Android使用SQLiteOpenHelper实现离线浏览

2019-11-08 00:17:54

字体：大中小

来源：转载

供稿：网友

相信在Android开发中，很多APP都会用到数据库来保存数据，今天我们来讨论一下它其中用法，用数据库实现离线浏览。数据库类型很多，Android默认是使用SQLite。

首先我们来看一下界面

这里写图片描述

很简单，经典的图文混排。

要想实现APP离线浏览，我们需要：

创建一个类继承SQLiteOpenHelper(该类帮我们封装好了很多数据库操作)将功能抽象出来创建数据库管理类使用我们创建的数据库来实现离线浏览。

首先我们根据模型类来创建SQLiteOpenHelper

模型类

public class News { PRivate int id; private String title; private String postdate; private String editor; private String icon; public News(int id, String title, String postdate, String editor, String icon) { this.id = id; this.title = title; this.postdate = postdate; this.editor = editor; this.icon = icon; } // getter and setter ...}public class NewsSQLiteOpenHelper extends SQLiteOpenHelper { // 数据库名 private static final String NAME = "learn.db"; // 数据库版本 private static final int VERSION = 1; // 建表语句 private static final String CREATE_NEWS = "create table if not exists news(" + "_id integer primary key," + "title text," + // 这个时间字段只是单纯的为了后面查询时好排序 "time integer," + "icon text," + "editor text," + "postdate text)"; public NewsSQLiteOpenHelper(Context context) { super(context, NAME, null, VERSION); } @Override public void onCreate(SQLiteDatabase db) { db.execSQL(CREATE_NEWS); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { }}

这里主要要实现的方法是onCreate，在里面完成我们的初始化建表工作，构造函数需要传入的四个参数分别为：上下文、数据库名、游标工厂(默认为null)、版本号。onUpgrade方法只会在数据库更新才会调用。这里我们不会用到。

将功能抽象出来

public interface INewsDB { List<News> getAllNews(); void deleteAllNews(); News getNewsById(int id); void insertAllNews(List<News> newsList); void insertNews(News news);}

这里首先说一下为什么要创建这么一个接口，很简单，解耦合，以后维护修改的时候方便。抽象出来的这些功能都是我们后面将用到的，都是基本的增删改查操作。

创建数据库管理类（重点）

public class NewsDBManager implements INewsDB { private static NewsDBManager sManager; private NewsSQLiteOpenHelper mHelper; private NewsDBManager(Context context) { mHelper = new NewsSQLiteOpenHelper(context); } public static NewsDBManager getInstance(Context context) { if (sManager == null) { synchronized (NewsDBManager.class) { if (sManager == null) { sManager = new NewsDBManager(context); } } } return sManager; } /** * 查询所有新闻 * * @return */ @Override public List<News> getAllNews() { SQLiteDatabase db = mHelper.getReadableDatabase(); Cursor cursor = db.rawQuery("select * from news order by time ASC", null); List<News> newsList = new ArrayList<>(); while (cursor.moveToNext()) { int id = cursor.getInt(cursor.getColumnIndex("_id")); String title = cursor.getString(cursor.getColumnIndex("title")); String icon = cursor.getString(cursor.getColumnIndex("icon")); String editor = cursor.getString(cursor.getColumnIndex("editor")); String postdate = cursor.getString(cursor.getColumnIndex("postdate")); News news = new News(id, title, postdate, editor, icon); newsList.add(news); } mHelper.close(); return newsList; } /** * 删除所有新闻 */ @Override public void deleteAllNews() { SQLiteDatabase db = mHelper.getWritableDatabase(); db.execSQL("delete from news"); mHelper.close(); } /** * 通过给定的id获取新闻 * * @param id * @return */ @Override public News getNewsById(int id) { SQLiteDatabase db = mHelper.getReadableDatabase(); Cursor cursor = db.rawQuery("select * from news where _id = ?", new String[]{String.valueOf(id)}); cursor.moveToNext(); String title = cursor.getString(cursor.getColumnIndex("title")); String icon = cursor.getString(cursor.getColumnIndex("icon")); String editor = cursor.getString(cursor.getColumnIndex("editor")); String postdate = cursor.getString(cursor.getColumnIndex("postdate")); News news = new News(id, title, postdate, editor, icon); mHelper.close(); return news; } @Override public void insertAllNews(List<News> newsList) { for (News news : newsList) { insertNews(news); } } @Override public void insertNews(News news) { SQLiteDatabase db = mHelper.getWritableDatabase(); db.execSQL("insert into news(_id,title,time,icon,editor,postdate) values(?,?,?,?,?,?)", new Object[]{news.getId(), news.getTitle(), System.currentTimeMillis(), news.getIcon(), news.getEditor(), news.getPostdate()}); }}

解析一下代码：这里我们创建一个News数据库管理类，专门进行数据库的增删改查操作，实现INewsDB抽象的功能。增删改查的代码就不多解释了，这里注意一下delete操作，Android的delete有一个蜜汁bug，本来数据库的删除操作应该是delete * from table，但是这个 * 无法通过编译，所以只能delete from table。

实际运用

好了，终于到最后一步了，有木有很激动。

private void initView() { mNewsRv = ((RecyclerView) findViewById(R.id.rv_news)); mLoading = findViewById(R.id.pb_loading); LinearLayoutManager layout = new LinearLayoutManager(this); mNewsRv.setLayoutManager(layout); mAdapter = new NewsAdapter(this, null); mNewsRv.setAdapter(mAdapter); mLoading.setVisibility(View.VISIBLE); // 加载数据的时候先检测网络 if (NetworkUtils.checkNetworkState(this)) { Log.d(TAG, "initView: 从网络获取数据"); getDataFromInternet(); } else { Log.d(TAG, "initView: 从数据库获取数据"); getDataFromDB(); } } private void getDataFromInternet() { HttpUtils.getAsynString(Urls.URL_NEWS_ITEM_SOFTWARE, new HttpUtils.ResultCallback() { @Override public void onFailure(Exception e) { Toast.makeText(MainActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show(); } @Override public void onFinish(String result) { Gson gson = new Gson(); List<News> data = gson.fromJson(result, new TypeToken<List<News>>() { }.getType()); mAdapter.update(data); NewsDBManager manager = NewsDBManager.getInstance(MainActivity.this); // 删除旧数据，保存最新数据 manager.deleteAllNews(); manager.insertAllNews(data); mLoading.setVisibility(View.GONE); } }); } private void getDataFromDB() { NewsDBManager manager = NewsDBManager.getInstance(this); List<News> data = manager.getAllNews(); if (data.size() > 0) { mAdapter.update(data); } mLoading.setVisibility(View.GONE); }