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Fixing the bugs: slow shadows, leaking UITableViewCells

It's time for us to use instruments to spot and fix some problems. Important: when making performance changes you should change only one thing at a time, then re-test to make sure your change helped. If you changed two or more things and performance got better, which one worked? Or, if performance got worse, perhaps one thing worked and one didn't!

Let's begin with the table view: you should have seen parts of the table view turn dark yellow when Color Offscreen-Rendered Yellow was selected. This is happening because the images are being rendered inefficiently: the rounded corners effect and the shadow are being done in real-time, which is computationally expensive.

You can find the code for this in SelectionViewController.swift, inside the cellForRowAt method:

let renderer = UIGraphicsImageRenderer(size: original.size)

let rounded = renderer.image { ctx in
    ctx.cgContext.addEllipse(in: CGRect(origin:, size: original.size))


cell.imageView?.image = rounded

// give the images a nice shadow to make them look a bit more dramatic
cell.imageView?.layer.shadowColor =
cell.imageView?.layer.shadowOpacity = 1
cell.imageView?.layer.shadowRadius = 10
cell.imageView?.layer.shadowOffset =

There are two new techniques being demonstrated here: creating a clipping path and rendering layer shadows.

We’ve used UIGraphicsImageRenderer before to create custom-rendered images, and the rendering here is made up of three commands: adding an ellipse and drawing a UIImage are both things you’ve seen before, but the call to clip() is new. As you know, you can create a path and draw it using two separate Core Graphics commands, but instead of running the draw command you can take the existing path and use it for clipping instead. This has the effect of only drawing things that lie inside the path, so when the UIImage is drawn only the parts that lie inside the elliptical clipping path are visible, thus rounding the corners.

The second new technique in this code is rendering layer shadows. iOS lets you add a basic shadow to any of its views, and it's a simple way to make something stand out on the screen. But it's not fast: it literally scans the pixels in the image to figure out what's transparent, then uses that information to draw the shadow correctly.

The combination of these two techniques creates a huge amount of work for iOS: it has to load the initial image, create a new image of the same size, render the first image into the second, the render the second image off-screen to calculate the shadow pixels, then render the whole finished product to the screen. When you hit a performance problem, you either drop the code that triggers the problem or you make it run faster.

In our case, we'll assume the designer insists the drop shadow is gorgeous so we need to make the code faster. There are several different approaches we could take, and I want to walk you through each of them so you can see the relative benefits of each.

The first possibility: Core Graphics is more than able of drawing shadows itself, which means we could handle the shadow rendering in our UIGraphicsImageRenderer pass rather than needing an extra render pass. To do that, we can use the Core Graphics setShadow() method, which takes three parameters: how far to offset the shadow, how much to blur it, and what color to use. You’ll notice there’s no way of specifying what shape the shadow should be, because Core Graphics has a simple but powerful solution: once you enable a shadow, it gets applied to everything you draw until you disable it by specifying a nil color.

So, we can replicate our current shadow like this:

let rounded = renderer.image { ctx in
    ctx.cgContext.setShadow(offset:, blur: 200, color:
    ctx.cgContext.fillEllipse(in: CGRect(origin:, size: original.size))
    ctx.cgContext.setShadow(offset:, blur: 0, color: nil)

    ctx.cgContext.addEllipse(in: CGRect(origin:, size: original.size))


Notice how the blur is 200 points, which is quite different from the shadow radius of 10 in the old code? The reason for this is important, because it highlights another significant problem in the code. When the original code set the shadow size using cell.imageView?.layer.shadowRadius it was specified in points relative to the size of the UIImageView. When the new code sets the shadow size using setShadow() it’s in points relative to the size of the image being drawn, which is created like this:

let renderer = UIGraphicsImageRenderer(size: original.size)

The problem is that the images being loaded are 750x750 pixels at 1x resolution, so 1500x1500 at 2x and 2250x2250 at 3x. If you look at viewDidLoad() you’ll see that the row height is 90 points, so we’re loading huge pictures into a tiny space. On iPhone 8, that means loading a 1500x1500 image, creating a second render buffer that size, rendering the image into it, and so on.

Clearly those images don’t need to be anything like that size, but sometimes you don’t have control over it. In this app you might be able to go back to the original designer and ask them to provide smaller assets, or if you were feeling ready for a fight you could resize them yourself, but what if you had fetched these assets from a remote server? And wait until you see the size of the images in the detail view – those images might only take up 500KB on disk, but when they are uncompressed by iOS they’ll need around 45 MB of RAM!

A second thing to notice is that the result of this new shadowing isn’t quite the same, because the shadow being rendered is now properly clipped inside the bounds of its image view. Although it’s more technically correct, it doesn’t look the same, and I’m going to assume that the original look – ugly as it was – was intentional.

