FMDB是SQLite C語(yǔ)言接口的Objective-C封裝，最近看的幾份代碼，CTPersistance, YYCache, 都是直接使用C語(yǔ)言接口，YTKKeyValueStore倒是使用了FMDB，順帶就把FMDB給看了。
代碼比較簡(jiǎn)單，主要類就
FMDatabase，負(fù)責(zé)管理數(shù)據(jù)庫(kù)連接，并且執(zhí)行SQL語(yǔ)句，并不對(duì)外提供FMStatement類，查詢結(jié)果返回FMResultSet.
FMResultSet當(dāng)然是負(fù)責(zé)查詢結(jié)果的。也沒(méi)有什么特別注意的內(nèi)容。
一個(gè)比較有意思的功能是提供FMDatabaseQueue來(lái)保證數(shù)據(jù)庫(kù)的多線程安全訪問(wèn)。
前兩周看的代碼，做了一些筆記，摘抄如下：
一些功能小結(jié)
1）FMDatabase的緩存stmt語(yǔ)句對(duì)象
2）FMStatement可以當(dāng)做FMDatabase類的私有類，并不對(duì)外提供
3）一個(gè)SQL語(yǔ)句的執(zhí)行過(guò)程
- (FMResultSet*)executeQuery:(NSString*)sql withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary*)dictionaryArgs orVAList:(va_list)args
FMDatabase
講SQL語(yǔ)句執(zhí)行的功能封裝在executeQuery和executeUpdate兩大方法里，內(nèi)部使用FMStatement類封裝sql語(yǔ)句對(duì)象sqlite3_stmt，不過(guò)也只是字符串而已。
- (FMResultSet*)executeQuery:(NSString*)sql withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary*)dictionaryArgs orVAList:(va_list)args
- (BOOL)executeUpdate:(NSString*)sql error:(NSError**)outErr withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary*)dictionaryArgs orVAList:(va_list)args
FMDatabaseQueue
不直接使用FMDatabase對(duì)象，而是使用這個(gè)類，的inDatabase方法來(lái)保證多線程安全。這個(gè)方法里使用的隊(duì)列的dispatch_sync分派方法。就能保證一個(gè)block執(zhí)行完之后，再執(zhí)行下一個(gè)block。
在queue中執(zhí)行的塊時(shí)，返回綁定的對(duì)象，如果不是在queue中執(zhí)行的代碼，返回nil
- (void)inDatabase:(void(^)(FMDatabase*db))block {
/* Get the currently executing queue (which should probably be nil, but in theory could be another DB queue
* and then check it against self to make sure we're not about to deadlock. */
FMDatabaseQueue*currentSyncQueue = (__bridgeid)dispatch_get_specific(kDispatchQueueSpecificKey);
assert(currentSyncQueue !=self&&"inDatabase: was called reentrantly on the same queue, which would lead to a deadlock");
FMDBRetain(self);
dispatch_sync(_queue, ^() {
FMDatabase*db = [selfdatabase];
block(db);
if([dbhasOpenResultSets]) {
NSLog(@"Warning: there is at least one open result set around after performing [FMDatabaseQueue inDatabase:]");
#ifdef DEBUG
NSSet*openSetCopy =FMDBReturnAutoreleased([[dbvalueForKey:@"_openResultSets"]copy]);
for(NSValue*rsInWrappedInATastyValueMealinopenSetCopy) {
FMResultSet*rs = (FMResultSet*)[rsInWrappedInATastyValueMealpointerValue];
NSLog(@"query: '%@'", [rsquery]);
}
#endif
}
});
FMDBRelease(self);
}
小知識(shí)點(diǎn)
void?dispatch_queue_set_specific(dispatch_queue_t?queue,?const?void?*key,?void?*context,?dispatch_function_t?destructor);
Description
Sets the key/value data for the specified dispatch queue.
Use this method to associate custom context data with a dispatch queue. Blocks executing on the queue can use thedispatch_get_specificfunction to retrieve this data while they are running.
void * dispatch_get_specific(const void *key);
DescriptionReturns the value for the key associated with the current dispatch queue.
This function is intended to be called from a block executing in a dispatch queue. You use it to obtain context data associated with the queue. Calling this method from code not running in a dispatch queue returnsNULLbecause there is no queue to provide context.
運(yùn)行在隊(duì)列的block，可以通過(guò)dispatch_get_specific方法來(lái)獲取context信息
FMDB的使用
Database Creation （數(shù)據(jù)庫(kù)創(chuàng)建的幾種模式，普通數(shù)據(jù)庫(kù)，臨時(shí)數(shù)據(jù)庫(kù)（用完即刪除），內(nèi)存數(shù)據(jù)庫(kù)）
AnFMDatabaseis created with a path to a SQLite database file. This path can be one of these three:
A file system path. The file does not have to exist on disk. If it does not exist, it is created for you.
