Getting UIScrollView to behave more like (small images are centred rather than shifting to the top left of the UIScrollView)
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NYOBetterZoom (NYOBetterZoomUIScrollView example project)


This little example project stemmed from the fact that by default UIScrollView chucks small images to the top left corner of it's frame. I (and a lot of other people) would prefer it centred in the scrollview (like Having spent far too long trying to find a solution without any strange issues I finally got it to behave. To save other people some time going round the houses (like I initially did) I'm uploading this example solution.

Feel free to fork and use/improve this (and push back improvements!). It's a start but the code could certainly be improved upon.

How it works

The solution I finally settled on is quite simple really (getting to this solution wasn't though...). I subclass UIScrollView and override it's setContentOffset: so that it returns a CGPoint that will result in the image being centred in it's UIScrollView superview if it's smaller than the UIScrollView.

How to use it

  1. Include NYOBetterZoomUIScrollView in your project
  2. #import the .h and switch you [[UIScrollView alloc] init...] out for [[NYOBetterZoomUIScrollView alloc] init...]
  3. Once the ScrollView is setup add your subview with setChildView: rather than addSubview:. (Alternatively you have an initWithChildView: and initWithFrame:andChildView: initialisers)
  4. Have your view controller implement UIScrollViewDelegate and have viewForZoomingInScrollView: return the childView.

Possible step 5 - call [yourScrollView setContentOffset:CGPointZero] if your small view is initially appearing in the top left of the NYOBetterZoomUIScrollView instead of centred. With the v2.0 code changes though you hopefully won't need to do this any more.

Example project

The included example project has all the setup required to get a functioning ScrollView that centres small images (and large images when they're zoomed out).

It also shows how to be a bit smarter in regard to device rotation too (recalculating the view's aspect ratio & new minimum zoom when it's rotated).

The project also includes a variety of test images to ensure it works correctly with images of different dimensions (you can switch which image is used in the top of NYOBetterZoomViewController.m's viewDidLoad()):

  • small.png (100x100) - Always smaller than the ScrollView frame, even when zoomed in 2x (maximumZoom).
  • big.png (1000x1000)
  • fit-portrait.png (320x460) - Same size as the portrait ScrollView's frame
  • fit-landscape.png (480x300) - Same size as the landscape ScrollView's frame
  • tall.png (500x1000)
  • wide.png (1000x500)