So, option 1 – making Core Graphics draw the shadow – helps eliminate the second render pass, but it has very different results and a result we should rule it out. However, it did at least point us to an interesting problem: we’re squeezing very large images into a tiny space. iOS doesn’t know or care that this is happening because it just does what its told, but we have more information: we know the image isn’t needed at that crazy size, so we can use that knowledge to deliver huge performance increases.

First, change the rendering code to this:

let renderRect = CGRect(origin:, size: CGSize(width: 90, height: 90))
let renderer = UIGraphicsImageRenderer(size: renderRect.size)

let rounded = renderer.image { ctx in
    ctx.cgContext.addEllipse(in: renderRect)

    original.draw(in: renderRect)

That still causes iOS to load and render a large image, but it now gets scaled down to the size it needs to be for actual usage, so it will immediately perform faster.

However, it still incurs a second rendering pass: iOS still needs to trace the resulting image to figure out where the shadow must be drawn. Calculating the shadow is hard, because iOS doesn’t know that we clipped it to be a circle so it needs to figure out what's transparent itself. Again, though, we have more information: the shadow is going to be a perfect circle, so why bother having iOS figure out the shadow for itself?

We can tell iOS not to automatically calculate the shadow path for our images by giving it the exact shadow path to use. The easiest way to do this is to create a new UIBezierPath that describes our image (an ellipse with width 90 and height 90), then convert it to a CGPath because CALayer doesn't understand what UIBezierPath is.

Here's the updated shadow code:

// give the images a nice shadow to make them look a bit more dramatic
cell.imageView?.layer.shadowColor =
cell.imageView?.layer.shadowOpacity = 1
cell.imageView?.layer.shadowRadius = 10
cell.imageView?.layer.shadowOffset =
cell.imageView?.layer.shadowPath = UIBezierPath(ovalIn: CGRect(x: 0, y: 0, width: 90, height: 90)).cgPath

When you run that, you'll still see the same shadows everywhere, but the dark yellow color is gone. This means we’ve successfully eliminated the second render pass by giving iOS the pre-calculated shadow path, and we’ve also sped up drawing by scaling down the amount of working being done. You can turn off Color Offscreen-Rendered Yellow now and quit Instruments.

Working with rounded corners and shadows can be tricky, as you’ve seen here. If it weren’t for the shadowing, we could eliminate the first render pass by setting layer.cornerRadius to have iOS round the corners for us – it’s a nice and easy way to create rounded rectangle shapes (or even circles!) without any custom rendering code.

Wasted allocations

Back in Xcode, press Cmd+I to launch a fresh instance of Instruments, and this time I want you to choose the Allocations instrument. This tells you how many objects you're creating and what happens to them. Press record, then scroll around the table view a few times to get a complete picture of the app running. At the very least, you should go all the way down to the bottom and back up two or three times.

What you'll see is a huge collection of information being shown – lots of "malloc", lots of "CFString", lots of "__NSArrayM” and more. Stuff we just don't care about right now, because most of the code we have is user interface work. Fortunately, there's a search box just below the detail pane – it should say "Instrument Detail" but if you type "UI" in there it will only show information that has "UI" somewhere in there, which just happens to be all of Apple's user interface libraries!

In the picture below you can see how filtering for "UI" inside Instruments shows only data that has "UI" in its name somewhere, which primarily restricts the view to things that come from Apple's UIKit libraries.

Use Instruments's filtering system to show only items that matter to you.

Once you filter by "UI" you'll see see UIImage, UIImageView, UITableViewCell and more. The allocations instrument will tell you how many of these objects are persistent (created and still exist) and how many are transient (created and since destroyed). Notice how just swiping around has created a large number of transient UIImageView and UITableViewCell objects?

This is happening because each time the app needs to show a cell, it creates it then creates all the subviews inside it – namely an image view and a label, plus some hidden views we don’t usually care about. iOS works around this cost by using the method dequeueReusableCell(withIdentifier:), but if you look at the cellForRowAt method you won’t find it there. This means iOS is forced to create a new cell from scratch, rather than re-using an existing cell. This is a common coding mistake to make when you're not using storyboards and prototype cells, and it's guaranteed to put a speed bump in your apps.

If you look inside cellForRowAt method you'll see this line:

let cell = UITableViewCell(style: .default, reuseIdentifier: "Cell")

That's the only place where table view cells are being created, so clearly it's the culprit because it creates a new cell every time the table view asks for one. This has been slow since the very first days of iOS development, and Apple has always had a solution: ask the table view to dequeue a cell, and if you get nil back then create a cell yourself.

This is different from when we were using prototype cells with a storyboard. With storyboards, if you dequeue a prototype cell then iOS automatically handles creating them as needed.

If you're creating table view cells in code, you have two options to fix this intense allocation of views. First, you could rewrite the above line to be this:

var cell: UITableViewCell! = tableView.dequeueReusableCell(withIdentifier: "Cell")

if cell == nil {
    cell = UITableViewCell(style: .default, reuseIdentifier: "Cell")

That dequeues a cell, but if it gets nil back then we create one. Note the force unwrapped optional at the end of the first line – we’re saying that cell might initially be empty, but it will definitely have a value by the time it’s used.

