MojoDatabase 源碼學習之對象映射

Mojo-database是我我的比較喜歡多開源項目，下文是該項目打介紹和地址：
mojo-database
簡介：
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，我將介紹一下mojo的對象映射機制。下面的代碼就是model執行findsql是運
行的代碼

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

從代碼的［self class］就是把本身的類傳遞過去，

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

在execute函數中，有以下代碼。

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];動態建立對象了，其中sqlite支持的類型有

-(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;
}

提取數值的代碼

-(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;
}

設置對象屬性的代碼

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