在IPhone中创建AVAudioRecorder实例

我在控制器实现文件中使用以下代码来创建AVAudioRecorder的实例。

-(NSError *) createAVAudioRecorder 
{
 [recorder release];
 recorder = nil; 

 NSDate *now = [NSDate dateWithTimeIntervalSinceNow:0]; 
 NSString *calDate = [now description]; 

 NSString *fileName = [NSString stringWithFormat:@"myfile"]; 


 NSLog(@"%@\%@",DOCUMENTS_FOLDER,calDate); 

 NSString *recorderFilePath = [[NSString stringWithFormat:@"%@/%@.caf",DOCUMENTS_FOLDER, fileName] retain]; 

 NSURL *url = [NSURL fileURLWithPath:recorderFilePath]; 

 NSMutableDictionary *recordSettings = [[NSMutableDictionary alloc]init]; 

 NSError *err = nil; 

 recorder = [[AVAudioRecorder alloc] initWithURL:url settings:recordSettings error:&err]; 

 return nil; 

}

它一直给我以下错误:

Undefined symbols:
  "_OBJC_CLASS_$_AVAudioRecorder", referenced from:
      objc-class-ref-to-AVAudioRecorder in myapp.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <CoreAudio/CoreAudioTypes.h> 



@interface MyController : UIViewController<AVAudioRecorderDelegate> {

 IBOutlet UIButton *recordButton; 
 AVAudioRecorder *recorder; 

}

-(IBAction) record: (id) sender; 
-(NSError *) createAVAudioRecorder; 


@end