I wanted to be able to evaluate arithmetic represented as an NSString without having to write too much code. Using NSExpression it's a two step operation.

stackoverflow answer

NSExpression *expression = [NSExpression expressionWithFormat:@"9+4"];
// result is a NSNumber
id result = [expression expressionValueWithObject:nil context:nil]; 

Check out JLPeekModal on Github.

There are a few options which I need to refactor to allow for more obvious customisation. I'm hoping the code is readable enough for that already :)

Please note that this is for iOS 6.0+. In iOS 5.0 you can only customise the Back button without having to create a subclass

UIImage* plainBtn = [[UIImage imageNamed:@"btn"] resizableImageWithCapInsets:UIEdgeInsetsMake(12, 12, 12, 12)];
UIImage* plainBtnHighlight = [[UIImage imageNamed:@"btn-high"] resizableImageWithCapInsets:UIEdgeInsetsMake(12, 12, 12, 12)];

UIImage* doneBtn = [[UIImage imageNamed:@"btn-done"] resizableImageWithCapInsets:UIEdgeInsetsMake(12, 12, 12, 12)];
UIImage* doneBtnHighlight = [[UIImage imageNamed:@"btn-done-high"] resizableImageWithCapInsets:UIEdgeInsetsMake(12, 12, 12, 12)];

// Set the background images for all plain style buttons
[[UIBarButtonItem appearance] setBackgroundImage:plainBtn forState:UIControlStateNormal style:UIBarButtonItemStylePlain barMetrics:UIBarMetricsDefault];
[[UIBarButtonItem appearance] setBackgroundImage:plainBtnHighlight forState:UIControlStateHighlighted style:UIBarButtonItemStylePlain barMetrics:UIBarMetricsDefault];

// Set the background images for all of our done style buttons
[[UIBarButtonItem appearance] setBackgroundImage:doneBtn forState:UIControlStateNormal style:UIBarButtonItemStyleDone barMetrics:UIBarMetricsDefault];
[[UIBarButtonItem appearance] setBackgroundImage:doneBtnHighlight forState:UIControlStateHighlighted style:UIBarButtonItemStyleDone barMetrics:UIBarMetricsDefault];

Documentation for this method is here: UIBarButtonItem - setBackgroundImage:forState:style:barMetrics:

Something to watch out for if you're doing custom text attributes for your UIBarButtonItem's. E.g.

[[UIBarButtonItem appearance] setTitleTextAttributes:attrsNormal forState:UIControlStateNormal];
[[UIBarButtonItem appearance] setTitleTextAttributes:attrsHighlighted forState:UIControlStateHighlighted];

This goes for all of your UIBarButtonItem's in your app. What this means is if you had a white plainBtn image and a very dark doneBtn image you may have to watch out what UITextAttributeTextColor you set. E.g. if you set a black colour then it might not be seen on the Done buttons.

One way I found to get around this is making your root view a UINavigationController and setting itself as a UINavigationControllerDelegate.

You can then implement the delegate method navigationController:willShowViewController:animated::

#pragma mark - UINavigationControllerDelegate
- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated
{
    NSDictionary* attrs = @{UITextAttributeTextColor: [UIColor whiteColor]};
    for (UIBarButtonItem* rightBarButton in viewController.navigationItem.rightBarButtonItems)
    {
        if (rightBarButton.style == UIBarButtonItemStylePlain || rightBarButton.style == UIBarButtonItemStyleDone)
        {
            [rightBarButton setTitleTextAttributes:attrs forState:UIControlStateNormal];
            [rightBarButton setTitleTextAttributes:attrs forState:UIControlStateHighlighted];
        }
    }
}

It's kind of a cheat; but I haven't found a better solution to making sure your Plain and Done buttons have different title text attributes.

With the UIAppearance protocol this is really quite simple:

[[UINavigationBar appearance] setTitleTextAttributes:attrs];

attrs is an NSDictionary that is made up of Keys For Text Attribute Dictionaries

NSString *const UITextAttributeFont;
NSString *const UITextAttributeTextColor;
NSString *const UITextAttributeTextShadowColor;
NSString *const UITextAttributeTextShadowOffset;

E.g. To change all the titles to blue.

NSDictionary* attrs = @{ UITextAttributeTextColor: [UIColor blueColor] };
[[UINavigationBar appearance] setTitleTextAttributes:attrs];

Something to note when setting a custom font; the base line may be higher than a system font. I think this partly due to how the font is created but don't hold me to that one. In order to adjust the base line height you just need to do this:

[[UINavigationBar appearance] setTitleVerticalPositionAdjustment:3.5f forBarMetrics:UIBarMetricsDefault];

Play around with the vertical adjustment value to get it looking correct again.

I've written a gist to save out a UILabel as a UIImage.

This category makes a few assumptions:

• You're only creating an image for a single letter from an icon font
• You want to save out the image to file and load from that

Once an image is created it will be loaded each and every time. If you're going to be actively changing the icon font I'd recommend commenting out the saving code. I'd also recommend putting in a job on app load to clear out the PNG files created every couple of weeks in case of updates.

The creation of a PNG file for loading later isn't really required as the creation of a UIImage is very fast.