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。
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;
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);
}
}
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
