Instantiating UIActivity subclass - ios

Question

When sharing an image from, eg Photos, I want my iOS app extension to appear in the list of apps available (next to Mail, Messages...) in order to receive the shared picture. Reading many sites reveals I have to subclass UIActivity and add it to an App Extension, what I did.
However I don't understand how I should instantiate my subclass, and it never gets called (No NSLog is displayed, item not in the sharing list).
How should I "instruct" iOS to use this subclass ?
myActivity.m:
#import "myActivity.h"
#implementation myActivity
- (NSString *)activityType {
// a unique identifier
return #"com.myapp.uniqueIdentifier";
}
- (NSString *)activityTitle {
// a title shown in the sharing menu
return #"Custom Activity";
}
- (UIImage *)activityImage {
// an image to go with our option
return [UIImage imageNamed:#"MyImage"];
}
+ (UIActivityCategory)activityCategory {
// which row our activity is shown in
// top row is sharing, bottom row is action
NSLog(#"UIActivityCategoryShare");
return UIActivityCategoryShare;
}
- (BOOL)canPerformWithActivityItems:(NSArray *)activityItems {
// return YES for anything that our activity can deal with
for (id item in activityItems) {
// we can deal with strings and images
if ([item isKindOfClass:[UIImage class]]) {
return YES;
}
}
// for everything else, return NO
return NO;
}
- (void)prepareWithActivityItems:(NSArray *)activityItems {
// anything we need to prepare, now's the chance
// custom UI, long running calculations, etc
// also: grab a reference to the objects our user wants to share/action
self.activityItems = activityItems;
}
- (UIViewController *)activityViewController {
// return a custom UI if we need it,
// or the standard activity view controller if we don't
return nil;
}
- (void)performActivity {
// the main thing our activity does
// act upon each item here
for (id item in self.activityItems) {
NSLog(#"YEY - someone wants to use our activity!");
NSLog(#"They used this object: %#", [item description]);
}
// notify iOS that we're done here
// return YES if we were successful, or NO if we were not
[self activityDidFinish:YES];
}
#end
And myActivity.h:
#import <UIKit/UIKit.h>
#interface myActivity : UIActivity
#property (nonatomic, strong) NSArray *activityItems;
#end