FMDB初探（一）

+ (instancetype)databaseWithPath:(NSString*)aPath {     return FMDBReturnAutoreleased([[self alloc] initWithPath:aPath]); }  - (instancetype)init {     return [self initWithPath:nil]; }  - (instancetype)initWithPath:(NSString*)aPath {     // SQLite支持三种线程模式，分别为单线程模式、多线程模式和串行模式     // sqlite3_threadsafe()的返回值可以确定编译时指定的线程模式，其中对于单线程模式，sqlite3_threadsafe()返回false，对于另外两个模式，则返回true。这是因为单线程模式下没有进行互斥，所以多线程下是不安全的     assert(sqlite3_threadsafe()); // whoa there big boy- gotta make sure sqlite it happy with what we're going to do.          self = [super init];          if (self) {         _databasePath               = [aPath copy];         _openResultSets             = [[NSMutableSet alloc] init];         // 此时并不创建数据库，真正创建是在open的时候        _db                         = nil;         _logsErrors                 = YES;         _crashOnErrors              = NO;         _maxBusyRetryTimeInterval   = 2;     }          return self; }

// 给定database路径 NSString *dbPath = @"/tmp/tmp.db"; // 调用[+ databaseWithPath:]方法 FMDatabase *db = [FMDatabase databaseWithPath:dbPath];

- (BOOL)open {     if (_db) {         return YES;     }     // 根据数据库路径打开数据库，如果数据库不存在就新建一个     int err = sqlite3_open([self sqlitePath], (sqlite3**)&_db );     if(err != SQLITE_OK) {         NSLog(@"error opening!: %d", err);         return NO;     }     // 设置最大繁忙重试时间间隔，默认为2秒     if (_maxBusyRetryTimeInterval > 0.0) {         // set the handler         [self setMaxBusyRetryTimeInterval:_maxBusyRetryTimeInterval];     }               return YES; } // 根据标记打开一个新的数据库连接 // flag可以是以下三种值之一： // SQLITE_OPEN_READONLY，只读模式，如果数据库不存在会报错 // SQLITE_OPEN_READWRITE，读写模式，如果该文件在操作系统中是写保护的，那就是以只读方式打开，如果数据库不存在会报错 // SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE，以读写方式打开，如果数据不存在则新建一个，这个用于open方法里面 // 以上还可以可选组合`SQLITE_OPEN_NOMUTEX`, `SQLITE_OPEN_FULLMUTEX`, `SQLITE_OPEN_SHAREDCACHE`, `SQLITE_OPEN_PRIVATECACHE`, and/or `SQLITE_OPEN_URI` - (BOOL)openWithFlags:(int)flags {     return [self openWithFlags:flags vfs:nil]; } - (BOOL)openWithFlags:(int)flags vfs:(NSString *)vfsName { #if SQLITE_VERSION_NUMBER >= 3005000     if (_db) {         return YES;     }     // 比sqlite3_open多传入了flags和vfsName，vfsName为使用的VFS模块的名称     // sqlite3_open_v2里zVfs参数允许客户应用程序命名一个虚拟文件系统（Virtual File System）模块，用来与数据库连接。VFS作为SQlite library和底层存储系统（如某个文件系统）之间的一个抽象层，通常客户应用程序可以简单的给该参数传递一个NULL指针，以使用默认的VFS模块。     // sqlite3_open_v2比sqlite3_open和sqlite3_open16强大在它可以指定连接方式     int err = sqlite3_open_v2([self sqlitePath], (sqlite3**)&_db, flags, [vfsName UTF8String]);     if(err != SQLITE_OK) {         NSLog(@"error opening!: %d", err);         return NO;     }          if (_maxBusyRetryTimeInterval > 0.0) {         // set the handler         [self setMaxBusyRetryTimeInterval:_maxBusyRetryTimeInterval];     }          return YES; #else     NSLog(@"openWithFlags requires SQLite 3.5");     return NO; #endif }