An empty string (@""). An empty database is created at a temporary location. This database is deleted with theFMDatabaseconnection is closed.
NULL. An in-memory database is created. This database will be destroyed with theFMDatabaseconnection is closed.
Multiple Statements and Batch Stuff
You can useFMDatabase's executeStatements:withResultBlock: to do multiple statements in a string:
NSString*sql?=@"create?table?bulktest1?(id?integer?primary?key?autoincrement,?x?text);"
"create?table?bulktest2?(id?integer?primary?key?autoincrement,?y?text);"
"create?table?bulktest3?(id?integer?primary?key?autoincrement,?z?text);"
"insert?into?bulktest1?(x)?values?('XXX');"
"insert?into?bulktest2?(y)?values?('YYY');"
"insert?into?bulktest3?(z)?values?('ZZZ');";
success?=?[dbexecuteStatements:sql];
sql?=@"select?count(*)?as?count?from?bulktest1;"
"select?count(*)?as?count?from?bulktest2;"
"select?count(*)?as?count?from?bulktest3;";
success?=?[self.dbexecuteStatements:sqlwithResultBlock:^int(NSDictionary*dictionary)?{
NSIntegercount?=?[dictionary[@"count"]integerValue];
XCTAssertEqual(count,1,@"expected?one?record?for?dictionary%@",?dictionary);
return0;
}];
Data Sanitization
When providing a SQL statement to FMDB, you should not attempt to "sanitize" any values before insertion. Instead, you should use the standard SQLite binding syntax:
INSERT?INTOmyTableVALUES(?,??,??,??)
Using FMDatabaseQueue and Thread Safety.（多線程安全）
Using a single instance ofFMDatabasefrom multiple threads at once is a bad idea. It has always been OK to make aFMDatabaseobjectper thread. Just don't share a single instance across threads, and definitely not across multiple threads at the same time. Bad things will eventually happen and you'll eventually get something to crash, or maybe get an exception, or maybe meteorites will fall out of the sky and hit your Mac Pro.This would suck.
So don't instantiate a singleFMDatabaseobject and use it across multiple threads.
Instead, useFMDatabaseQueue. Instantiate a singleFMDatabaseQueueand use it across multiple threads. TheFMDatabaseQueueobject will synchronize and coordinate access across the multiple threads. Here's how to use it:
First, make your queue.
FMDatabaseQueue *queue = [FMDatabaseQueuedatabaseQueueWithPath:aPath];
Then use it like so:
[queueinDatabase:^(FMDatabase *db) {? ? [dbexecuteUpdate:@"INSERT INTO myTable VALUES (?)", @1];? ? [dbexecuteUpdate:@"INSERT INTO myTable VALUES (?)", @2];? ? [dbexecuteUpdate:@"INSERT INTO myTable VALUES (?)", @3];? ? FMResultSet *rs = [dbexecuteQuery:@"select * from foo"];while([rsnext]) {? ? ? ? …? ? }}];
An easy way to wrap things up in a transaction can be done like this:
[queueinTransaction:^(FMDatabase?*db,BOOL*rollback)?{
[dbexecuteUpdate:@"INSERT?INTO?myTable?VALUES?(?)",?@1];
[dbexecuteUpdate:@"INSERT?INTO?myTable?VALUES?(?)",?@2];
[dbexecuteUpdate:@"INSERT?INTO?myTable?VALUES?(?)",?@3];
if(whoopsSomethingWrongHappened)?{
*rollback?=YES;
return;
}
//?etc…
[dbexecuteUpdate:@"INSERT?INTO?myTable?VALUES?(?)",?@4];
}];
源碼剖析
對(duì)ARC和非ARC的兼容，OS_OBJECT_USE_OBJC和objc_instancetype的判斷
#if ! __has_feature(objc_arc)
#define FMDBAutorelease(__v) ([__v autorelease]);
#define FMDBReturnAutoreleased FMDBAutorelease
#define FMDBRetain(__v) ([__v retain]);
#define FMDBReturnRetained FMDBRetain
#define FMDBRelease(__v) ([__v release]);
#define FMDBDispatchQueueRelease(__v) (dispatch_release(__v));
#else
// -fobjc-arc
#define FMDBAutorelease(__v)
#define FMDBReturnAutoreleased(__v) (__v)
#define FMDBRetain(__v)
#define FMDBReturnRetained(__v) (__v)
#define FMDBRelease(__v)
// If OS_OBJECT_USE_OBJC=1, then the dispatch objects will be treated like ObjC objects
// and will participate in ARC.
// See the section on "Dispatch Queues and Automatic Reference Counting" in "Grand Central Dispatch (GCD) Reference" for details.
#if OS_OBJECT_USE_OBJC
#define FMDBDispatchQueueRelease(__v)
#else
#define FMDBDispatchQueueRelease(__v) (dispatch_release(__v));
#endif
#endif
#if !__has_feature(objc_instancetype)
#define instancetype id
#endif
