《第一行代码》第七章 数据持久化

7.2 文件存储

7.2.1 将数据存储到文件中

输入一些信息，然后按back键退出.

按下快捷键 Ctrl + Shift + A, 在搜索框输入Device File Explorer (或者在Android Studio右下角找) 在data/data/com.example.<项目名称>/files/ 的目录下可以找到一个data文件。

7.2.2 从文件中读取数据

退出后重新进入
MainActivity.kt

package com.example.persistence_7_1

import android.content.Context
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_main.*
import java.io.*
import java.lang.StringBuilder

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val inputText = load()
        if (inputText.isNotEmpty()) {
            editText.setText(inputText)
            editText.setSelection(inputText.length)
            Toast.makeText(this, "Restoring succeeded", Toast.LENGTH_SHORT).show()
        }
    }

    override fun onDestroy() {
        super.onDestroy()
        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()
    }
}

activity_main.xml和7.2.1一样

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

   <EditText
       android:id="@+id/editText"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:hint="Type something here"
       tools:ignore="HardcodedText" />

</androidx.constraintlayout.widget.ConstraintLayout>

7.3 SharedPreferences存储

7.3.1-7.3.2 SharedPreferences存储和读取数据

点击save data按钮
在/data/data/com.example.<项目名>/shared_preds/ 目录下

点击restore data按钮

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

   <Button
      android:id="@+id/saveButton"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:text="Save Data"
       tools:ignore="HardcodedText" />

   <Button
      android:id="@+id/restoreButton"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
      android:text="Restore Data"
       tools:ignore="HardcodedText" />

</LinearLayout>

MainActivity.kt

package com.example.persistence_7_1

import android.annotation.SuppressLint
import android.content.Context
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_main.*
import java.io.*
import java.lang.StringBuilder

class MainActivity : AppCompatActivity() {

    @SuppressLint("CommitPrefEdits")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        saveButton.setOnClickListener{
            val editor = getSharedPreferences("data", Context.MODE_PRIVATE).edit()
            editor.putString("name", "Tom")
            editor.putInt("age", 28)
            editor.putBoolean("married", false)
            editor.apply()
            Toast.makeText(this, "Save data successfully", Toast.LENGTH_SHORT).show()
        }

        restoreButton.setOnClickListener{
            val prefs = getSharedPreferences("data", Context.MODE_PRIVATE)
            val name = prefs.getString("name", "")
            val age = prefs.getInt("age", 0)
            val married = prefs.getBoolean("married", false)
            Log.d("MainActivity", "name is $name")
            Log.d("MainActivity", "age is $age")
            Log.d("MainActivity", "married is $married")
        }
    }
}

7.3.3 实现记住用户名和密码的功能

这里我们拿6.4强制实现下线功能的项目再进行改进.先看看效果

退出后会发现用户名密码都在，然后复选框也被选上了。
主要改动的两个文件的代码:
activity_login.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    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:"
            tools:ignore="HardcodedText" />

        <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"
            tools:ignore="HardcodedText" />
    </LinearLayout>

    <Button
        android:id="@+id/login"
        android:layout_width="200dp"
        android:layout_height="60dp"
        android:layout_gravity="center_horizontal"
        android:text="Login"
        tools:ignore="HardcodedText" />
</LinearLayout>

LoginActivity.kt

package com.example.broadcasttest

import android.content.Context
import android.content.Intent
import android.os.Bundle
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_login.*


class LoginActivity : BaseActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        // 7.3.3 更改部分1（共两部分）start
        val prefs = getPreferences(Context.MODE_PRIVATE)
        val isRemember = prefs.getBoolean("remember_password", false)
        if(isRemember) {
            // 获取已经保存的账号和密码
            val account = prefs.getString("account", "")
            val password = prefs.getString("password", "")
            // 账号和密码都设置到文本框里面
            accountEdit.setText(account)
            passwordEdit.setText(password)
            // 将记住密码的checkBox选上
            rememberPass.isChecked = true
        }
        // end of updated part1

        login.setOnClickListener {
            val account = accountEdit.text.toString()
            val password = passwordEdit.text.toString()
            //用户名是admin，密码是123456
            if (account == "admin" && password == "123456"){
                // 7.3.3 更改部分2（共两部分）start
                val editor = prefs.edit()
                // 检查复选框是否被选中
                if(rememberPass.isChecked){
                    // 将记住密码的状态和用户名密码保存起来，实际开发项目中还要加密
                    editor.putBoolean("remember_password", true)
                    editor.putString("account", account)
                    editor.putString("password", password)
                } else {
                    editor.clear()
                }
                editor.apply()
                // end of updated part2

                val intent = Intent(this, MainActivity::class.java)
                startActivity(intent)
                finish()
            } else {
                Toast.makeText(this, "account or password is invalid", Toast.LENGTH_SHORT).show()
            }
        }

    }
}

7.4 SQLite 数据库存储

7.4.1 创建数据库

本文地址：https://blog.csdn.net/weixin_43850253/article/details/107567633