- (void)setMaxBusyRetryTimeInterval:(NSTimeInterval)timeout {          _maxBusyRetryTimeInterval = timeout;          if (!_db) {         return;     }     /*      int sqlite3_busy_handler(sqlite3*, int(*)(void*,int), void*);      第二个参数是回调函数（busy handler），如果设置了回调函数，那就需要设置sqlite3_busy_handler的第三个参数，这里传递给它的是一个void*的参数的拷贝；如果回调函数返回０时，将不再尝试再次访问数据库而返回SQLITE_BUSY或者SQLITE_IOERR_BLOCKED。如果回调函数返回非０,将会不断尝试操作数据库。      也就是说，程序运行过程中，如果有其他进程或者线程在读写数据库，那么sqlite3_busy_handler会不断调用回调函数，直到其他进程或者线程释放锁。获得锁之后，不会再调用回调函数，从而向下执行，进行数据库操作。该函数是在获取不到锁的时候，以执行回调函数的次数来进行延迟，等待其他进程或者线程操作数据库结束，从而获得锁操作数据库。      */     if (timeout > 0) {         // busy handler设置为FMDBDatabaseBusyHandler函数         sqlite3_busy_handler(_db, &FMDBDatabaseBusyHandler, (__bridge void *)(self));     }     else {         // turn it off otherwise         // 关闭busy handler         sqlite3_busy_handler(_db, nil, nil);     } }  // 这就是回调函数 static int FMDBDatabaseBusyHandler(void *f, int count) {     FMDatabase *self = (__bridge FMDatabase*)f;          // 如果是第一次调用，调用[+ timeIntervalSinceReferenceDate]方法，获取当前时间与2001年1月1日00:00:00 UTC的时间间隔，并赋值给startBusyRetryTime     if (count == 0) {         self->_startBusyRetryTime = [NSDate timeIntervalSinceReferenceDate];         // 返回1，则将不断尝试操作数据库         return 1;     }     // 计算当前时间与2001年1月1日00:00:00 UTC的时间间隔和startBusyRetryTime的间隔     NSTimeInterval delta = [NSDate timeIntervalSinceReferenceDate] - (self->_startBusyRetryTime);     // 如果间隔时间小于最大的重试间隔时间     if (delta < [self maxBusyRetryTimeInterval]) {         // 产生一个从50-99的随机整数作为需要挂起毫秒时间         int requestedSleepInMillseconds = (int) arc4random_uniform(50) + 50;         // 调用sqlite3_sleep返回实际挂起毫秒时间，如果不一致，可能是因为SQLite构建时没有将HAVE_USLEEP设置为1         int actualSleepInMilliseconds = sqlite3_sleep(requestedSleepInMillseconds);         if (actualSleepInMilliseconds != requestedSleepInMillseconds) {             NSLog(@"WARNING: Requested sleep of %i milliseconds, but SQLite returned %i. Maybe SQLite wasn't built with HAVE_USLEEP=1?", requestedSleepInMillseconds, actualSleepInMilliseconds);         }         return 1;     }          return 0; }

