文件存储

存储数据

通过openFileOutput()得到FileOutputStream对象
借助它构建一个OutputStreamWriter对象在借助这个构建一个BufferedWriter对象

利用BufferedWriter将内容写入文件

private fun save(inputText: String) {
        try {
           val output=openFileOutput("data",Context.MODE_PRIVATE)
            val writer=BufferedWriter(OutputStreamWriter(output))
            writer.use{
                it.write(inputText)
            }
        } catch (e: IOException) {
            e.printStackTrace()
        }
    }

读取数据

通过openFileInput()方法获取了一个FileInputStream对象
利用它构建InputStreamReader()对象再借助这个构建BufferedReader对象

最后利用这个对象将文件一行行读取出来并拼接到StringBuilder对象中

private fun load(): String {
       val content = StringBuilder()
       try {
               val input=openFileInput("data")
               val reader=BufferedReader(InputStreamReader(input))
                   reader.use{
                       reader.forEachLine{
                           content.append(it)
                       }
                   }
       } catch (e: IOException) {
           e.printStackTrace()       
       }
       return content.toString()
   }

SharedPreferences

存储数据

得到SharedPreferences对象

(1)Context类中的getSharedPreferences()方法得到（第一个参数是文件名，第二个是模式）

(2)Activity类中的getPreferences()方法得到（只接收模式参数）
调用其edit()方法得到SharedPreferences.Editor对象
像这个对象中添加数据

调用apply()提交数据

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

读取数据

得到SharedPreferences对象

调用get系列方法获取数据

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

SQLite数据库

创建和升级

创建
1. 新建一个类继承自SQLiteOpenHelper
2. 建立表格
3. 在onCreate方法中使用execSQL()方法在数据库中创建表格
4. 在活动中使用writableDatabase方法创建数据库
升级
1. 在帮助类中建立新的表格或作出一些更改
2. 如果建立新的表格需要在oncreate方法中用execSQL()方法创建表格
3. 在onUpgrade()方法中使用execSQL()方法进行更新再调用onCreate()
4. 活动中数据库版本号改为比原来大的数字

class MyDatabaseHelper(val context:Context,name:String,version: Int) :SQLiteOpenHelper(context,name,null,version){
   private val createBook="create table Book("+
           "id integer primary key autoincrement,"+
           "author text,"+
           "price real,"+
           "pages integer,"+
           "name text)"
    private val createCategory="create table Category("+
            "id integer primary key autoincrement,"+
            "category_name text,"+
            "category_code integer)"
    override fun onCreate(db: SQLiteDatabase) {
        db.execSQL(createBook)
        db.execSQL(createCategory)
        Toast.makeText(context,"Create succeeded",Toast.LENGTH_SHORT).show()
    }
    override fun onUpgrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
         db.execSQL("drop table if exists Book")
        db.execSQL("drop table if exists Category")
        onCreate(db)
    }
}

添加，更新，删除，查询

添加

调用SQLiteOpenHelper的writableDatabase方法获取SQLiteDataBase对象
利用ContentValues()对象组装数据

利用insert方法插入数据

binding.addData.setOnClickListener {
            //获取SQLiteDatabase对象
            val db=dbHelper.writableDatabase
            //组装第一条数据
            val values1=ContentValues().apply {
                put("name","The Da Vinci Code")
                put("author","Dan Brown")
                put("pages",454)
                put("price",16.96)
            }
            //插入第一条数据
            db.insert("Book",null,values1)
            //组装第二条数据
            val values2=ContentValues().apply {
                put("name","The Lost Symbol")
                put("author","Dan Brown")
                put("pages",510)
                put("price",19.95)
            }
            //插入第二条数据
            db.insert("Book",null,values2)
        }

更新

获取SQLiteDataBase对象
获取ContentValues对象
指定更新的数据

利用update更新数据

binding.updateData.setOnClickListener {
            //获取SQLiteDatabase对象
            val db=dbHelper.writableDatabase
            val value=ContentValues()
            value.put("price",10.99)
            db.update("Book",value,"name=?",arrayOf("The Da Vinci Code"))
        Toast.makeText(this,"更新成功",Toast.LENGTH_SHORT).show()
        }

删除

获取SQLiteDataBase对象

利用delete删除数据

binding.deleteData.setOnClickListener {
            //获取SQLiteDatabase对象
            val db=dbHelper.writableDatabase
            db.delete("Book","pages>?",arrayOf("500"))
            Toast.makeText(this,"删除成功",Toast.LENGTH_SHORT).show()
        }

查询

获取SQLiteDataBase对象
调用query方法查询数据
查询后得到Cursor对象,遍历查询每一行数据在循环中通过Cursor的getColumnIndex()方法索引

查询完成后关闭Cursor

binding.queryData.setOnClickListener {
           //获取SQLiteDatabase对象
           val db=dbHelper.writableDatabase
           //查询Book中所有数据
           val cursor=db.query("Book",null,null,null,null,null,null)
           if(cursor.moveToFirst()){
               do{//遍历Cursor对象，取出数据并打印
                   val name=cursor.getString(cursor.getColumnIndex("name"))
                   val author=cursor.getString(cursor.getColumnIndex("author"))
                   val pages=cursor.getInt(cursor.getColumnIndex("pages"))
                   val price=cursor.getDouble(cursor.getColumnIndex("price"))
                   Log.d("MainActivity", "book name is $name ")
                   Log.d("MainActivity", "book author is $author ")
                   Log.d("MainActivity", "book pages is $pages ")
                   Log.d("MainActivity", "book price is $price ")
               }while (cursor.moveToNext())
     
           }
           cursor.close()
       }

SQL操作数据库

//添加数据
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")
          )
//更新数据
db.execSQL("update Book set price=? where name=? ",arrayOf("10.99","The Da Vinci Code"))
//删除数据
db.execSQL("delete from book where pages>?",arrayOf("500"))
//查询数据
val cursor=db.rawQuery("select * from Book",null)

使用事务

特点：操作要失败一起失败要成功一起成功

binding.replaceData.setOnClickListener {
           //获取SQLiteDatabase对象
           val db=dbHelper.writableDatabase
           //开启事务
           db.beginTransaction()
           try {
               db.delete("Book",null,null)
               /*if(true){
                   //手动抛出异常，让事务失败
                   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)
               //事务已经执行成功
               db.setTransactionSuccessful()
           }catch (e:Exception){
               e.printStackTrace()
           }finally {
               //结束事务
               db.endTransaction()
           }
       }

升级数据库最佳写法

每进行一次更新都在onUpgrade()方法中加入一个if判断语句

这样就算跨版本升级也不会出现数据丢失

class MyDatabaseHelper(val context:Context,name:String,version: Int) :SQLiteOpenHelper(context,name,null,version){
   private val createBook="create table Book("+
           "id integer primary key autoincrement,"+
           "author text,"+
           "price real,"+
           "pages integer,"+
           "name text,"+
           "category_id)"
    private val createCategory="create table Category("+
            "id integer primary key autoincrement,"+
            "category_name text,"+
            "category_code integer)"
    override fun onCreate(db: SQLiteDatabase) {
        db.execSQL(createBook)
        db.execSQL(createCategory)
        Toast.makeText(context,"Create succeeded",Toast.LENGTH_SHORT).show()
    }


    override fun onUpgrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
        if(oldVersion<=1){
            db.execSQL(createCategory)
        }
        if(oldVersion<=2){
            db.execSQL("alter table Book add column category_id integer")
        }
    }
}