Throughout this tutorial, we won't dive too deep into detecting and fixing memory issues. However, this step will discuss a specific issue of our Makestagram app - that should make you aware of potential problems in your own app.

What are Memory Warnings?

If you spent some time using the current version of Makestagram and upload a large amount of photos you will likely see a memory warning show up in the Xcode console. Potentially your app will even crash. Despite the fact that today's iPhones are powerful devices with lots of memory and processing power, we still have limited resources to work with.

Because users of today's mobile devices expect apps to be extremely responsive, mobile operating systems must divide the available resources efficiently between the different apps running on a phone. What does this mean for you? If your app is too greedy in its memory consumption, iOS will terminate it to prevent your app from causing the entire phone to slow down.

If you see a memory warning in the console or if your app even crashes due to a memory warning, it's time to act.

What Are Typical Causes of Memory Warnings?

Most apps run into memory problems when objects are retained too long. This means you have objects or resources in your program, that stay around even though you no longer need access to them. Further, most memory problems are caused by large resources such as videos or images.

What's the Problem in Makestagram?

Since we won't discuss the debugging of memory issues as part of this tutorial, I'll provide the answer for you; however, first I'd like to give you a chance to guess which part of our code could be responsible for our memory issues.

It is the code that is handling the images files! Currently, images are stored together with Posts. These images are stored as long as the post images stick around. If we load 100 posts for our timeline and a user scrolls through all posts, we have 100 images stored in memory, which takes up a lot of space! Ideally, we only want to keep the images that are currently displayed in memory.

How Can We Fix the Problem?

We can discard the image data that is stored in the image property of a Post, when the Post is no longer displayed. Where can we find out about posts that are no longer displayed? Inside of the PostTableViewCell!

Whenever the PostTableViewCell receives a new post that should be displayed, we can check if the image of the old post needs to be retained or if we can free that memory up.

Let's add the code and then discuss it in detail.

Update the didSet observer of the post property in the PostTableViewCell class to look as following:

var post:Post? {
  didSet {

    // free memory of image stored with post that is no longer displayed
    // 1
    if let oldValue = oldValue where oldValue != post {
        oldValue.image.value = nil

      if let post = post {
        postDisposable = post.image.bindTo(postImageView.bnd_image)

        likeDisposable = post.likes.observe { (value: [PFUser]?) -> () in
          if let value = value {
            self.likesLabel.text = self.stringFromUserList(value)
            self.likeButton.selected = value.contains(PFUser.currentUser()!)
            self.likesIconImageView.hidden = (value.count == 0)
          } else {
            self.likesLabel.text = ""
            self.likeButton.selected = false
            self.likesIconImageView.hidden = true
  1. The oldValue variable is available automatically in the didSet property observer. It provides us with a way to access the previous value of a property. We check if an oldValue exists and if that oldValue is different from the new post. If that's the case, we know that we need to do some cleanup.
  2. By setting oldValue.image.value to nil we are secretly fixing an issue that we haven't even discussed yet. Without this code in place, we are adding a new binding whenever a new post gets assigned to our PostTableViewCell. In most cases, where you create a binding, you should also have code that destroys that binding when it is no longer needed. In the case of the PostTableViewCell we don't need the binding anymore if the cell is displaying a new post. By setting oldValue.image.value to nil, we unsubscribe from future updates of the old post.

Great! Whenever an image is no longer displayed, we are freeing up the memory. That means we'll only have about 3 images in memory at any point in time. Our memory issue is fixed!

Won't This Make Our App Slower?

Yes and No. You might wonder if resetting the image to nil means that we need to download the image again, every single time a post is displayed, even if we have loaded the image previously.

Luckily that is not the case! Let's take a short look at the downloadImage method in the Post class:

func downloadImage() {
     imageFile?.getDataInBackgroundWithBlock { (data, error) -> Void in
      if let data = data {
        let image = UIImage(data: data, scale:1.0)!
        self.image.value = image

You can see that we are calling imageFile?.getDataInBackgroundWithBlock whenever the image.value == nil condition is true. However, Parse automatically takes care of caching our downloads and storing them on disk. This means if we request an image that has been downloaded recently, it will be fetched from the iPhone's local storage instead of from the Parse server.

However, if you run the app in the current version you might be able to realize small freezes and delays when scrolling up and down the timeline. That's because even loading image files from disk can take a noticeable amount of time.

So now we have traded our memory problem for a performance problem; can we have the best of both worlds?

Improving Performance

Yes we can! Thanks to NSCacheSwift. NSCacheSwift is very similar to a Dictionary it allows us to store key-value pairs. However, when our app receives a memory warning, entries are automatically removed from NSCacheSwift. That prevents our app from being shut down by the operating system.

So how can we use NSCacheSwift to solve our memory problem?

Whenever we download an image, we store it in the cache. We use the filename of the PFFile as a key for the cache. Next time we need to download a PFFile, we check if that file has already been downloaded and if the file content is stored in our cache. If that's the case, we get the file content from the cache instead of loading it from disk.

What we've just described is a very typical caching mechanism. Let's implement it in our Post class!

We start by importing the ConvenienceKit framework that provides the NSCacheSwift class.

Add the following import statement to the Post class:

import ConvenienceKit

Next, we'll define a static variable that will store our cache. We define it as static because the cache does not belong to a particular instance of the Post class, but is instead shared between all posts.

Add the following variable definition to the Post class:

static var imageCache: NSCacheSwift<String, UIImage>!

As you can see, we need to provide two pieces of type information: the first is the type of the keys we want to store in the cache, the second is the type of the values. We want to store image files and access them through their filename so we choose <String, UIImage>.

As soon as the Post class is loaded, we want to create an empty imageCache. The initialize method is the right place for class-level initialization code.

Extend the initialize method to look as following:

override class func initialize() {
  var onceToken : dispatch_once_t = 0;
  dispatch_once(&onceToken) {
    // inform Parse about this subclass
    // 1
    Post.imageCache = NSCacheSwift<String, UIImage>()
  1. We create an empty cache. Remember that the other lines in this method are primarily Parse boilerplate code.

Now we can extend the downloadImage method to use our cache.

Extend the downloadImage method to look as following:

func downloadImage() {
  // 1
  image.value = Post.imageCache[self.imageFile!.name]

  // if image is not downloaded yet, get it
  if (image.value == nil) {

    imageFile?.getDataInBackgroundWithBlock { (data: NSData?, error: NSError?) -> Void in
      if let data = data {
        let image = UIImage(data: data, scale:1.0)!
        self.image.value = image
        // 2
        Post.imageCache[self.imageFile!.name] = image
  1. We attempt to assign a value to image.value directly from the cache, using as key. If this assignment is successful the entire download block will be skipped.
  2. If we didn't have the image cached, we proceed as usual. Then, when the image is downloaded, we add it to the cache.

Now we have a solution in place that is fast and responds correctly to low-memory situations!


In this step, you learned a bunch of things that are important to build a stable app!

First, you learned that we should break up bindings when we know that they are no longer needed. We can do that by setting the binding to nil. If you end up using Observable in your own project, make sure to check for situations where you should break existing bindings.

You've also learned how to use NSCacheSwift to cache content that is expensive to fetch and generate. Even loading image files from disk can cause small freezes in your app - and your users will notice. NSCacheSwift is easy to use and will free memory automatically when necessary.

In the next chapter, we'll discuss another form of making your app more stable - implementing basic error handling.


If you have feedback on this tutorial or find any mistakes, please open issues on the GitHub Repository.

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