if (![db open]) {  // do somthing         }

- (FMResultSet *)executeQuery:(NSString *)sql withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary *)dictionaryArgs orVAList:(va_list)args {     // 检查数据库是否存在     if (![self databaseExists]) {         return 0x00;     }     // 检查是否正在执行操作     if (_isExecutingStatement) {         [self warnInUse];         return 0x00;     }          _isExecutingStatement = YES;          int rc                  = 0x00;     sqlite3_stmt *pStmt     = 0x00;     FMStatement *statement  = 0x00;     FMResultSet *rs         = 0x00;          if (_traceExecution && sql) {         NSLog(@"%@ executeQuery: %@", self, sql);     }     // 将sql语句转换成prepared语句     // 由于使用sqlite3_prepare_v2来生成sql对应的prepared语句代价很大     // 所以使用缓存机制来减少sqlite3_prepare_v2的使用     if (_shouldCacheStatements) {         // 根据sql获取到缓存中的prepared语句         statement = [self cachedStatementForQuery:sql];         pStmt = statement ? [statement statement] : 0x00;         // 重置prepared语句         [statement reset];     }     // 如果缓存中没有sql对应的prepared语句，那么需要用sqlite3_prepare_v2生成对应的prepared语句     if (!pStmt) {                  rc = sqlite3_prepare_v2(_db, [sql UTF8String], -1, &pStmt, 0);                  // 如果生成失败，则打印错误日志         if (SQLITE_OK != rc) {             if (_logsErrors) {                 NSLog(@"DB Error: %d /"%@/"", [self lastErrorCode], [self lastErrorMessage]);                 NSLog(@"DB Query: %@", sql);                 NSLog(@"DB Path: %@", _databasePath);             }                          if (_crashOnErrors) {                 NSAssert(false, @"DB Error: %d /"%@/"", [self lastErrorCode], [self lastErrorMessage]);                 // 停止程序                 abort();             }             // 释放所有内部资源和FMStatement             sqlite3_finalize(pStmt);             _isExecutingStatement = NO;             return nil;         }     }          id obj;     int idx = 0;     // sqlite3_bind_parameter_count 返回SQL语句参数的数量     int queryCount = sqlite3_bind_parameter_count(pStmt); // pointed out by Dominic Yu (thanks!)          // If dictionaryArgs is passed in, that means we are using sqlite's named parameter support     if (dictionaryArgs) {                  for (NSString *dictionaryKey in [dictionaryArgs allKeys]) {                          // Prefix the key with a colon.             NSString *parameterName = [[NSString alloc] initWithFormat:@":%@", dictionaryKey];                          if (_traceExecution) {                 NSLog(@"%@ = %@", parameterName, [dictionaryArgs objectForKey:dictionaryKey]);             }                          // Get the index for the parameter name.             // 通过传入参数名来获取该参数的索引             int namedIdx = sqlite3_bind_parameter_index(pStmt, [parameterName UTF8String]);                          FMDBRelease(parameterName);             // 如果索引大于0             if (namedIdx > 0) {                 // Standard binding from here.                 // 在prepared语句里将值绑定到索引位置                 [self bindObject:[dictionaryArgs objectForKey:dictionaryKey] toColumn:namedIdx inStatement:pStmt];                 // increment the binding count, so our check below works out                 // 绑定数量加一                 idx++;             }             else {                 NSLog(@"Could not find index for %@", dictionaryKey);             }         }     }     else {                  while (idx < queryCount) {             // 当调用下面三个方法时             // - (FMResultSet *)executeQueryWithFormat:(NSString*)format, ...             // - (FMResultSet *)executeQuery:(NSString *)sql withArgumentsInArray:(NSArray *)arguments             // - (FMResultSet *)executeQuery:(NSString *)sql values:(NSArray *)values error:(NSError * __autoreleasing *)error             // 值是放在NSArray里面，循环取出来绑定             if (arrayArgs && idx < (int)[arrayArgs count]) {                 obj = [arrayArgs objectAtIndex:(NSUInteger)idx];             }             // 当调用下面两个方法时             // - (FMResultSet *)executeQuery:(NSString*)sql, ...             // - (FMResultSet *)executeQuery:(NSString*)sql withVAList:(va_list)args             // 值是放在va_list里面，循环取出来绑定             else if (args) {                 obj = va_arg(args, id);             }             else {                 //We ran out of arguments                 break;             }                          if (_traceExecution) {                 if ([obj isKindOfClass:[NSData class]]) {                     NSLog(@"data: %ld bytes", (unsigned long)[(NSData*)obj length]);                 }                 else {                     NSLog(@"obj: %@", obj);                 }             }                          idx++;             // 绑定参数             [self bindObject:obj toColumn:idx inStatement:pStmt];         }     }     // 如果绑定数量和参数数量不一致，打印错误，并释放资源     if (idx != queryCount) {         NSLog(@"Error: the bind count is not correct for the # of variables (executeQuery)");         sqlite3_finalize(pStmt);         _isExecutingStatement = NO;         return nil;     }          FMDBRetain(statement); // to balance the release below          // 如果statement不为空，则缓存     if (!statement) {         statement = [[FMStatement alloc] init];         [statement setStatement:pStmt];                  if (_shouldCacheStatements && sql) {             // 以sql作为key来缓存statement             [self setCachedStatement:statement forQuery:sql];         }     }          // the statement gets closed in rs's dealloc or [rs close];     // 根据statement和FMDataBase对象来初始化FMResultSet对象     rs = [FMResultSet resultSetWithStatement:statement usingParentDatabase:self];     [rs setQuery:sql];          // 将FMResultSet对象加到_openResultSets里     NSValue *openResultSet = [NSValue valueWithNonretainedObject:rs];     [_openResultSets addObject:openResultSet];     // useCount+1     [statement setUseCount:[statement useCount] + 1];          FMDBRelease(statement);     // 设置_isExecutingStatement操作结束     _isExecutingStatement = NO;          return rs; }

rs = [newDb executeQuery:@"select rowid,* from test where a = ?", @"hi'"];  rs = [db executeQueryWithFormat:@"select * from t5 where a = %s and a = %@ and b = %d", "text", @"text", 42];  rs = [db executeQuery:@"select * from testOneHundredTwelvePointTwo where b > ?" withArgumentsInArray:[NSArray arrayWithObject:[NSNumber numberWithInteger:1]]];  rs = [db executeQuery:@"select * from namedparamcounttest where a = :a" withParameterDictionary:dictionaryArgs];

