Pro Android學習筆記（二）——ContentProvider（下）
阿新 • • 發佈：2018-12-31
上一章講解了ContentProvider的一些基本概念和涉及到的知識。這一章就來實現它。
要實現ContentProvider，我們繼承自ContentProvider這個抽象類，實現其中的抽象方法就可以了，其中的抽象方法包括：
query
insert
update
delete
getType
在實現他們之前需要大量的設定。按照以下步驟設計：
（1)計劃資料庫
（2）擴充套件抽象類ContentProvider
（3）實現方法：query，insert，update，delete，getType
（4）描述檔案中註冊程式
OK，按照以上步驟按部就班
（1）計劃資料庫
建立一個圖書資料庫，包含一張books表，表中的列包括name，isbn和author。這些列名對應著元資料，相關的元資料在java類中定義。定義的元資料java類為BookProviderMetaData。如下：
public class BookProviderMetaData {
public static final String AUTHORITY = "com.zxn.BookProvider";
public static final String DATABASE_NAME = "book.db";
public static final int DATABASE_VERSION = 1;
public static final String BOOKS_TABLE_NAME = "books";
private BookProviderMetaData BookProviderMetaData類首先定義其授權為“com.zxn.BookProvider”，我們就可以使用這個字串在Android的描述檔案中註冊這段程式了。這段程式形成了讓這段程式使用的URI的前面部分。就像這樣：
content://com.zxn.BookProvider/books
我們可以使用一下常量標識：
BookProviderMetaData.BookTableMetaData.CONTENT_URI
BookTableMetaData類定義了圖書合集和一本圖書的MIME型別，然後定義了一組列：name，isbn，author，created，modified。
BookTableMetaData繼承自BaseColumns,提供了標準的_id欄位。
有了這些元資料之後，我們就能實現我們的程式了。
（2）拓展ContentProvider
拓展ContentProvider，重寫onCreate()來建立資料庫，然後實現query，insert，update，delete，getType等方法。
程式如下：
public class BookProvider extends ContentProvider {
// Logging helper tag. No significance to providers.
private static final String TAG = "BookProvider";
// 定義一個投影對映以便我們可以重新命名列，這裡均設定的相同
// Projection maps are similar to "as" construct in an sql statement where by you can rename the Columns;
private static HashMap<String, String> sBooksProjectionMap;
static {
sBooksProjectionMap = new HashMap<String, String>();
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData._ID, BookProviderMetaData.BookTableMetaData._ID);
// name, isbn, author
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData.BOOK_NAME, BookProviderMetaData.BookTableMetaData.BOOK_NAME);
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData.BOOK_ISBN, BookProviderMetaData.BookTableMetaData.BOOK_ISBN);
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData.BOOK_AUTHOR, BookProviderMetaData.BookTableMetaData.BOOK_AUTHOR);
// created date, modified date
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData.CREATED_DATE, BookProviderMetaData.BookTableMetaData.CREATED_DATE);
sBooksProjectionMap.put(BookProviderMetaData.BookTableMetaData.MODIFIED_DATE, BookProviderMetaData.BookTableMetaData.MODIFIED_DATE);
}
// Provide a mechanism to identify
// all the incoming uri patterns
private static final UriMatcher sUriMatcher; // 定義一個URI匹配器，用於之後的URI匹配
private static final int INCOMING_BOOK_COLLECTION_URI_INDICATOR = 1;
private static final int INCOMING_SINGLE_BOOK_URI_INDICATOR = 2;
static {
sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
sUriMatcher.addURI(BookProviderMetaData.AUTHORITY, "books", INCOMING_BOOK_COLLECTION_URI_INDICATOR);
sUriMatcher.addURI(BookProviderMetaData.AUTHORITY, "books/#", INCOMING_SINGLE_BOOK_URI_INDICATOR);
}
// 定義一個DatabaseHelper以便我們能夠方便的開啟，建立，更新資料庫檔案
// This class helps open, create, and upgrade the database file
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, BookProviderMetaData.DATABASE_NAME, null, BookProviderMetaData.DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
Log.d(TAG, "inner oncreate called");
sqLiteDatabase.execSQL("CREATE TABLE " + BookProviderMetaData.BookTableMetaData.TABLE_NAME + " ("
