Grokking Cocoa

This is one of those errors that just make you go, duh!

Can you spot the problem with the following two snippets of code?

Snippet 1

    NSError *error;

    if (![someObject performFetch:&error])

        NSLog(@"%@: %@", [error description], [error userInfo]);

Snippet 2

    NSError *error;

    if (![self.someObject performFetch:&error])

        NSLog(@"%@: %@", [error description], [error userInfo]);

At first this does not appear to be a problem, but what if someObject is nil in the first case or the accessor self.someObject returns nil in the second?  If either is nil then the message is sent to the nil object which will cause the conditional to fire (a message sent to the nil object returns 0 or nil). Now when error is used it has an uninitialized value, which will probably cause things to blow up, but now always.

The solution to initialize error to nil,

    NSError *error = nil;

    if (![self.someObject performFetch:&error])

        NSLog(@"%@: %@", [error description], [error userInfo]);

Or in some cases add a check for the NSError also,

My first pass solution to animate a UIBarButtonItem was to initialize it's custom view with a UIImageView. The UIImageView can then be set up for animation with multiple images. Calling startAnimaiton/stopAnimation on the UIImaveView will then animate. This is great for animating buttons in tool and tab bars.

One problem is that the when initializing the UIImageView into the UIBarButtonItem, the UIBarButtonItem does not respond to touch events. Yuk.

A simple solution is to initialize the UIBarButtonItem custom view with a UIButton, then touch events for the UIBarButtonItem item are handle correctly.

But the UIButton does not have a way to use a UIImageView only UIImages. We want to use the UIImageView for its simple animation.

The solution is to add the UIImageView as a subview of the UIButton.

Here is the code.

NSArray *images = [NSArray arrayWithObjects:

        [UIImage imageNamed:@"image0.png"],

        [UIImage imageNamed:@"image1.png"],

        nil];

imageView = [[UIImageView alloc] initWithImage:[UIImage  imageNamed:@"image0.png"]];