- (BOOL)executeUpdate:(NSString*)sql error:(NSError**)outErr withArgumentsInArray:(NSArray*)arrayArgs orDictionary:(NSDictionary *)dictionaryArgs orVAList:(va_list)args {          if (![self databaseExists]) {         return NO;     }          if (_isExecutingStatement) {         [self warnInUse];         return NO;     }          _isExecutingStatement = YES;          int rc                   = 0x00;     sqlite3_stmt *pStmt      = 0x00;     FMStatement *cachedStmt  = 0x00;          if (_traceExecution && sql) {         NSLog(@"%@ executeUpdate: %@", self, sql);     }          if (_shouldCacheStatements) {         cachedStmt = [self cachedStatementForQuery:sql];         pStmt = cachedStmt ? [cachedStmt statement] : 0x00;         [cachedStmt reset];     }          if (!pStmt) {         rc = sqlite3_prepare_v2(_db, [sql UTF8String], -1, &pStmt, 0);                  if (SQLITE_OK != rc) {             if (_logsErrors) {                 NSLog(@"DB Error: %d /"%@/"", [self lastErrorCode], [self lastErrorMessage]);                 NSLog(@"DB Query: %@", sql);                 NSLog(@"DB Path: %@", _databasePath);             }                          if (_crashOnErrors) {                 NSAssert(false, @"DB Error: %d /"%@/"", [self lastErrorCode], [self lastErrorMessage]);                 abort();             }                          if (outErr) {                 *outErr = [self errorWithMessage:[NSString stringWithUTF8String:sqlite3_errmsg(_db)]];             }                          sqlite3_finalize(pStmt);                          _isExecutingStatement = NO;             return NO;         }     }          id obj;     int idx = 0;     int queryCount = sqlite3_bind_parameter_count(pStmt);          // If dictionaryArgs is passed in, that means we are using sqlite's named parameter support     if (dictionaryArgs) {                  for (NSString *dictionaryKey in [dictionaryArgs allKeys]) {                          // Prefix the key with a colon.             NSString *parameterName = [[NSString alloc] initWithFormat:@":%@", dictionaryKey];                          if (_traceExecution) {                 NSLog(@"%@ = %@", parameterName, [dictionaryArgs objectForKey:dictionaryKey]);             }             // Get the index for the parameter name.             int namedIdx = sqlite3_bind_parameter_index(pStmt, [parameterName UTF8String]);                          FMDBRelease(parameterName);                          if (namedIdx > 0) {                 // Standard binding from here.                 [self bindObject:[dictionaryArgs objectForKey:dictionaryKey] toColumn:namedIdx inStatement:pStmt];                                  // increment the binding count, so our check below works out                 idx++;             }             else {                 // 由于多了outErr，所以绑定时出错需要将error抛出                 NSString *message = [NSString stringWithFormat:@"Could not find index for %@", dictionaryKey];                                  if (_logsErrors) {                     NSLog(@"%@", message);                 }                 if (outErr) {                     *outErr = [self errorWithMessage:message];                 }             }         }     }     else {                  while (idx < queryCount) {                          if (arrayArgs && idx < (int)[arrayArgs count]) {                 obj = [arrayArgs objectAtIndex:(NSUInteger)idx];             }             else if (args) {                 obj = va_arg(args, id);             }             else {                 //We ran out of arguments                 break;             }                          if (_traceExecution) {                 if ([obj isKindOfClass:[NSData class]]) {                     NSLog(@"data: %ld bytes", (unsigned long)[(NSData*)obj length]);                 }                 else {                     NSLog(@"obj: %@", obj);                 }             }                          idx++;                          [self bindObject:obj toColumn:idx inStatement:pStmt];         }     }               if (idx != queryCount) {         // 同样也是组装error抛出         NSString *message = [NSString stringWithFormat:@"Error: the bind count (%d) is not correct for the # of variables in the query (%d) (%@) (executeUpdate)", idx, queryCount, sql];         if (_logsErrors) {             NSLog(@"%@", message);         }         if (outErr) {             *outErr = [self errorWithMessage:message];         }                  sqlite3_finalize(pStmt);         _isExecutingStatement = NO;         return NO;     }          /* Call sqlite3_step() to run the virtual machine. Since the SQL being      ** executed is not a SELECT statement, we assume no data will be returned.      */     // sqlite3_prepare函数将SQL命令字符串解析并转换为一系列的命令字节码，这些字节码最终被传送到SQlite3的虚拟数据库引擎（VDBE: Virtual Database Engine）中执行，完成这项工作的是sqlite3_step函数。比如一个SELECT查询操作，sqlite3_step函数的每次调用都会返回结果集中的其中一行，直到再没有有效数据行了。每次调用sqlite3_step函数如果返回SQLITE_ROW，代表获得了有效数据行，可以通过sqlite3_column函数提取某列的值。如果调用sqlite3_step函数返回SQLITE_DONE，则代表prepared语句已经执行到终点了，没有有效数据了。很多命令第一次调用sqlite3_step函数就会返回SQLITE_DONE，因为这些SQL命令不会返回数据。对于INSERT，UPDATE，DELETE命令，会返回它们所修改的行号——一个单行单列的值。     /**      SQLITE_BUSY 数据库文件有锁      SQLITE_LOCKED 数据库中的某张表有锁      SQLITE_DONE sqlite3_step()执行完毕      SQLITE_ROW sqlite3_step()获取到下一行数据      SQLITE_ERROR 一般用于没有特别指定错误码的错误，就是说函数在执行过程中发生了错误，但无法知道错误发生的原因。      SQLITE_MISUSE 没有正确使用SQLite接口，比如一条语句在sqlite3_step函数执行之后，没有被重置之前，再次给其绑定参数，这时bind函数就会返回SQLITE_MISUSE。      **/     rc      = sqlite3_step(pStmt);          if (SQLITE_DONE == rc) {         // all is well, let's return.     }     // sql操作被sqlite3_interrupt()函数终止     else if (SQLITE_INTERRUPT == rc) {         if (_logsErrors) {             NSLog(@"Error calling sqlite3_step. Query was interrupted (%d: %s) SQLITE_INTERRUPT", rc, sqlite3_errmsg(_db));             NSLog(@"DB Query: %@", sql);         }     }     else if (rc == SQLITE_ROW) {         NSString *message = [NSString stringWithFormat:@"A executeUpdate is being called with a query string '%@'", sql];         if (_logsErrors) {             NSLog(@"%@", message);             NSLog(@"DB Query: %@", sql);         }         if (outErr) {             *outErr = [self errorWithMessage:message];         }     }     else {         if (outErr) {             *outErr = [self errorWithMessage:[NSString stringWithUTF8String:sqlite3_errmsg(_db)]];         }                  if (SQLITE_ERROR == rc) {             if (_logsErrors) {                 NSLog(@"Error calling sqlite3_step (%d: %s) SQLITE_ERROR", rc, sqlite3_errmsg(_db));                 NSLog(@"DB Query: %@", sql);             }         }         else if (SQLITE_MISUSE == rc) {             // uh oh.             if (_logsErrors) {                 NSLog(@"Error calling sqlite3_step (%d: %s) SQLITE_MISUSE", rc, sqlite3_errmsg(_db));                 NSLog(@"DB Query: %@", sql);             }         }         else {             // wtf?             if (_logsErrors) {                 NSLog(@"Unknown error calling sqlite3_step (%d: %s) eu", rc, sqlite3_errmsg(_db));                 NSLog(@"DB Query: %@", sql);             }         }     }          if (_shouldCacheStatements && !cachedStmt) {         cachedStmt = [[FMStatement alloc] init];                  [cachedStmt setStatement:pStmt];                  [self setCachedStatement:cachedStmt forQuery:sql];                  FMDBRelease(cachedStmt);     }          int closeErrorCode;          if (cachedStmt) {         [cachedStmt setUseCount:[cachedStmt useCount] + 1];         closeErrorCode = sqlite3_reset(pStmt);     }     else {         /* Finalize the virtual machine. This releases all memory and other          ** resources allocated by the sqlite3_prepare() call above.          */         closeErrorCode = sqlite3_finalize(pStmt);     }          if (closeErrorCode != SQLITE_OK) {         if (_logsErrors) {             NSLog(@"Unknown error finalizing or resetting statement (%d: %s)", closeErrorCode, sqlite3_errmsg(_db));             NSLog(@"DB Query: %@", sql);         }     }          _isExecutingStatement = NO;     return (rc == SQLITE_DONE || rc == SQLITE_OK); }

