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This chapter is from the book

Recipe: Adding a Simple Direct Manipulation Interface

Before moving on to more modern (and commonly used) gesture recognizers, take time to understand and explore the traditional method of responding to a user’s touch. You’ll gain a deeper understanding of the touch interface by learning how simple UIResponder touch event handling works.

When you work with direct manipulation, your design focus moves from the UIViewController to the UIView. The view, or more precisely the UIResponder, forms the heart of direct manipulation development. You create touch-based interfaces by customizing methods that derive from the UIResponder class.

Recipe 1-1 centers on touches in action. This example creates a child of UIImageView called DragView and adds touch responsiveness to the class. Because this is an image view, it’s important to enable user interaction (that is, set setUserInteractionEnabled to YES). This property affects all the view’s children as well as the view itself. User interaction is generally enabled for most views, but UIImageView is the one exception that stumps most beginners; Apple apparently didn’t think people would generally use touches with UIImageView.

The recipe works by updating a view’s center to match the movement of an onscreen touch. When a user first touches any DragView, the object stores the start location as an offset from the view’s origin. As the user drags, the view moves along with the finger—always maintaining the same origin offset so that the movement feels natural. Movement occurs by updating the object’s center. Recipe 1-1 calculates x and y offsets and adjusts the view center by those offsets after each touch movement.

Upon being touched, the view pops to the front. That’s due to a call in the touchesBegan:withEvent: method. The code tells the superview that owns the DragView to bring that view to the front. This allows the active element to always appear foremost in the interface.

This recipe does not implement touches-ended or touches-cancelled methods. Its interests lie only in the movement of onscreen objects. When the user stops interacting with the screen, the class has no further work to do.

Recipe 1-1 Creating a Draggable View

@implementation DragView
    CGPoint startLocation;

- (instancetype)initWithImage:(UIImage *)anImage
    self = [super initWithImage:anImage];
    if (self)
        self.userInteractionEnabled = YES;
    return self;

- (void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event
    // Calculate and store offset, pop view into front if needed
    startLocation = [[touches anyObject] locationInView:self];
    [self.superview bringSubviewToFront:self];

- (void)touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event
    // Calculate offset
    CGPoint pt = [[touches anyObject] locationInView:self];
    float dx = pt.x - startLocation.x;
    float dy = pt.y - startLocation.y;
    CGPoint newcenter = CGPointMake(
        self.center.x + dx,
        self.center.y + dy);

    // Set new location
    self.center = newcenter;
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