您可以通过多种方式存储数据，例如存储到在线数据库、本地 SQLite 数据库甚至是文本文件中。您可以自行决定最适合您的应用的方案。本课程介绍了如何创建能够提供强大的在全文内搜索功能的 SQLite 虚拟表。该表会填充文本文件（文件中的每一行均包含一个字词和定义对）中的数据。
创建虚拟表
虚拟表的行为方式与 SQLite 表类似，但它可以通过回调在内存（而不是数据库文件）中读取和写入对象。要创建虚拟表，请为它创建一个类：
Kotlin
class DatabaseTable(context: Context) {
private val databaseOpenHelper = DatabaseOpenHelper(context)
}
Java
public class DatabaseTable {
private final DatabaseOpenHelper databaseOpenHelper;
public DatabaseTable(Context context) {
databaseOpenHelper = new DatabaseOpenHelper(context);
}
}
在 DatabaseTable 中创建一个扩展 SQLiteOpenHelper 的内部类。SQLiteOpenHelper 类定义了您必须替换的抽象方法，以便在必要时可以创建和升级您的数据库表。例如，以下代码声明了一个数据库表，其中包含某字典应用的字词：
Kotlin
private const val TAG = "DictionaryDatabase"
//The columns we'll include in the dictionary table
const val COL_WORD = "WORD"
const val COL_DEFINITION = "DEFINITION"
private const val DATABASE_NAME = "DICTIONARY"
private const val FTS_VIRTUAL_TABLE = "FTS"
private const val DATABASE_VERSION = 1
private const val FTS_TABLE_CREATE =
"CREATE VIRTUAL TABLE $FTS_VIRTUAL_TABLE USING fts3 ($COL_WORD, $COL_DEFINITION)"
class DatabaseTable(context: Context) {
private val databaseOpenHelper: DatabaseOpenHelper
init {
databaseOpenHelper = DatabaseOpenHelper(context)
}
private class DatabaseOpenHelper internal constructor(private val helperContext: Context) :
SQLiteOpenHelper(helperContext, DATABASE_NAME, null, DATABASE_VERSION) {
private lateinit var mDatabase: SQLiteDatabase
override fun onCreate(db: SQLiteDatabase) {
mDatabase = db
mDatabase.execSQL(FTS_TABLE_CREATE)
}
override fun onUpgrade(db: SQLiteDatabase, oldVersion: Int, newVersion: Int) {
Log.w(
TAG,
"Upgrading database from version $oldVersion to $newVersion , which will " +
"destroy all old data"
)
db.execSQL("DROP TABLE IF EXISTS $FTS_VIRTUAL_TABLE")
onCreate(db)
}
}
}
Java
public class DatabaseTable {
private static final String TAG = "DictionaryDatabase";
//The columns we'll include in the dictionary table
public static final String COL_WORD = "WORD";
public static final String COL_DEFINITION = "DEFINITION";
private static final String DATABASE_NAME = "DICTIONARY";
private static final String FTS_VIRTUAL_TABLE = "FTS";
private static final int DATABASE_VERSION = 1;
private final DatabaseOpenHelper databaseOpenHelper;
public DatabaseTable(Context context) {
databaseOpenHelper = new DatabaseOpenHelper(context);
}
private static class DatabaseOpenHelper extends SQLiteOpenHelper {
private final Context helperContext;
private SQLiteDatabase mDatabase;
private static final String FTS_TABLE_CREATE =
"CREATE VIRTUAL TABLE " + FTS_VIRTUAL_TABLE +
" USING fts3 (" +
COL_WORD + ", " +
COL_DEFINITION + ")";
DatabaseOpenHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
helperContext = context;
}
@Override
public void onCreate(SQLiteDatabase db) {
mDatabase = db;
mDatabase.execSQL(FTS_TABLE_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
+ newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS " + FTS_VIRTUAL_TABLE);
onCreate(db);
}
}
}
填充虚拟表
现在该表需要存储的数据。以下代码展示了如何读取一个包含字词及其定义的文本文件（位于 res/raw/definitions.txt 中），如何解析该文件，以及如何将该文件的每一行插入为虚拟表中的行。所有这些均在另一个线程中完成，以防止界面被锁定。将以下代码添加到您的 DatabaseOpenHelper 内部类中。
提示：您可能还需要设置一个回调，以在该线程完成时通知您的界面 Activity。
Kotlin
private fun loadDictionary() {
Thread(Runnable {
try {
loadWords()
} catch (e: IOException) {
throw RuntimeException(e)
}
}).start()
}
@Throws(IOException::class)
private fun loadWords() {
val inputStream = helperContext.resources.openRawResource(R.raw.definitions)
BufferedReader(InputStreamReader(inputStream)).use { reader ->
var line: String? = reader.readLine()
while (line != null) {
val strings: List<String> = line.split("-").map { it.trim() }
if (strings.size < 2) continue