[db executeUpdate:@"insert into t5 values (?, ?, ?, ?, ?)" withErrorAndBindings:&err, @"text", [NSNumber numberWithInt:42], @"BLOB", @"d", [NSNumber numberWithInt:0]];  [dbB executeUpdate:@"create table attached (a text)"];  [dbB executeUpdate:@"insert into attached values (?)", @"test"];  [db executeUpdateWithFormat:@"insert into t55 values (%c, %hi, %g)", 'a', testShort, testFloat];  [db executeUpdate:@"insert into testOneHundredTwelvePointTwo values (?, ?)" withArgumentsInArray:[NSArray arrayWithObjects:@"one", [NSNumber numberWithInteger:2], nil]];  [db executeUpdate:@"insert into namedparamtest values (:a, :b, :c, :d)" withParameterDictionary:dictionaryArgs];

// 使用executeStatements:函数可以将多个SQL执行语句写在一个字符串中，并执行 - (BOOL)executeStatements:(NSString *)sql {     return [self executeStatements:sql withResultBlock:nil]; }  - (BOOL)executeStatements:(NSString *)sql withResultBlock:(FMDBExecuteStatementsCallbackBlock)block {          int rc;     char *errmsg = nil;     /*      sqlite3_exec(sqlite3*, const char *sql, sqlite_callback, void *data, char **errmsg)            该例程提供了一个执行 SQL 命令的快捷方式，SQL 命令由 sql 参数提供，可以由多个 SQL 命令组成。            在这里，第一个参数 sqlite3 是打开的数据库对象，sqlite_callback 是一个回调，data 作为其第一个参数，errmsg 将被返回用来获取程序生成的任何错误。            sqlite3_exec() 程序解析并执行由 sql 参数所给的每个命令，直到字符串结束或者遇到错误为止。      */     rc = sqlite3_exec([self sqliteHandle], [sql UTF8String], block ? FMDBExecuteBulkSQLCallback : nil, (__bridge void *)(block), &errmsg);          if (errmsg && [self logsErrors]) {         NSLog(@"Error inserting batch: %s", errmsg);         sqlite3_free(errmsg);     }          return (rc == SQLITE_OK); }

NSString *sql = @"create table messages1 (id integer primary key messageId, x text);"                  "create table messages2 (id integer primary key messageId, y text);"      "insert into messages1 (x) values ('X');"                  "insert into messages2 (y) values ('Y');";  success = [db executeStatements:sql];  sql = @"select count(*) as count from messages1;"        "select count(*) as count from messages2;";  success = [self.db executeStatements:sql withResultBlock:^int(NSDictionary *dictionary) {     // do something }];

- (sqlite_int64)lastInsertRowId {     // 如果有正在执行语句，返回     if (_isExecutingStatement) {         [self warnInUse];         return NO;     }          _isExecutingStatement = YES;     // sqlite3_last_insert_rowid 获取指定数据库最后一个插入的rowid     sqlite_int64 ret = sqlite3_last_insert_rowid(_db);          _isExecutingStatement = NO;          return ret; }

- (int)changes {     if (_isExecutingStatement) {         [self warnInUse];         return 0;     }          _isExecutingStatement = YES;          int ret = sqlite3_changes(_db);          _isExecutingStatement = NO;          return ret; }

