SharedPreferences Storage

SharedPreferences It uses key value pairs to store data . in other words , When saving a piece of data , You need to provide a key for this data , In this way, when reading data, you can pass Use this key to get the corresponding value . and SharedPreferences It also supports different types of data storage , If The stored data type is integer , So the data read out is also integer ; If the data stored is a string , that The read data is still a string .

Store data to SharedPreferences in

adopt getSharedPreferences() Method specification SharedPreferences The file name of is data, And got SharedPreferences.Editor object . Then add... To the object 3 Different types of data , most After the call apply() Method to submit , Thus, the operation of data storage is completed

val editor = getSharedPreferences("data", Context.MODE_PRIVATE).edit()
editor.putString("name", "Tom")
editor.putInt("age", 28)
editor.putBoolean("married", false)
editor.apply()

from SharedPreferences Read data from

from SharedPreferences Reading data from a file will be easier .SharedPreferences In the object A series of get Method , Used to read stored data , Each of these get All methods correspond SharedPreferences.Editor One of the put Method , For example, read a Boolean data to use getBoolean() Method , Read a string and use getString() Method . these get Methods both receive two arguments Count : The first parameter is the key , You can get the corresponding value by passing in the key used to store the data ; The second parameter is the default value , That is, what default value will be returned when the incoming key cannot find the corresponding value .
Call its... Respectively getString()、getInt() and getBoolean() Method , To get the name stored in front of 、 Age and whether married , If no corresponding value is found , The default value passed in the method will be used instead of

val prefs = getSharedPreferences("data", Context.MODE_PRIVATE)
val name = prefs.getString("name", "")
val age = prefs.getInt("age", 0)
val married = prefs.getBoolean("married", false)

App remember password

You can see , Here first onCreate() Method SharedPreferences object , And then call it. getBoolean() Method to get remember_password The value of this key . At first, of course, there was no corresponding The value of , So default values are used false, So that nothing happens . Then after the login is successful , Would call CheckBox Of isChecked() Method to check if the check box is selected . If it's selected , It means that the user wants to remember Live code , This will be remember_password Set to true, And then put account and password The corresponding values are Deposit in SharedPreferences Document and submit ; If not selected , Just call clear() Method , take SharedPreferences Erase all the data in the file .
When the user selects the remember password check box , And after a successful login ,remember_password The value of the bond is true 了 , If you restart the login interface at this time , It will start from SharedPreferences The account number and... Will be saved in the file The passwords are read out , And fill in the text input box , Then select the remember password check box , This completes remembering the password The function of .

val prefs = getPreferences(Context.MODE_PRIVATE)
val isRemember = prefs.getBoolean("remember_password", false)
if (isRemember) {
    
	//  Set both account and password in the text box 
	val account = prefs.getString("account", "") 
	val password = prefs.getString("password", "") 		
	accountEdit.setText(account) 	
	passwordEdit.setText(password) 
	rememberPass.isChecked = true
}
login.setOnClickListener {
    
	val account = accountEdit.text.toString()
	val password = passwordEdit.text.toString()
	//  If the account number is admin And the code is 123456, I think the login is successful 
	if (account == "admin" && password == "123456") {
    
		val editor = prefs.edit()
		if (rememberPass.isChecked) {
     //  Check that the check box is selected 
            editor.putBoolean("remember_password", true)
            editor.putString("account", account)
            editor.putString("password", password)
        } else {
    
            editor.clear()
        }
        editor.apply()
        val intent = Intent(this, MainActivity::class.java)
        startActivity(intent)
        finish()
    } else {
    
        Toast.makeText(this, "account or password is invalid", Toast.LENGTH_SHORT).show()}

SQLite Database storage

Add data :

db.execSQL("insert into Book (name, author, pages, price) values(?, ?, ?, ?)",arrayOf("The Da Vinci Code", "Dan Brown", "454", "16.96"))
db.execSQL("insert into Book (name, author, pages, price) values(?, ?, ?, ?)",arrayOf("The Lost Symbol", "Dan Brown", "510", "19.95"))

Update data :

db.execSQL("update Book set price = ? where name = ?", arrayOf("10.99", "The Da Vinci Code"))

Delete data :

db.execSQL("delete from Book where pages > ?", arrayOf("500"))

Query data :

val cursor = db.rawQuery("select * from Book", null)

Business

Android Standard usage of transactions in , First call SQLiteDatabase Of beginTransaction() Method to open a transaction , Then execute the specific database in an exception caught code block operation , When all operations are completed , call setTransactionSuccessful() Indicates that the transaction has been executed into Thanks , Last in finally Call in code block endTransaction() End the business . observe , We're deleting old After the data operation is completed, a... Is thrown manually NullPointerException, In this way, the code to add new data will execute Not yet . However, due to the existence of transactions , An exception in the middle will lead to the failure of the transaction , At this time, the old data should not be deleted Of .

val db = dbHelper.writableDatabase db.beginTransaction() //  Open transaction  
try {
    
	db.delete("Book", null, null)
	if (true) {
    
		//  Manually throw an exception , Let the transaction fail  
		throw NullPointerException()
	}
	val values = ContentValues().apply {
    
		put("name", "Game of Thrones")
        put("author", "George Martin")
        put("pages", 720)
        put("price", 20.85)
	}
	db.insert("Book", null, values) 
	db.setTransactionSuccessful() //  Transaction executed successfully 
} catch (e: Exception) {
    
	e.printStackTrace()
} finally {
    
db.endTransaction() //  End the business 
}

Room

Google Another one has been launched specifically for Android The database framework of the platform ——Room.