When a system is running low on available memory can it ask programs in general to give up any non-vital memory without shutting down. Like a process signal which causes the processes to yield any memory they can which does not render them unusable.
Think of it as a stage before caching them to disk or terminating them altogether.
Any Operating systems that do this?
iOS does. First, it will ask apps to give up memory when it gets tight (and apps that give up memory when needed tend to live longer), and there is a class (I think) NSCache which contains cached objects that can disappear any time. MacOS 9 did similar things. On MacOS X the assumption is that there is unlimited memory.
In addition to gnasher729's answer, iOS also has
a delegate method on UIViewController called didReceiveMemoryWarning().
Discussion
Your app never calls this method directly. Instead, this method is called when the system determines that the amount of available memory is low.
You can override this method to release any additional memory used by your view controller. If you do, your implementation of this method must call the super implementation at some point.
Unfortunately, 99% of the implementations I've seen are this default stub:
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
It's a bit of a tragedy of the commons scenario. Every app would benefit from implementing this behavior correctly (due to a lower chance of being force-stopped or paged to storage), but each app individually benefits from not implementing it (snappier response time, due to not needing to recompute/reload purged resources).
