iOS-ESpeak API proposal (UPDATE)

I would like to know your opinion of the following API proposal.
Instead of using an API like the one defined in NSSpeechSynthesizer where the strings are parsed every time you want to speech it, I decided to divide the 'parsing' and 'speaking' responsibility in two different classes:

• ESSpeechSynthesizer
• ESSpeechSynthesis

Here you have the API.

/*
* ESSpeechSynthesis
* =================
* This class represent an unmutable synthesized message ready to be played.
*/
@interface ESSpeechSynthesis : NSObject 
{ .. }

@property(nonatomic, assign) id delegate;
@property(nonatomic, readonly) NSString *string;
@property(nonatomic, retain) NSString *identifier;
@property(nonatomic, readonly) NSDictionary *properties;

- (BOOL)isSpeaking;
- (BOOL)startSpeaking;
- (void)stopSpeaking;
- (void)pauseSpeaking;
- (void)continueSpeaking;

@end

/*
* ESSpeechSynthesisDelegate
* =========================
* This protocol represent the Synthesis delegate.
*/
@protocol ESSpeechSynthesisDelegate 

@optional
- (void)speechSynthesisBeginInterruption:(ESSpeechSynthesis *)sender;
- (void)speechSynthesisEndInterruption:(ESSpeechSynthesis *)sender;

- (void)speechSynthesisDecodeErrorDidOccur:(ESSpeechSynthesis *)sender error:(NSError *)error;
- (void)speechSynthesisDidFinishPlaying:(ESSpeechSynthesis *)sender successfully:(BOOL)flag;

@end

/*
* ESSpeechSynthesizer
* ===================
* This class represent a singleton that can convert a string into a sound message.
*/
@interface ESSpeechSynthesizer: NSObject
{ ... }

@property(nonatomic, readonly) NSString* defaultVoice;
@property(nonatomic, retain) NSString* voice;

// ...

+ (ESSpeechSynthesizer *)sharedESSpeechSynthesizer;

- (NSArray *)availableVoices;
- (NSDictionary *)attributesForVoice:(NSString *)voice;

- (ESSpeechSynthesis *)synthesize:(NSString *)text;

@end

Here you have an example of how to use this API:

// First obtain the synthesize singleton instance.
ESSpeechSynthesizer *synth = [ESSpeechSynthesizer sharedESSpeechSynthesizer];

// Then configure it, in this case just setup the voice language.
[synth setVoice:@"english"];

// Convert string into sound
ESpeechSynthesis speech = [synth synthesize:msg];

// Some log
NSLog(@"Available voices: %@", [synth availableVoices]);
NSLog(@"Start speacking: %@", speech);

// Set the delegate for notification
[speech setDelegate:self];

// Start speaking
[speech startSpeaking];

// This method will be invoked when the speaking is terminated
- (void)speechSynthesizerDidFinishPlaying:(ESSpeechSynthesis *)sender successfully:(BOOL)flag
{
  // Release here if sound is not needed anymore
  [speech release];
  speech = nil;
}

All comments are welcome.

UPDATE: Please notice that the ESSpeechSynthesizer API is not thread safe, What do you think about a new thread safe method like the following:

@interface ESSpeechSynthesizer: NSObject

- (ESSpeechSynthesis *)synthesize:(NSString *)text properties:(NSDictionary *)props;

@end

Where the dictionary instance is used to specify the synthesis attributes like: voice, gender, pitch, ....

As usual all comments are welcome !

--
John