The other solution you could use is to register a class with the table view for the reuse identifier "Cell". Using this method you could leave the original line of code as-is, but add this to viewDidLoad():

tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")

With that line of code, you will never get nil when dequeuing a cell with the identifier "Cell". As with prototype cells, if there isn't one to dequeue a new cell will be created automatically.

The second solution is substantially newer than the first and can really help cut down the amount of code you need. But it has two drawbacks: with the first solution you can specify different kinds of cell styles than just .default, not least the .subtitle option we used in project 7; also, with the first solution you explicitly know when a cell has just been created, so it's easy to force any one-off work into the if cell == nil { block.

Regardless of which solution you chose (you'll use both in your production code, I expect), you should be able to run the allocations instrument again and see far fewer table view cell allocations. With this small change, iOS will just reuse cells as they are needed, which makes your code run faster and operate more efficiently.

Running out of memory

Now, why does the app crash when you go the detail view controller enough times? There are two answers to this question, one code related and one not. For the first answer, open one of the images in Finder, such as LOL-Large.jpg, and you should notice that it’s extremely large – just like the so-called thumbnails we were working with earlier.

But there's something else subtle here, and it's something we haven't covered yet so this is the perfect time. When you create a UIImage using UIImage(named:) iOS loads the image and puts it into an image cache for reuse later. This is sometimes helpful, particularly if you know the image will be used again. But if you know it's unlikely to be reused or if it's quite large, then don't bother putting it into the cache – it will just add memory pressure to your app and probably flush out other more useful images!

If you look in the viewDidLoad() method of ImageViewController you'll see this line of code:

let original = UIImage(named: image)!

How likely is it that users will go back and forward to the same image again and again? Not likely at all, so we can skip the image cache by creating our images using the UIImage(contentsOfFile:) initializer instead. This isn't as friendly as UIImage(named:) because you need to specify the exact path to an image rather than just its filename in your app bundle, but you already know how to use path(forResource:) so it's not so hard:

let path = Bundle.main.path(forResource: image, ofType: nil)!
let original = UIImage(contentsOfFile: path)

Retain cycles

Let's take a look at one more problem, this time quite subtle. Loading the images was slow because they were so big, and iOS was caching them unnecessarily. But UIImage's cache is intelligent: if it senses memory pressure, it automatically clears itself to make room for other stuff. So why does our app run out of memory?

To find another problems, profile the app using Instruments and select the allocations instrument again. This time filter on "imageviewcontroller" and to begin with you'll see nothing because the app starts on the table view. But if you tap into a detail view then go back, you'll see one is created and remains persistent – it hasn't been destroyed. Which means the image it's showing also hasn't been destroyed, hence the massive memory usage.

What's causing the image view controller to never be destroyed? If you read through SelectionViewController.swift and ImageViewController.swift you might spot these two things:

  1. The selection view controller has a viewControllers array that claims to be a cache of the detail view controllers. This cache is never actually used, and even if it were used it really isn't needed.
  2. The image view controller has a property var owner: SelectionViewController! – that makes it a strong reference to the view controller that created it.

The first problem is easily fixed: just delete the viewControllers array and any code that uses it, because it's just not needed. The second problem smells like a strong reference cycle, so you should probably change it to this:

weak var owner: SelectionViewController!

Run Instruments again and you'll see that the problem is… still there?! That's right: those two were either red herrings or weren't enough to solve the problem, because something far more sneaky is happening.

The view controllers aren't destroyed because of this line of code in ImageViewController.swift:

animTimer = Timer.scheduledTimer(withTimeInterval: 5, repeats: true) { timer in

That timer does a hacky animation on the image, and it could easily be replaced with better animations as done inside project 15. But even so, why does that cause the image view controllers to never leak?

The reason is that when you provide code for your timer to run, the timer holds a strong reference to it so it can definitely be called when the timer is up. We're using self inside our timer’s code, which means our view controller owns the timer strongly and the timer owns the view controller strongly, so we have a strong reference cycle.

There are two solutions here: rewrite the code using smarter animations, or destroy the timer when it's no longer needed, thus breaking the cycle. The second option is easier, because it avoids having to write too much new code. In fact, all we need to do is detect when the image view controller is about to disappear and stop the timer. We'll do this in viewWillDisappear():

override func viewWillDisappear(_ animated: Bool) {

Calling invalidate() on a timer stops it immediately, which also forces it to release its strong reference on the view controller it belongs to, thus breaking the strong reference cycle. If you profile again, you'll see all the ImageViewController objects are now transient, and the app should no longer be quite so crash-prone.

That being said, the app might still crash sometimes because despite our best efforts we’re still juggling pictures that are far too big. However, the code is at least a great deal more efficient now, and none of the problems were too hard to find.

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