Objective C object not recognized in C function

Question

I am adding an Audio session property listener for kAudioSessionProperty_AudioRouteChange and in the callback I want to call the takePicture function of UIImagePickerController. The problem is that I cannot access my picker in the callback.

I am initializing my picker in viewDidLoad. I have tried declaring the picker as both a private and public object and accessing with picker or self.picker but it always gives the "Use of undeclared identifier" error. I use this code to add the listener:

AudioSessionInitialize(nil, nil, nil, nil);
AudioSessionSetActive(true);
AudioSessionAddPropertyListener(
                                kAudioSessionProperty_AudioRouteChange,
                                applicationAudioRouteDidChange,
                                self);

This is my callback:

void applicationVolumeDidChange(void *inClientData,
                                AudioSessionPropertyID inID,
                                UInt32 inDataSize, const void *inData)
{
    NSLog(@"Volume changed");
    //[picker takePicture]; Error

}

I also declared an NSArray to see if this was the problem with UIImagePickerController only but the array also gives the same error.

score 2 · Accepted Answer · edited Aug 03 '11 at 07:49

2

The last parameter of AudioSessionAddPropertyListener() is there so you can pass whatever you like into the callback.

 OSStatus AudioSessionAddPropertyListener (
       AudioSessionPropertyID         inID,
       AudioSessionPropertyListener   inProc,
       void                           *inClientData
    );

You are passing self so within the callback the void *inClientData parameter is a pointer to whichever object self is.

If self was an instance of UIImagePickerController then,

UIImagePickerController *picker = inClientData;
[picker takePicture];

edited Aug 03 '11 at 07:49

Bushra Shahid

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answered Aug 02 '11 at 11:02

hooleyhoop

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self is my view controller so in my case i use `UIImagePickerController *myPicker = (RootViewController*)inClientData.picker;` Your direction was correct though. – Bushra Shahid Aug 03 '11 at 07:07

score 1 · Answer 2 · answered Aug 02 '11 at 09:57

1

Your callbacks must be C functions, not Objective-C methods. These are not compatible. See this link.

answered Aug 02 '11 at 09:57

Rudy Velthuis

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my callback is a c function, im trying to access an objective c object from that function. Please read carefully before answering – Bushra Shahid Aug 03 '11 at 07:00
Hmmm... I was sure I saw a method there. Sorry. So in effect, you have a scope problem. What is the "self" you are passing? – Rudy Velthuis Aug 03 '11 at 08:08
problem solved....have a look at the accepted answer. 'self' is a viewcontroller class – Bushra Shahid Aug 03 '11 at 08:10
I will, thanks. Sorry again, don't know why I saw a method there. Glad it was solved. – Rudy Velthuis Aug 03 '11 at 08:13

Objective C object not recognized in C function

2 Answers2