+ BookProviderMetaData.BookTableMetaData._ID + " INTEGER PRIMARY KEY,"
+ BookProviderMetaData.BookTableMetaData.BOOK_NAME + " TEXT,"
+ BookProviderMetaData.BookTableMetaData.BOOK_ISBN + " TEXT,"
+ BookProviderMetaData.BookTableMetaData.BOOK_AUTHOR + " TEXT,"
+ BookProviderMetaData.BookTableMetaData.CREATED_DATE + " INTEGER,"
+ BookProviderMetaData.BookTableMetaData.MODIFIED_DATE + " INTEGER"
+ ");");
}
@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i2) {
Log.d(TAG, "inner onupgrade called");
Log.w(TAG, "Upgrading database from version" + i + " to " + i2 + ", which will destroy all old data");
sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + BookProviderMetaData.BookTableMetaData.TABLE_NAME);
onCreate(sqLiteDatabase);
}
}
private DatabaseHelper mOpenHelper;
@Override
public boolean onCreate() {
Log.d(TAG, "main onCreate called");
mOpenHelper = new DatabaseHelper(getContext());
return true;
}
@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
switch(sUriMatcher.match(uri)) { // 匹配URI
case INCOMING_BOOK_COLLECTION_URI_INDICATOR: // 如果是圖書集合
qb.setTables(BookProviderMetaData.BookTableMetaData.TABLE_NAME); // 設定查詢表
qb.setProjectionMap(sBooksProjectionMap); // 設定查詢的投影對映，官方說明是：投影對映將從呼叫方傳入查詢的列名稱對映到資料庫列名稱。
// 這對於重新命名列以及在進行聯結時消除列名稱歧義非常有用。
break;
case INCOMING_SINGLE_BOOK_URI_INDICATOR: // 如果是單個圖書
qb.setTables(BookProviderMetaData.BookTableMetaData.TABLE_NAME);
qb.setProjectionMap(sBooksProjectionMap);
qb.appendWhere(BookProviderMetaData.BookTableMetaData._ID + "=" + uri.getPathSegments().get(1)); // 給一些條件到查詢中的Where子句
break;
default:
throw new IllegalArgumentException("Unknow URI" + uri); // 丟擲引數異常
}
// if no sort order is specified use the default
String orderBy;
if(TextUtils.isEmpty(sortOrder)) {
orderBy = BookProviderMetaData.BookTableMetaData.DEFAULT_SORT_ORDER;
} else {
orderBy = sortOrder;
}
// Get the databases and run query
SQLiteDatabase db = mOpenHelper.getReadableDatabase();
Cursor c = qb.query(db, projection, selection, selectionArgs, null, null, orderBy); // 進行查詢操作，返回當前查詢結果的cursor
// example of getting a count
int i = c.getCount();
// Tell the cursor what uri to watch
// so it knows when its source data changes
c.setNotificationUri(getContext().getContentResolver(), uri);
return c;
}
@Override
public String getType(Uri uri) {
switch(sUriMatcher.match(uri)) {
case INCOMING_BOOK_COLLECTION_URI_INDICATOR:
return BookProviderMetaData.BookTableMetaData.CONTENT_TYPE; // 返回圖書集合URI的MIME型別
case INCOMING_SINGLE_BOOK_URI_INDICATOR:
return BookProviderMetaData.BookTableMetaData.CONTENT_ITEM_TYPE; // 返回單個圖書URI的MIME型別
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
}
@Override
public Uri insert(Uri uri, ContentValues contentValues) {
// Validate the requested uri
if(sUriMatcher.match(uri) != INCOMING_BOOK_COLLECTION_URI_INDICATOR) {
throw new IllegalArgumentException("Unknown URI " + uri);
}
// contentValues可以儲存一個鍵值對映用於ContentResolver處理
ContentValues values;
if(contentValues != null) {
values = new ContentValues(contentValues);
} else {
values = new ContentValues();
}
Long now = Long.valueOf(System.currentTimeMillis());
// 插入操作要保證每個列必須存在
// Make sure that the fields are all set
if(values.containsKey(BookProviderMetaData.BookTableMetaData.CREATED_DATE) == false) {
values.put(BookProviderMetaData.BookTableMetaData.CREATED_DATE, now);
}
if(values.containsKey(BookProviderMetaData.BookTableMetaData.MODIFIED_DATE) == false) {
