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NSUndoManager to undo UIImage rotation using rotation gesture
- (void)handleRotate:(UIRotationGestureRecognizer *)recognizer
{
    if (recognizer.state == UIGestureRecognizerStateBegan) {
        prevRotation = 0.0;
    }

    float thisRotate = recognizer.rotation - prevRotation;
    prevRotation = recognizer.rotation;
    recognizer.view.transform = CGAffineTransformRotate(recognizer.view.transform, thisRotate);

    CGPoint lastpoint = point;
}

@implementation YourViewController {
    CGAffineTransform _originalImageViewTransform;
}

- (void)setTransform:(CGAffineTransform)newTransform ofView:(UIView *)view
    undoTransform:(CGAffineTransform)undoTransform
{
    // If I'm called because the gesture ended, this pushes an undo action.
    // If I'm called because the user requested an undo, this pushes a redo action.
    [[self.undoManager prepareWithInvocationTarget:self]
        setTransform:undoTransform ofView:view undoTransform:newTransform];

    // Now actually set the transform.
    view.transform = newTransform;
}

- (void)handleRotate:(UIRotationGestureRecognizer *)recognizer {
    UIView *view = recognizer.view;
    UIGestureRecognizerState state = recognizer.state;

    if (state == UIGestureRecognizerStateCancelled) {
        view.transform = _originalImageViewTransform;
        return;
    }

    if (state == UIGestureRecognizerStateBegan) {
        _originalImageViewTransform = view.transform;
    }

    CGAffineTransform transform = view.transform;
    transform = CGAffineTransformRotate(transform, recognizer.rotation);
    recognizer.rotation = 0; // This line means we don't need prevRotation

    if (state == UIGestureRecognizerStateEnded) {
        [[ The gesture ended, so push an undo action before setting the transform.
        [self setTransform:transform ofView:view undoTransform:_originalImageViewTransform];
    } else {
        // The gesture changed but didn't end, so don't push an undo action.
        view.transform = transform;
    }
}