2022-11-8学习内容

1.案例-购物车-商品列表展示

1.1MyApplication.java

package com.example.chapter06;

import android.app.Application;
import android.content.res.Configuration;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Environment;
import android.util.Log;

import androidx.annotation.NonNull;
import androidx.room.Room;

import com.example.chapter06.database.BookDatabase;
import com.example.chapter06.database.ShoppingDBHelper;
import com.example.chapter06.entity.GoodsInfo;
import com.example.chapter06.util.FileUtil;
import com.example.chapter06.util.SharedUtil;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;

public class MyApplication extends Application {

    private static MyApplication myApp;
    // 声明一个公共的信息映射对象，可当做全局变量使用
    public HashMap<String, String> infoMap = new HashMap<>();

    // 声明一个书籍数据库对象
    private BookDatabase bookDatabase;

    public static MyApplication getInstance() {
        return myApp;
    }

    // 在APP启动时调用
    @Override
    public void onCreate() {
        super.onCreate();
        myApp = this;
        Log.d("ning", "MyApplication onCreate");

        // 构建书籍数据库的实例
        bookDatabase = Room.databaseBuilder(this, BookDatabase.class, "book")
                // 允许迁移数据库（发生数据库变更时，Room默认删除原数据库再创建新数据库，如此一来原来的记录会丢失，故而要改为迁移方式以便保存原有记录
                .addMigrations()
                // 允许在主线程中操作数据库（Room默认不能在主线程中操作数据库）
                .allowMainThreadQueries()
                .build();

        // 初始化商品信息
        initGoodsInfo();
    }

    private void initGoodsInfo() {
        // 获取共享参数保存的是否首次打开参数
        boolean isFirst = SharedUtil.getInstance(this).readBoolean("first", true);
        // 获取当前APP的私有下载路径
        String directory = getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS).toString() + File.separatorChar;
        if (isFirst) {
            // 模拟网络图片下载
            ArrayList<GoodsInfo> list = GoodsInfo.getDefaultList();
            for (GoodsInfo info : list) {
                Bitmap bitmap = BitmapFactory.decodeResource(getResources(), info.pic);
                String path = directory + info.id + ".jpg";
                // 往存储卡保存商品图片
                FileUtil.saveImage(path, bitmap);
                // 回收位图对象
                bitmap.recycle();
                info.picPath = path;
            }
            // 打开数据库，把商品信息插入到表中
            ShoppingDBHelper dbHelper = ShoppingDBHelper.getInstance(this);
            dbHelper.openWriteLink();
            dbHelper.insertGoodsInfos(list);
            dbHelper.closeLink();
            // 把是否首次打开写入共享参数
            SharedUtil.getInstance(this).writeBoolean("first", false);
        }
    }

    // 在APP终止时调用
    @Override
    public void onTerminate() {
        super.onTerminate();
        Log.d("ning", "onTerminate");
    }

    // 在配置改变时调用，例如从竖屏变为横屏
    @Override
    public void onConfigurationChanged(@NonNull Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        Log.d("ning", "onConfigurationChanged");
    }

    // 获取书籍数据库的实例
    public BookDatabase getBookDB() {
        return bookDatabase;
    }
}

1.2SharedUtil.java

package com.example.chapter06.util;

import android.content.Context;
import android.content.SharedPreferences;

public class SharedUtil {

    private static SharedUtil mUtil;
    private SharedPreferences preferences;

    public static SharedUtil getInstance(Context ctx) {
        if (mUtil == null) {
            mUtil = new SharedUtil();
            mUtil.preferences = ctx.getSharedPreferences("shopping", Context.MODE_PRIVATE);
        }
        return mUtil;
    }

    public void writeBoolean(String key, boolean value) {
        SharedPreferences.Editor editor = preferences.edit();
        editor.putBoolean(key, value);
        editor.commit();
    }

    public boolean readBoolean(String key, boolean defaultValue) {
        return preferences.getBoolean(key, defaultValue);
    }
}

1.3ShoppingChannelActivity.java

package com.example.chapter06;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import com.example.chapter06.database.ShoppingDBHelper;

public class ShoppingChannelActivity extends AppCompatActivity {

    // 声明一个商品数据库的帮助器对象
    private ShoppingDBHelper mDBHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_shopping_channel);
        mDBHelper = ShoppingDBHelper.getInstance(this);
        mDBHelper.openReadLink();
        mDBHelper.openWriteLink();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mDBHelper.closeLink();
    }
}