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メモ

   __weak typeof(self) bself = self;
    UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] initWithHandler:^(UIGestureRecognizer *sender, UIGestureRecognizerState state, CGPoint location) {
        [bself closeKeyboard];
    }];
    
    NSArray* recognizers = [self.userCollectionView gestureRecognizers];
    // Make the default gesture recognizer wait until the custom one fails.
    for (UIGestureRecognizer* aRecognizer in recognizers) {
        if ([aRecognizer isKindOfClass:[UITapGestureRecognizer class]])
            [tapGesture requireGestureRecognizerToFail:aRecognizer];
    }
    // Now add the gesture recognizer to the collection view.
    tapGesture.numberOfTapsRequired = 1;
    [self.userCollectionView addGestureRecognizer:tapGesture];