I am experimenting with some new properties in iOS 5 regarding UIProgressView. They are:

@property(nonatomic, retain) UIImage *progressImage;
@property(nonatomic, retain) UIImage *trackImage;

These new properties enable the customisation of the "progress" and the "track" image, so that you can make fancy progress bars without having to roll-your-own.

I however cannot understand how Apple "stretches" the progress images, because documentation is a little flakey / OR there is some standard I am not aware of. Regardless, I am asking if someone can help me understand how to make appropriate progress and tracking images.

I get results like this when I load my custom images, no matter which sizes I try:

My measurements are as follows:

• UIProgressView length: 226 units
• trackingImage.png: 10px
• progressImage.png: 7px

Lastly, here are my custom images:

progressImage.png

trackImage.png

Here's what's going on:

The images you provide to the UIProgressView are basically being shoved in to UIImageViews, and the UIImageView is stretching the image to fill the space.

If you simply do:

[progressView setTrackImage:[UIImage imageNamed:@"track.png"]];

Then you're going to get weird results, because it's trying to stretch a 10px wide image to fill (for example) a 100px wide image view. This means (roughly) that every pixel in the image will be repeated 10 times. So if the pixels in our image were:

0123456789

Then putting that image straight into a 100px wide image view would stretch it something like this:

000000000011111111112222222222333333333344444444445555555555...

This is what's happening to you.

What you really want to have happen is this:

01234567812345678123456781234567812345678...123456789

In other words, you want the image to have a 1 point left edge that is never stretched, the center to be tiled, and to have a 1 point right edge that is also never stretched. To do this, you'll need to make the image resizable:

UIImage *track = [[UIImage imageNamed:@"track"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 1, 0, 1)];
[progressView setTrackImage:track];

If you want this to tile appropriately vertically as well, then the edge insets should be {1, 1, 1, 1} (assuming you want a 1 point border).

Do the same to the progressImage, and you'll end up with something that looks correct:

tl;dr:

Your images need to be resizable.