// 开始延迟事务 - (BOOL)beginDeferredTransaction {          BOOL b = [self executeUpdate:@"begin deferred transaction"];     if (b) {         _inTransaction = YES;     }          return b; }  //Exclusive事务会试着获取对数据库的EXCLUSIVE锁。这与IMMEDIATE类似，但是一旦成功，EXCLUSIVE事务保证没有其它的连接，所以就可对数据库进行读写操作了。  // 开始事务 - (BOOL)beginTransaction {          BOOL b = [self executeUpdate:@"begin exclusive transaction"];     if (b) {         _inTransaction = YES;     }          return b; }  // 回滚事务 - (BOOL)rollback {     BOOL b = [self executeUpdate:@"rollback transaction"];          if (b) {         _inTransaction = NO;     }          return b; } // 提交事务 - (BOOL)commit {     BOOL b =  [self executeUpdate:@"commit transaction"];          if (b) {         _inTransaction = NO;     }          return b; }

- (BOOL)setKeyWithData:(NSData *)keyData { // SQLITE_HAS_CODEC 用来确定是否支持加密 #ifdef SQLITE_HAS_CODEC     if (!keyData) {         return NO;     }          // int sqlite3_key( sqlite3 *db, const void *pKey, int nKey)     // db是指定的数据库，pKey是密钥，nKey是密钥的长度     // 例如：sqlite3_key(_db, "gongcheng", 9);     int rc = sqlite3_key(_db, [keyData bytes], (int)[keyData length]);          return (rc == SQLITE_OK); #else #pragma unused(keyData)     return NO; #endif }

- (BOOL)rekeyWithData:(NSData *)keyData { #ifdef SQLITE_HAS_CODEC     if (!keyData) {         return NO;     }          // int sqlite3_rekey( sqlite3 *db, const void *pKey, int nKey)     // db是指定的数据库，pKey是密钥，nKey是密钥的长度     // 例如：sqlite3_rekey(_db, "yzy", 3);     int rc = sqlite3_rekey(_db, [keyData bytes], (int)[keyData length]);          if (rc != SQLITE_OK) {         NSLog(@"error on rekey: %d", rc);         NSLog(@"%@", [self lastErrorMessage]);     }          return (rc == SQLITE_OK); #else #pragma unused(keyData)     return NO; #endif }

- (NSError*)inSavePoint:(void (^)(BOOL *rollback))block { #if SQLITE_VERSION_NUMBER >= 3007000     static unsigned long savePointIdx = 0;     // 设置节点名称     NSString *name = [NSString stringWithFormat:@"dbSavePoint%ld", savePointIdx++];     // 默认不回滚     BOOL shouldRollback = NO;          NSError *err = 0x00;     // 先对当前状态进行保存     if (![self startSavePointWithName:name error:&err]) {         return err;     }          if (block) {         block(&shouldRollback);     }     // 如果需要回滚，则回滚当上一个节点     if (shouldRollback) {         // We need to rollback and release this savepoint to remove it         [self rollbackToSavePointWithName:name error:&err];     }     // 释放节点     [self releaseSavePointWithName:name error:&err];          return err; #else     NSString *errorMessage = NSLocalizedString(@"Save point functions require SQLite 3.7", nil);     if (self.logsErrors) NSLog(@"%@", errorMessage);     return [NSError errorWithDomain:@"FMDatabase" code:0 userInfo:@{NSLocalizedDescriptionKey : errorMessage}]; #endif }

- (BOOL)close {     // 清除缓存的prepared语句     [self clearCachedStatements];     // 关闭所有打开的FMResultSet对象     [self closeOpenResultSets];          if (!_db) {         return YES;     }          int  rc;     BOOL retry;     BOOL triedFinalizingOpenStatements = NO;          do {         retry   = NO;         // 调用sqlite3_close尝试关闭数据库         rc      = sqlite3_close(_db);         // 当返回结果是数据库繁忙或者被锁住了         if (SQLITE_BUSY == rc || SQLITE_LOCKED == rc) {             if (!triedFinalizingOpenStatements) {                 triedFinalizingOpenStatements = YES;                 sqlite3_stmt *pStmt;                 // 从关联的pDb数据里面对应的prepared语句开始往下找相应的prepared语句，                 // 如果pStmt是NULL，则从pDb的第一个prepared语句开始找，                 // 如果没有找到，则返回NULL                 while ((pStmt = sqlite3_next_stmt(_db, nil)) !=0) {                     // 找到之后，释放资源                     NSLog(@"Closing leaked statement");                     sqlite3_finalize(pStmt);                     retry = YES;                 }             }         }         else if (SQLITE_OK != rc) {             NSLog(@"error closing!: %d", rc);         }     }     while (retry);          _db = nil;     return YES; }

[newDb close];