iOS - UIImageWriteToSavedPhotosAlbum

Question

I wanted to use the following void API to write a captured image to the photo album but I am not very clear about 2 of the parameters

UIImageWriteToSavedPhotosAlbum (    UIImage  *image,    id       completionTarget,    SEL      completionSelector,    void     *contextInfo ); 

From the explanation from ADC:

completionTarget: optional; the object whose selector should be called after the image has been written to the Camera Roll album.

completionSelector: the method selector of the completionTarget object. This optional method should conform to the following signature:

- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo: (void *) contextInfo; 

What is the significance of completionTarget here? Can someone explain with an example how this parameter should be used? Or any resource which can guide me through it.

Answer 1

• The completionSelector is the selector (method) to call when the writing of the image has finished.
• The completionTarget is the object on which to call this method.

Generally:

• Either you don't need to be notified when the writing of the image is finished (in many cases that's not useful), so you use nil for both parameters
• Or you really want to be notified when the image file has been written to the photo album (or ended up with a writing error), and in such case, you generally implement the callback (= the method to call on completion) in the same class that you called the UIImageWriteToSavedPhotosAlbum function from, so the completionTarget will generally be self

As the documentation states, the completionSelector is a selector representing a method with the signature described in the documentation, so it has to have a signature like:

- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo: (void *) contextInfo; 

It does not have to have this exact name, but it has to use the same signature, namely take 3 parameters (the first being an UIImage, the second an NSError and the third being of void* type) and return nothing (void).

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Example

You may for example declare and implement a method that you could call anything like this :

- (void)thisImage:(UIImage *)image hasBeenSavedInPhotoAlbumWithError:(NSError *)error usingContextInfo:(void*)ctxInfo {     if (error) {         // Do anything needed to handle the error or display it to the user     } else {         // .... do anything you want here to handle         // .... when the image has been saved in the photo album     } } 

And when you call UIImageWriteToSavedPhotosAlbum you will use it like this:

UIImageWriteToSavedPhotosAlbum(theImage,    self, // send the message to 'self' when calling the callback    @selector(thisImage:hasBeenSavedInPhotoAlbumWithError:usingContextInfo:), // the selector to tell the method to call on completion    NULL); // you generally won't need a contextInfo here 

Note the multiple ':' in the @selector(...) syntax. The colons are part of the method name so don't forget to add these ':' in the @selector (event the trainling one) when you write this line!