7. 持久化技术

• 持久化技术可以让数据在瞬时状态和持久状态之间进行转换

  • 文件存储
  • SharedPreferences 存储
  • 数据区存储

• MainActivity2

  • package com.example.helloworld
    
    import android.content.Context
    import android.content.Intent
    import android.os.Bundle
    import android.util.Log
    import android.widget.ArrayAdapter
    import android.widget.Button
    import android.widget.ListView
    import androidx.appcompat.app.AppCompatActivity
    import androidx.appcompat.widget.Toolbar
    
    
    class MainActivity2 : BaseActivity() {
    
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.second_layout)
    
            // toolbar 部分
            val toolbar: Toolbar = findViewById(R.id.toolbar2)
            setSupportActionBar(toolbar)
            supportActionBar?.setDisplayHomeAsUpEnabled(true)            // 设置返回按钮
            toolbar.setNavigationOnClickListener { finish() }             // 设置结束 act2
    
            ActivityCollector.addActivity(this)
    
            // 从前一个 Activity 接收数据
            val extraData1 = intent.getStringExtra("param1")
            val extraData2 = intent.getStringExtra("param2")
            Log.d("Activity2", "param1 is $extraData1, param2 is $extraData2")
    
    
            // 返回数据（但是未实现）
            val button2: Button = findViewById(R.id.button2)        // 通过 id 找到 view, 并指明类型为 button
            button2.setOnClickListener {
                val intent = Intent().apply { putExtra("data_return", "Hello Activity1") }
                setResult(RESULT_OK, intent)
                finish()
            }
    
            // 关闭所有 Activity
            val button3: Button = findViewById(R.id.button3)
            button3.setOnClickListener {
                ActivityCollector.finishAll()
            }
    
            val button4: Button = findViewById(R.id.button4)
            button4.setOnClickListener {
                startNewActivity(this, MainActivity3::class.java)
            }
    
            val button5: Button = findViewById(R.id.button5)
            button5.setOnClickListener {
                startNewActivity(this, MainActivity4::class.java)
            }
    
            val button7: Button = findViewById(R.id.button7)
            button7.setOnClickListener {
                startNewActivity(this, FragmentActivity::class.java)
            }
            val button8: Button = findViewById(R.id.button8)
            button8.setOnClickListener {
                startNewActivity(this, FileStorage::class.java)
            }
    
            val button9: Button = findViewById(R.id.button9)
            button9.setOnClickListener {
                startNewActivity(this, DataPreference::class.java)
            }
    
            val button10: Button = findViewById(R.id.button10)
            button10.setOnClickListener {
                startNewActivity(this, SqliteDB::class.java)
            }
    
        }
    
    }
    
    
    

• second_layout

  • <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    
        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            android:minHeight="?attr/actionBarSize"
            android:theme="?attr/actionBarTheme" />
    
        <Button
            android:id="@+id/button2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Back" />
    
        <Button
            android:id="@+id/button3"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Quit App" />
    
        <Button
            android:id="@+id/button4"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Go third" />
    
        <Button
            android:id="@+id/button5"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Go forth" />
    
        <Button
            android:id="@+id/button7"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Go Fragment" />
    
        <Button
            android:id="@+id/button8"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="File Storage" />
    
        <Button
            android:id="@+id/button9"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="SharedPreferences" />
    
        <Button
            android:id="@+id/button10"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="SQLite" />
    
    </LinearLayout>
    

1. 文件存储

• FileStorage

  • package com.example.helloworld
    
    import android.content.Context
    import android.os.Bundle
    import android.widget.EditText
    import android.widget.Toast
    import java.io.BufferedReader
    import java.io.BufferedWriter
    import java.io.IOException
    import java.io.InputStreamReader
    import java.io.OutputStreamWriter
    
    class FileStorage : BaseActivity() {
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            // 设置 layout
            setContentView(R.layout.file_storage_layout)
            ActivityCollector.addActivity(this)
    
            val editText : EditText = findViewById(R.id.editText)
            val inputText = load()      // 启动时读取数据
            if (inputText.isNotEmpty()) {
                editText.setText(inputText)
                editText.setSelection(inputText.length)
                Toast.makeText(this, "Restoring succeeded", Toast.LENGTH_SHORT).show()
            }
        }
    
        override fun onDestroy() {      // 退出程序时自动保存字符串
            super.onDestroy()
            val editText : EditText = findViewById(R.id.editText)
            val inputText = editText.text.toString()
            save(inputText)
        }
        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()
            }
        }
    
        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()
        }
    }
    

• file_storage_layout

  • <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
    
        <EditText
            android:id="@+id/editText"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="Type something here"
            />
    
    </LinearLayout>
    

• DataPerference

  • package com.example.helloworld
    
    import android.content.Context
    import android.content.Intent
    import android.os.Bundle
    import android.widget.Button
    import android.widget.CheckBox
    import android.widget.EditText
    import android.widget.Toast
    import androidx.core.content.edit
    
    class DataPreference : BaseActivity() {
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.data_perference)
    
            val prefs = getPreferences(Context.MODE_PRIVATE)
            val isRemember = prefs.getBoolean("remember_password", false)
            val accountEdit: EditText = findViewById(R.id.accountEdit)
            val passwordEdit: EditText = findViewById(R.id.passwordEdit)
            val rememberPass: CheckBox = findViewById(R.id.rememberPass)
            val login: Button = findViewById(R.id.login)
            if (isRemember) {
                // 将账号和密码都设置到文本框中
                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()
                // 如果账号是admin且密码是123456，就认为登录成功
                if (account == "admin" && password == "123456") {
                    prefs.edit {
                        if (rememberPass.isChecked) { // 检查复选框是否被选中
                            putBoolean("remember_password", true)
                            putString("account", account)
                            putString("password", password)
                        } else {
                            clear()
                        }
                    }
                    val intent = Intent(this, MainActivity::class.java)
                    startActivity(intent)
                    finish()
                } else {
                    Toast.makeText(this, "account or password is invalid",
                        Toast.LENGTH_SHORT).show()
                }
            }
        }
    
