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Highlighting NSImageView the right way

Category: misc

Tags: cocoa

Recently I have been working on a small Cocoa app and one of the things I needed to do was highlight an NSImageView when the user is hovering above it while dragging a file. You would think that it's a very simple task, and you would be right, but judging by some of the things on the internet it looks like an unsolved problem.

The difficult part about Mac development is not Objective-C, if you know C then you can learn Objective-C in a few days. The hard part is Cocoa. Cocoa is a very old framework, older than .NET, Java and macOS. A framework this old is guaranteed to have grown a lot over the years and finding what you are looking for can be a daunting task. Even Apple's own documentation is filled with deprecated API calls or downright bad practice.

The wrong way

Here is how Apple's own documentation highlights an NSImageView instance. The code is from the official CocoaDragAndDrop example.

        draw method is overridden to do drop highlighing
    //do the usual draw operation to display the image
    [super drawRect:rect];

    if ( highlight ) {
        //highlight by overlaying a gray border
        [[NSColor grayColor] set];
        [NSBezierPath setDefaultLineWidth: 5];
        [NSBezierPath strokeRect: rect];

There are other implementations on the internet, but those can be excused for not being official. They all have the same fundamental problem anyway, so I'll just use this one for illustration.

The implementation looks simple enough and works, but only as long as your NSImageView instance satisfies the assumptions made here. We assume that our view object will always have the outline of a rectangle and we assume that the chosen colour will always be adequate. In my case the first assumption was already broken from the start because I was using the Bezel style for my view objects, a style that is part of Cocoa itself. There is also no telling when in the future Apple might change the default colours, making your choice of colour inadequate. And that's not even going into portability if you want to use GNUStep to port your app to other operating systems.

What we want is an abstraction. Abstractions provide us with a clean interface that can remain stable while the underlying implementation can change wildly. Cocoa has abstractions everywhere, such as using NSURL for file paths instead of C strings or even NSString. Doing things your own way should only be reserved for when you actually have control over the underlying code. So if I were to draw my own shapes for an NSView I would also be comfortable hard-coding my own highlighting.

The right way™

As it turns out Cocoa already provides us with the means of abstraction for highlighting. The NSControl class has the highlighted property, which has the getter isHighlighted, so we don't need a new property. For drawing the actual highlight NSView provides the method drawFocusRingMask. Subclasses can override the property to implement their own highlighting and luckily NSImageView does exacly that.

- (void)drawRect:(NSRect)dirtyRect {
    [super drawRect:dirtyRect];

    if ([self isHighlighted]) {
        [self drawFocusRingMask];

Now your code is future-proof and will work regardless of the shape you choose. Well, at least until Apple deprecates this method.