Swift Arrays Holding Elements With Weak References
This tutorial talks about when it's actually better to use weak references with arrays in Swift arrays for iOS development.
Join the DZone community and get the full member experience.
Join For FreeIn iOS development, there are moments where you ask yourself: “To weak, or not to weak, that is the question.” Let’s see how “to weak” with the arrays.
Overview
In this article, I speak about memory management without explaining it since it would be beyond the goal of this article. The official documentation is a good starting point to learn this subject. Then, if you have other doubts, please leave a comment and I’ll reply as soon as possible.
Array
is the most popular collection in Swift. By default, it maintains strong references of its elements. Even if this behavior is useful most of the time, you might have some scenarios where you would want to use weak references. For this reason, Apple provides an alternative to Array
which maintains weak references of its elements: NSPointerArray.
Before looking at this class, let’s see an example to understand why we should use it.
Why Weak References?
Let’s use, as an example, a ViewManager
class which has two properties of type View
. In its constructor, we add these views in an array to inject inside Drawer
—which uses this array to draw something inside the views. Finally, we have a method destroyViews
to destroy the two View
s:
class View { }
class Drawer {
private let views: [View]
init(views: [View]) {
self.views = views
}
func draw() {
// draw something in views
}
}
class ViewManager {
private var viewA: View? = View()
private var viewB: View? = View()
private var drawer: Drawer
init() {
self.drawer = Drawer(views: [viewA!, viewB!])
}
func destroyViews() {
viewA = nil
viewB = nil
}
}
Unfortunately, destroyViews
doesn’t destroy the two views because the array inside Drawer
is maintaining a strong reference of the views. We can avoid this problem replacing the array with anNSPointerArray
.
NSPointerArray
NSPointerArray
is an alternative to Array
with the main difference that it doesn’t store an object but its pointer (UnsafeMutableRawPointer
).
This type of array can store the pointer maintaining either a weak or a strong reference depending on how it’s initialized. It provides two static methods to be initialized in different ways:
let strongRefarray = NSPointerArray.strongObjects() // Maintains strong references
let weakRefarray = NSPointerArray.weakObjects() // Maintains weak references
Since we want an array of weak references, we’ll use NSPointerArray.weakObjects()
.
Now, we can add a new object in this array:
class MyClass { }
var array = NSPointerArray.weakObjects()
let obj = MyClass()
let pointer = Unmanaged.passUnretained(obj).toOpaque()
array.addPointer(pointer)
Since using the pointer may be annoying, you can use this extension which I made to simplify the NSPointerArray
:
extension NSPointerArray {
func addObject(_ object: AnyObject?) {
guard let strongObject = object else { return }
let pointer = Unmanaged.passUnretained(strongObject).toOpaque()
addPointer(pointer)
}
func insertObject(_ object: AnyObject?, at index: Int) {
guard index < count, let strongObject = object else { return }
let pointer = Unmanaged.passUnretained(strongObject).toOpaque()
insertPointer(pointer, at: index)
}
func replaceObject(at index: Int, withObject object: AnyObject?) {
guard index < count, let strongObject = object else { return }
let pointer = Unmanaged.passUnretained(strongObject).toOpaque()
replacePointer(at: index, withPointer: pointer)
}
func object(at index: Int) -> AnyObject? {
guard index < count, let pointer = self.pointer(at: index) else { return nil }
return Unmanaged<AnyObject>.fromOpaque(pointer).takeUnretainedValue()
}
func removeObject(at index: Int) {
guard index < count else { return }
removePointer(at: index)
}
}
Thanks to this extension, you can replace the previous example with:
var array = NSPointerArray.weakObjects()
let obj = MyClass()
array.addObject(obj)
If you want to clean the array removing the objects with value nil
, you can call the method compact()
:
array.compact()
At this point, we can refactor the example used in “Why Weak References?” with the following code:
class View { }
class Drawer {
private let views: NSPointerArray
init(views: NSPointerArray) {
self.views = views
}
func draw() {
// draw something in views
}
}
class ViewManager {
private var viewA: View? = View()
private var viewB: View? = View()
private var drawer: Drawer
init() {
let array = NSPointerArray.weakObjects()
array.addObject(viewA)
array.addObject(viewB)
self.drawer = Drawer(views: array)
}
func destroyViews() {
viewA = nil
viewB = nil
}
}
Note:
You may have noticed that NSPointerArray
stores pointers of AnyObject
only, it means that you can store just classes—so neither structs nor enums. You can store protocols if they have the keyword class
:
protocol MyProtocol: class { }
If you want to play with NSPointerArray
, I suggest you avoid the Playground since you may have odd behaviors with the retain count. A sample app would be better.
Alternatives
NSPointerArray
is very useful to store objects maintaining weak references, but it has a problem: it’s not type-safe.
For “not type-safe”, I mean that the compiler is not able to infer the type of the objects inside NSPointerArray
, since it uses pointers of objects AnyObject
. For this reason, when you get an object from the array, you must cast it to your object type:
object(at:)
comes from my NSPointerArray
extension which I shown previously.
If we want to use a type-safe alternative we can’t use NSPointerArray
anymore.
A possible workaround is creating a new class WeakRef
with a generic weak property value
:
private(set)
exposes value
in read-only mode, in this way no one can set its value from outside the class.
Then, we can create an array of WeakRef
, where value
is your MyClass
object to store:
Now, we have an array type-safe which maintains a weak reference of your MyClass
objects. The disadvantage of this approach is that we must add an extra layer in our code (WeakRef
) to wrap the weak reference in a type-safe way.
If you want to clean the array removing the objects with value nil
, you can write the following method:
filter
returns a new array with the elements that satisfy the given predicate. You can find more details in the documentation.
Now, we can refactor the example used in “Why Weak References?” with the following code:
A cleaner version using the typealias:
Dictionary and Set
This article has the main focus on Array
, if you need something similar to NSPointerArray
for Dictionary
you can have a look at NSMapTable, whereas for Set
you can use NSHashTable.
If you want a type-safe Dictionary
/Set
, you can achieve it storing a WeakRef
object.
Conclusion
I guess you are not going to use arrays with weak references very often, but it’s not an excuse not to know how to achieve it. In iOS development, the memory management is very important to avoid memory leaks, since iOS doesn’t have a garbage collector.
Published at DZone with permission of Marco Santarossa, DZone MVB. See the original article here.
Opinions expressed by DZone contributors are their own.
Comments