    }
    

• data_perference

  • <?xml version="1.0" encoding="utf-8"?>
    
    
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="60dp">
        <TextView
            android:layout_width="90dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:textSize="18sp"
            android:text="Account:" />
        <EditText
            android:id="@+id/accountEdit"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_gravity="center_vertical" />
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="60dp">
        <TextView
            android:layout_width="90dp"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:textSize="18sp"
            android:text="Password:" />
        <EditText
            android:id="@+id/passwordEdit"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_gravity="center_vertical"
            android:inputType="textPassword" />
    </LinearLayout>
        <LinearLayout
            android:orientation="horizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
            <CheckBox
                android:id="@+id/rememberPass"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textSize="18sp"
                android:text="Remember password" />
        </LinearLayout>
    <Button
        android:id="@+id/login"
        android:layout_width="200dp"
        android:layout_height="60dp"
        android:layout_gravity="center_horizontal"
        android:text="Login" />
    </LinearLayout>
    

3. SQLite

• SqliteDB.kt

  • package com.example.helloworld
    
    import android.content.ContentValues
    import android.content.Context
    import android.database.sqlite.SQLiteDatabase
    import android.database.sqlite.SQLiteOpenHelper
    import android.os.Bundle
    import android.util.Log
    import android.widget.Button
    import android.widget.Toast
    
    
    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 integer)"
        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)
        }
    
        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")
            }
        }
    }
    
    class SqliteDB : BaseActivity() {
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.sql_layout)
            ActivityCollector.addActivity(this)
    
            // 调用 helper 根据版本号创建或者升级数据库
            val dbHelper = MyDatabaseHelper(this, "BookStore.db", 2)
            val createDatabase: Button = findViewById(R.id.createDatabase)
            createDatabase.setOnClickListener {
                dbHelper.writableDatabase
            }
    
            // 添加数据
            val addData: Button = findViewById(R.id.addData)
            addData.setOnClickListener {
                val db = dbHelper.writableDatabase
                val values1 = ContentValues().apply {
                    // 开始组装第一条数据
                    put("name", "The Da Vinci Code")
                    put("author", "Dan Brown")
                    put("pages", 454)
                    put("price", 16.96)
                }
                db.insert("Book", null, values1) // 插入第一条数据
                val values2 = ContentValues().apply {
                    // 开始组装第二条数据
                    put("name", "The Lost Symbol")
                    put("author", "Dan Brown")
                    put("pages", 510)
                    put("price", 19.95)
                }
                db.insert("Book", null, values2) // 插入第二条数据
            }
    
            // 更新数据
            val updateData: Button = findViewById(R.id.updateData)
            updateData.setOnClickListener {
                val db = dbHelper.writableDatabase
                val values = ContentValues()
                values.put("price", 10.99)
                db.update("Book", values, "name = ?", arrayOf("The Da Vinci Code"))
            }
    
            // 删除数据
            val deleteData: Button = findViewById(R.id.deleteData)
            deleteData.setOnClickListener {
                val db = dbHelper.writableDatabase
                db.delete("Book", "pages > ?", arrayOf("500"))
            }
    
            // 查询数据
            val queryData: Button = findViewById(R.id.queryData)
            queryData.setOnClickListener {
                val db = dbHelper.writableDatabase
                // 查询Book表中所有的数据
                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("SqliteDB", "book name is $name")
                        Log.d("SqliteDB", "book author is $author")
                        Log.d("SqliteDB", "book pages is $pages")
                        Log.d("SqliteDB", "book price is $price")
                    } while (cursor.moveToNext())
                }
                cursor.close()
            }
    
            // 使用事务确保原子性进行数据替换
            val replaceData: Button = findViewById(R.id.replaceData)
            replaceData.setOnClickListener {
                val db = dbHelper.writableDatabase
                db.beginTransaction() // 开启事务
                try {
                    db.delete("Book", null, null)
                    if (true) {
                        // 手动抛出一个异常，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)
                    db.setTransactionSuccessful() // 事务已经执行成功
                } catch (e: Exception) {
                    e.printStackTrace()
                } finally {
                    db.endTransaction() // 结束事务
                }
            }
        }
    }
    
    

• sql_layout

  • <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        >
    
        <Button
            android:id="@+id/createDatabase"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Create Database"
            />
    
        <Button
            android:id="@+id/addData"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Add Data"
            />
    
        <Button
            android:id="@+id/updateData"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Update Data"
            />
    
        <Button
            android:id="@+id/deleteData"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Delete Data"
            />
    
        <Button
            android:id="@+id/queryData"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Query Data"
            />
    
        <Button
            android:id="@+id/replaceData"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Replace Data"
            />
    </LinearLayout>
    

// 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)