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POObject subclassing approaches
 

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    <p>Suppose I was designing a component representing grips on a UI control for resizing it. </p> <p><img src="https://i.stack.imgur.com/Hgplp.jpg" alt="Control"></p> <p>now obviously each grip is not exactly the same in terms of behavior, for instance the top left grip will have to display a North West South East mouse cursor and dragging it will change the size of the control whereas the top middle grip needs to show a north south mouse cursor and dragging it will only allow you to change the height.</p> <p>In terms of implementation there are two basic approaches:-</p> <p><strong>(1)</strong> a constructor which requires you to pass in information as to what kip of grip you are designing and then have a huge switch statement inside to define the actual behavior of the grip like so :-</p> <pre><code>- initWithGripType(int)gripType { switch(gripType { case TOP_MIDDLE_GRIP: cursorType = northSouthCursor; draggedMovement = upDown; break; case TOP_LEFT_GRIP: cursorType = northWestSouthEastCursor; draggedMovement = upDownLeftRight; break; ... ... ... } } </code></pre> <p><strong>(2)</strong> have a grip base class that defines the common attributes. e.g painting a grip is the same no matter which grip it is. And then sub classes for each grip type liks so :-</p> <pre><code>@interface Grip : NSObject { NSCursor _cursorType; int _draggedMovement; } // force init to be unavilable for base class - (id)init __attribute__((unavailable("Instantiate subclass instead."))); - (void)drawGrip; @end; @interface GripTopLeft : Grip { } - (id)init; @end; @interface GripBottomLeft : Grip { } - (id)init; @end; ... ... ... </code></pre> <p>Approach two has the advantage that I don't have to maintain any switch statements and can add new grip types at will, the downside is that if I have many grip types, each one is now a separate source and header file.</p> <p>Is there a third approach?</p>
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