MojoDatabase 源码学习之对象映射

Mojo-database是我个人比较喜欢多开源项目，下文是该项目打介绍和地址：
mojo-database
简介：
MojoDatabase is an ActiveRecord-like ORM for SQLite written in Objective-C for use in iOS applications.

The goal is to provide a nicer user interface for using SQLite with Objective-C applications via the ActiveRecord pattern.

下面，我将介绍一下mojo的对象映射机制。下面的代码就是model执行findsql是运
行的代码

NSArray *results = [database executeSql:sql
withParameters:parameters withClassForRow:[self class]];

从代码的［self class］就是把自己的类传递过去，

withParameters:(NSArray *)parameters 
withClassForRow:(Class )rowClass 

在execute函数中，有如下代码。

      if (needsToFetchColumnTypesAndNames) {
        columnTypes = [self columnTypesForStatement:statement];
        columnNames = [self columnNamesForStatement:statement];
        needsToFetchColumnTypesAndNames = NO;
      }

      id row = [[rowClass alloc] init];
      [self copyValuesFromStatement:statement 
      toRow:row queryInfo:queryInfo 
      columnTypes:columnTypes columnNames:columnNames];
      [rows addObject:row];
    }

其中id row = [[rowClass alloc] init];动态创建对象了，其中sqlite支持的类型有

-(int)columnTypeToInt:(NSString *)columnType {
  if ([columnType isEqualToString:@"INTEGER"]) {
    return SQLITE_INTEGER;
  } else if ([columnType isEqualToString:@"REAL"]) {
    return SQLITE_FLOAT;
  } else if ([columnType isEqualToString:@"TEXT"]) {
    return SQLITE_TEXT;
  } else if ([columnType isEqualToString:@"BLOB"]) {
    return SQLITE_BLOB;
  } else if ([columnType isEqualToString:@"NULL"]) {
    return SQLITE_NULL;
  }
  return SQLITE_TEXT;
}

提取数值的代码

-(id)valueFromStatement:(sqlite3_stmt *)statement 
column:(int)column queryInfo:(NSDictionary *)queryInfo columnTypes:(NSArray *)columnTypes 
{
  int columnType = [[columnTypes objectAtIndex:column] intValue];

  /*
   * force conversion to the declared type using sql conversions; this saves
   * some problems with NSNull being assigned to non-object values
   */
  if (columnType == SQLITE_INTEGER) {
    return [NSNumber numberWithInt:sqlite3_column_int(statement, column)];
  } else if (columnType == SQLITE_FLOAT) {
    return [NSNumber numberWithDouble:sqlite3_column_double(statement, column)];
  } else if (columnType == SQLITE_TEXT) {
    const char* text = (const char *) sqlite3_column_text(statement, column);
    if (text != nil) {
      return [NSString stringWithUTF8String:text];
    } else {
      return @"";
    }
  } else if (columnType == SQLITE_BLOB) {
    // create an NSData object with the same size as the blob
    return [NSData dataWithBytes:sqlite3_column_blob(statement, column) length:sqlite3_column_bytes(statement, column)];
  } else if (columnType == SQLITE_NULL) {
    return nil;
  }


  NSLog(@"Unrecognized SQL column type: %i for sql %@", columnType, [queryInfo objectForKey:@"sql"]);
  return nil;
}

设置对象属性的代码

      [row setValue:value forKey:[columnNames objectAtIndex:i]];
    }