Whenever I create a new View Controller subclass, Xcode automatically adds the method
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated
}
Usually I just delete it or ignore it. This is what all the tutorials I have seen do, too. But I assume that since Xcode gives it to me every time, it should be somewhat important, right? What should I be doing here? I assume that disposing of resources means setting them to nil, but what exactly are "resources that can be recreated"?
I have seen these questions:
But they are all pre-Swift. Although I don't know much about Objective-C, I have heard that the memory management is different. How does that affect what I should do in didReceiveMemoryWarning
?
Other notes:
didReceiveMemoryWarning
that I found was rather brief.Swift uses ARC just like Objective-C does (source to Apple Docs). The same kind of rules apply for freeing memory, remove all of the references to an object and it will be deallocated.
I assume that disposing of resources means setting them to nil, but what exactly are "resources that can be recreated"?
"resources that can be recreated" really depends on your application.
Say you are a social media app that deals with a lot of pictures. You want a snappy user interface so you cache the next 20 pictures in memory to make scrolling fast. Those images are always saved on the local file system.
You are creating an amazing game that has a number of different levels. Loading a level into your fancy game engine takes a while so if the user has enough memory you can load level 3 while they are playing level 2.
LevelCache.sharedCache().nextLevel = nil
frees up all that memoryNever deallocate the stuff that is on screen. I've seen some answers to related questions deallocate the view of the UIViewController. If you remove everything from the screen you might at well crash (in my opinion).
If the user has a document open that they are editing, DON'T deallocate it. Users will get exceptional angry at you if your app deletes their work without ever being saved. (In fact, you should probably have some emergency save mechanism for when that happens)
If your user is writing a post for your fabulous social media app, don't let their work go to waste. Save it and try to restore it when they open the app again. Although it's a lot of work to setup I love apps that do this.
Most modern devices rarely run out of memory. The system does a pretty good job of killing apps in the background to free up memory for the app running in the foreground. You have probably seen an app "open" in the App Switcher yet when you tapped on the app it opened to its initial state. The OS killed the app in the background to free up memory. See State Restoration for info on how to avoid this problem.
If your app is getting consistent memory warnings when you aren't doing a huge amount of processing make sure that you aren't leaking memory first. Detecting memory leaks is outside the scope of this answer. Docs and a tutorial.