val id = addWord(strings[0], strings[1])
if (id < 0) {
Log.e(TAG, "unable to add word: ${strings[0]}")
}
line = reader.readLine()
}
}
}
fun addWord(word: String, definition: String): Long {
val initialValues = ContentValues().apply {
put(COL_WORD, word)
put(COL_DEFINITION, definition)
}
return database.insert(FTS_VIRTUAL_TABLE, null, initialValues)
}
Java
private void loadDictionary() {
new Thread(new Runnable() {
public void run() {
try {
loadWords();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}).start();
}
private void loadWords() throws IOException {
final Resources resources = helperContext.getResources();
InputStream inputStream = resources.openRawResource(R.raw.definitions);
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
try {
String line;
while ((line = reader.readLine()) != null) {
String[] strings = TextUtils.split(line, "-");
if (strings.length < 2) continue;
long id = addWord(strings[0].trim(), strings[1].trim());
if (id < 0) {
Log.e(TAG, "unable to add word: " + strings[0].trim());
}
}
} finally {
reader.close();
}
}
public long addWord(String word, String definition) {
ContentValues initialValues = new ContentValues();
initialValues.put(COL_WORD, word);
initialValues.put(COL_DEFINITION, definition);
return database.insert(FTS_VIRTUAL_TABLE, null, initialValues);
}
在适当的位置调用 loadDictionary() 方法以填充表。建议的位置是创建表后在 DatabaseOpenHelper 类的 onCreate() 方法中：
Kotlin
override fun onCreate(db: SQLiteDatabase) {
database = db
database.execSQL(FTS_TABLE_CREATE)
loadDictionary()
}
Java
@Override
public void onCreate(SQLiteDatabase db) {
database = db;
database.execSQL(FTS_TABLE_CREATE);
loadDictionary();
}
搜索查询
创建并填充虚拟表后，可以使用您的 SearchView 提供的查询来搜索数据。将以下方法添加到 DatabaseTable 类，以构建一个用于搜索查询的 SQL 语句：
Kotlin
fun getWordMatches(query: String, columns: Array<String>?): Cursor? {
val selection = "$COL_WORD MATCH ?"
val selectionArgs = arrayOf("$query*")
return query(selection, selectionArgs, columns)
}
private fun query(
selection: String,
selectionArgs: Array<String>,
columns: Array<String>?
): Cursor? {
val cursor: Cursor? = SQLiteQueryBuilder().run {
tables = FTS_VIRTUAL_TABLE
query(databaseOpenHelper.readableDatabase,
columns, selection, selectionArgs, null, null, null)
}
return cursor?.run {
if (!moveToFirst()) {
close()
null
} else {
this
}
} ?: null
}
Java
public Cursor getWordMatches(String query, String[] columns) {
String selection = COL_WORD + " MATCH ?";
String[] selectionArgs = new String[] {query+"*"};
return query(selection, selectionArgs, columns);
}
private Cursor query(String selection, String[] selectionArgs, String[] columns) {
SQLiteQueryBuilder builder = new SQLiteQueryBuilder();
builder.setTables(FTS_VIRTUAL_TABLE);
Cursor cursor = builder.query(databaseOpenHelper.getReadableDatabase(),
columns, selection, selectionArgs, null, null, null);
if (cursor == null) {
return null;
} else if (!cursor.moveToFirst()) {
cursor.close();
return null;
}
return cursor;
}
通过调用 getWordMatches() 搜索查询。所有匹配的结果都会在一个 Cursor 中返回，您可以遍历该 Cursor 或用其构建 ListView。此示例在可搜索 Activity 的 handleIntent() 方法中调用 getWordMatches()。请注意，由于您之前创建的 intent 过滤器，可搜索 Activity 会将 ACTION_SEARCH intent 内部的查询接收为 extra：
Kotlin
private val db = DatabaseTable(this)
...
private fun handleIntent(intent: Intent) {
if (Intent.ACTION_SEARCH == intent.action) {
val query = intent.getStringExtra(SearchManager.QUERY)
val c = db.getWordMatches(query, null)
//process Cursor and display results
}
}
Java
DatabaseTable db = new DatabaseTable(this);
...
private void handleIntent(Intent intent) {
if (Intent.ACTION_SEARCH.equals(intent.getAction())) {
String query = intent.getStringExtra(SearchManager.QUERY);
Cursor c = db.getWordMatches(query, null);
//process Cursor and display results
}
}