FMDB是SQLite C语言接口的Objective-C封装，最近看的几份代码，CTPersistance, YYCache, 都是直接使用C语言接口，YTKKeyValueStore倒是使用了FMDB，顺带就把FMDB给看了。

代码比较简单，主要类就

FMDatabase，负责管理数据库连接，并且执行SQL语句，并不对外提供FMStatement类，查询结果返回FMResultSet.

FMResultSet当然是负责查询结果的。也没有什么特别注意的内容。

一个比较有意思的功能是提供FMDatabaseQueue来保证数据库的多线程安全访问。

前两周看的代码，做了一些笔记，摘抄如下：

一些功能小结

1）FMDatabase的缓存stmt语句对象

2）FMStatement可以当做FMDatabase类的私有类，并不对外提供

3）一个SQL语句的执行过程

- (FMResultSet*)executeQuery:(NSString*)sql withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary*)dictionaryArgs orVAList:(va_list)args

FMDatabase

讲SQL语句执行的功能封装在executeQuery和executeUpdate两大方法里，内部使用FMStatement类封装sql语句对象sqlite3_stmt，不过也只是字符串而已。

- (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对象，而是使用这个类，的inDatabase方法来保证多线程安全。这个方法里使用的队列的dispatch_sync分派方法。就能保证一个block执行完之后，再执行下一个block。

在queue中执行的块时，返回绑定的对象，如果不是在queue中执行的代码，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);

}

小知识点

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.

运行在队列的block，可以通过dispatch_get_specific方法来获取context信息

FMDB的使用

Database Creation （数据库创建的几种模式，普通数据库，临时数据库（用完即删除），内存数据库）

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.（多线程安全）

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];

}];

源码剖析

对ARC和非ARC的兼容，OS_OBJECT_USE_OBJC和objc_instancetype的判断

#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