return (long)0;
}
return Long.parseLong(codeValue);
}
else if (codeDataType.toLowerCase().trim().equals("int") == true)
{
if (codeValue.equals("") == true)
{
return (int)0;
}
return Integer.parseInt(codeValue);
}
else if (codeDataType.toLowerCase().trim().equals("date") == true)
{
if (codeValue.equals("") == true)
{
return null;
}
return new Date(Long.parseLong(codeValue));
}
else if (codeDataType.toLowerCase().trim().equals("boolean") == true)
{
if (codeValue.equals("") == true)
{
return false;
}
return Boolean.parseBoolean(codeValue);
}
else
{
return (String)codeValue;
}
}
private static class OpenHelper extends SQLiteOpenHelper {
OpenHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE IF NOT EXISTS code" +
"(id INTEGER PRIMARY KEY, codeName TEXT, codeValue TEXT, codeDataType TEXT)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
}
之后打个简单的电话
dataHelper dh = new dataHelper(getBaseContext());
String status = (String) dh.getCode("appState", "safetyDisabled");
Date serviceStart = (Date) dh.getCode("serviceStartTime", null);
dh.close();
dh = null;
