一、Room是什么？

Room是Jetpack組件中一個物件關系映射（ORM）庫，可以很容易將 SQLite 表資料轉換為 Java 物件，
Room 在 SQLite 上提供了一個抽象層，以便在充分利用 SQLite 的強大功能的同時，能夠流暢地訪問資料庫，
支持與LiveData、RxJava、Kotlin協成組合使用，
Google 官方強烈推薦使用Room，

二、使用步驟

1.引入庫

代碼如下（示例）：

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    //下面這兩句不要忘記加不然會報錯
    id 'kotlin-kapt'
    id 'kotlin-android-extensions'
}

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.example.practice"
        minSdkVersion 25
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
}

dependencies {

    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation 'androidx.core:core-ktx:1.2.0'
    implementation 'androidx.appcompat:appcompat:1.3.0'
    implementation 'com.google.android.material:material:1.3.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

    //下面的庫根據自己需求添加
    def room_version = "2.3.0"
    implementation("androidx.room:room-runtime:$room_version")
    annotationProcessor "androidx.room:room-compiler:$room_version"

    // To use Kotlin annotation processing tool (kapt)
    kapt("androidx.room:room-compiler:$room_version")
    // To use Kotlin Symbolic Processing (KSP)
    kapt("androidx.room:room-compiler:$room_version")

    // optional - Kotlin Extensions and Coroutines support for Room
    implementation("androidx.room:room-ktx:$room_version")

    // optional - RxJava2 support for Room
    implementation "androidx.room:room-rxjava2:$room_version"

    // optional - RxJava3 support for Room
    implementation "androidx.room:room-rxjava3:$room_version"

    // optional - Guava support for Room, including Optional and ListenableFuture
    implementation "androidx.room:room-guava:$room_version"

    // optional - Test helpers
    testImplementation("androidx.room:room-testing:$room_version")
}

2.@Entity 資料庫中表對應的物體

代碼如下（示例）：

package com.example.practice;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

/**
 * @description:
 * @author: 黃劍橋
 * @email: 1437337158@qq.com
 * @date :   2021/8/12 11:45
 */
@Entity
public class Table {
    @PrimaryKey(autoGenerate = true)
    private  int id;
    @ColumnInfo(name = "name")
    private String name;
    @ColumnInfo(name = "work")
    private String work;
    @ColumnInfo(name = "attendance")
    private int attendance;
    @ColumnInfo(name = "pay")
    private int pay;

    public Table(String name, String work, int attendance, int pay) {
        this.name = name;
        this.work = work;
        this.attendance = attendance;
        this.pay = pay;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getWork() {
        return work;
    }

    public void setWork(String work) {
        this.work = work;
    }

    public int getAttendance() {
        return attendance;
    }

    public void setAttendance(int attendance) {
        this.attendance = attendance;
    }

    public int getPay() {
        return pay;
    }

    public void setPay(int pay) {
        this.pay = pay;
    }
}

我以一個員工的工資單為例子，

3.Dao： 操作資料庫的方法

package com.example.practice;

import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;


/**
 * @author 黃劍橋
 */
@Dao //database access object
public interface WordDao {
   //插入  引數Table代表上面自己創建的資料庫中表對應的物體  引數中間三個 ... 代表可以加很多
    @Insert
    void insertWord(Table...tables);
    //更新
    @Update
    void updateWord(Table...tables);
    //洗掉指定的
    @Delete
    void deleteWord(Table...tables);
    //洗掉所有
    @Query("DELETE FROM `Table`")
    void deleteAllWord();
    //就是從名字叫做jdal的表里面選出所有資訊，并且把選出來的資訊按照id降序排列的順序排列出來
    @Query("SELECT * FROM `Table` ORDER BY ID DESC")
    List<Table> getAllWords();
}

4. 創建Database

package com.example.practice;
import androidx.room.Database;
import androidx.room.RoomDatabase;


/**
 * @author 黃劍橋
 */ //singleton
@Database(entities = {Table.class}, version = 1, exportSchema = false)
public abstract class WordDatabase extends RoomDatabase {
    public abstract WordDao getWordDao();
}

5. 簡單的操作

package com.example.practice;

import androidx.appcompat.app.AppCompatActivity;
import androidx.room.Room;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.List;

/**
 * @author 黃劍橋
 */
public class MainActivity2 extends AppCompatActivity {
    WordDatabase wordDatabase;
    WordDao wordDao;
    private TextView showTv;
    private Button clearBtn;
    private Button insertBtn;
    private Button deleteBtn;
    private Button updateBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        //初始化
        wordDatabase=Room.databaseBuilder(this,WordDatabase.class,"word")
        //資料庫操作比較耗時主要目的是熟悉資料庫Room這句話代替了切換執行緒問題
                .allowMainThreadQueries()
                .build();
        wordDao = wordDatabase.getWordDao();
        showTv = (TextView)findViewById(R.id.textView2);
        clearBtn = (Button)findViewById(R.id.button);
        insertBtn = (Button)findViewById(R.id.button2);
        deleteBtn = (Button)findViewById(R.id.button3);
        updateBtn = (Button)findViewById(R.id.button5);
        updateView();
        event();
    }

    private void event() {
        //插入
        insertBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //傳入對應資料
                Table table = new Table("張三", "web工程師", 25, 30000);
                wordDao.insertWord(table);
                updateView();
            }
        });
        //清除
        clearBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
              //清除所有資料
                wordDao.deleteAllWord();
                updateView();
            }
        });
        //修改
        updateBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
            修改第20條資料為
                Table table = new Table("張三", "web工程師",1,20);
                table.setId(20);
                wordDao.updateWord(table);
                updateView();
            }
        });
        //洗掉指定
        deleteBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
            洗掉第6條資料
                Table table = new Table("張三", "web工程師",1,20);
                table.setId(6);
                wordDao.deleteWord(table);
                updateView();
            }
        });

    }

    void updateView(){
        List<Table> list = wordDao.getAllWords();
        String text="";
        for (int i=0;i<list.size();i++){
            Table table = list.get(i);
            //這種方式這是簡單演示字串拼接不建議這樣搞   建議StringBuilder() 
            text +=table.getId()+"："+"姓名："+table.getName()+"\t"+"職位："+table.getWork()+"\t"+"出勤天數："+table.getAttendance()+"\t"+"工資："+table.getPay()+"\n";
        }
        showTv.setText(text);
    }

}

總結

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