values.put(BookProviderMetaData.BookTableMetaData.MODIFIED_DATE, now);
}
if(values.containsKey(BookProviderMetaData.BookTableMetaData.BOOK_NAME) == false) {
throw new SQLException("Failed to insert row because Book Name is needed " + uri);
}
if(values.containsKey(BookProviderMetaData.BookTableMetaData.BOOK_ISBN) == false) {
values.put(BookProviderMetaData.BookTableMetaData.BOOK_ISBN, "Unknown ISBN");
}
if(values.containsKey(BookProviderMetaData.BookTableMetaData.BOOK_AUTHOR) == false) {
values.put(BookProviderMetaData.BookTableMetaData.BOOK_AUTHOR, "Unknown Author");
}
SQLiteDatabase db = mOpenHelper.getWritableDatabase();
long rowId = db.insert(BookProviderMetaData.BookTableMetaData.TABLE_NAME, BookProviderMetaData.BookTableMetaData.BOOK_NAME, values);
if(rowId > 0) {
Uri insertedBookUri = ContentUris.withAppendedId(BookProviderMetaData.BookTableMetaData.CONTENT_URI, rowId);
getContext().getContentResolver().notifyChange(insertedBookUri, null);
return insertedBookUri;
}
throw new SQLException("Failed to insert row into " + uri);
}
@Override
public int delete(Uri uri, String where, String[] whereArgs) {
SQLiteDatabase db = mOpenHelper.getWritableDatabase();
int count;
switch(sUriMatcher.match(uri)) {
case INCOMING_BOOK_COLLECTION_URI_INDICATOR:
count = db.delete(BookProviderMetaData.BookTableMetaData.TABLE_NAME, where, whereArgs);
break;
case INCOMING_SINGLE_BOOK_URI_INDICATOR:
String rowId = uri.getPathSegments().get(1);
count = db.delete(BookProviderMetaData.BookTableMetaData.TABLE_NAME, BookProviderMetaData.BookTableMetaData._ID + "=" + rowId
+ (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
getContext().getContentResolver().notifyChange(uri, null);
return count;
}
@Override
public int update(Uri uri, ContentValues values, String where, String[] whereArgs) {
SQLiteDatabase db = mOpenHelper.getWritableDatabase();
int count;
switch(sUriMatcher.match(uri)) {
case INCOMING_BOOK_COLLECTION_URI_INDICATOR:
count = db.update(BookProviderMetaData.BookTableMetaData.TABLE_NAME, values, where, whereArgs);
break;
case INCOMING_SINGLE_BOOK_URI_INDICATOR:
String rowId = uri.getPathSegments().get(1);
count = db.update(BookProviderMetaData.BookTableMetaData.TABLE_NAME, values, BookProviderMetaData.BookTableMetaData._ID + "=" + rowId
+ (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
getContext().getContentResolver().notifyChange(uri, null);
return count;
}
}getType方法中，通過判斷返回的URI的MIME型別從而知道是一個圖書的集合還是一本圖書。
query方法中，首先根據傳入的URI判斷型別，如果是單個型別，則要獲取傳入ID。然後通過SQLiteQueryBuilder.query方法查詢，返回遊標。
insert方法中，首先判斷URI型別，然後驗證列引數，然後通過SQLiteDatabase.insert方法插入，返回新插入的ID。
update方法中，首先判斷URI型別，如果是集合，則傳入where子句，使用SQLiteDatabase.update方法更新；如果是單個圖書，則首先從URI中獲取ID，然後指派一條新的where子句，最後執行update。該方法返回更新的數量。
delete方法中，首先判斷URI型別，如果是圖書集合，然後傳入where子句，執行SqliteDatabase.delete刪除操作。如果是單個圖書，首先獲取URI中的ID，然後指派一條新的where子句，最後執行delelte。該方法返回刪除的數量。
注意以上程式中的sBooksProjectionMap。由於ContentProvider充當著抽象列集和資料庫中真實的列集之間的媒介，這些列集可能是不同的。所以在構造查詢的時候，必須在客戶端指定的where子句列與真實的資料庫列之間建立對映。藉助於SQLiteQueryBuilder類來構建對映。
（3）註冊程式
最後，記得在AndroidManifest.xml檔案中註冊ContentProvider
<provider
android:authorities="com.androidbook.provider.BookProvider"
android:name=".BookProvider" />依據以上步驟就可以實現ContentProvider。參考Pro Android書本原始碼。書本原始碼我已上傳到CSDN地址
http://download.csdn.net/detail/ithink213/8878935
以上程式碼在我的GitHub也有
https://github.com/EvidenceKiller/ProAndroid