Ok so I have a a bunch of helper functions in my project that I originally had in a class called Animate. I was wonder what are the benefits of declaring func vc class func.
Lets use this as an example class:
class Animate{
func moveView(...){
...
}
}
So I believe if I have a class func I don't have to instantiate the class as so.
Animate.moveView(...)
And if I just declare the function with func it would be:
Animate().moveView(...)
However if I don't declare the file as a class at all as so:
func moveView(...){
...
}
When I call the function it is just:
moveView(...)
With no indication where the code came from and it can be just used like this anywhere in the project.
What are the pros and cons of these three ways? Is not declaring a class bad practice? Or, is there some edge case that this is very useful? For example in my situation I have no need for a class since I am just creating helper functions and not an object.
Thanks in advance for any insight on this!
Ok. Instance methods vs class methods vs global methods.
(The term method and function are interchangeable. Method implies a function implemented by an object, so I tend to prefer the term method to the term function.)
An instance method is a method that is performed by instances of a class. You must have an instance of that class to talk to in order to invoke an instance method.
Instance methods have access to the instance variables of the object they belong to, so the object can save state information between calls. (In a networking class you could create multiple download objects, each of which manages an individual file download of a different file from a different URL, and each might have a different delegate it notifies when it's download is complete)
Class methods are invoked by the class itself, not by an instance. This can make it simple to invoke helper functions without having to manage an object to do that work for you. Since class methods don't talk to an instance of the class, they can't preserve different state information for each object. You might have a utilities class that performs localization functions on strings for example. The localization process is self-contained. You call a class function and pass in a string and the language you want it localized to, and it hands you back a result. No need to keep state between calls. Such a call might look like
let frenchString =
LocalizationUtils.localizeString("English String",
toLanguage: "French")
Global functions do not belong to any particular class. They are global to the entire module in which they are defined. They are similar to class functions, except that they are not specific to a particular class.
I agree with (and upvoted) #Duncan C's answer, but just thought I'd throw in a couple of other pros/cons.
I tend to like global functions over class methods because global functions don't clutter up my classes. I like to keep my classes lean and thin. The global functions can be kept in a separate file that I can copy and paste, or import, into a given project as I need them. So I might have a file in my project called AnimateHelperFunctions that is just global functions related to that class. A given project may only need a couple of them, or most of them, or those plus a couple more that I discover I need. I can delete the ones I don't use in a given project from the file so as to keep that file neat and trim.
I just think that global functions are more modular and encourage me to factor out single tasks for a single function - a good global helper function that does exactly one thing and does it perfectly can also sometimes be abstracted or made generic and used in other contexts as well. You might have a project sometime where you realize you don't need the class - you just need its helper function, and there it is.
I would prefer a hundred simple global functions that I can pick and choose from over a giant bloated class.
You can accomplish much the same thing with extensions, of course, and to some degree it is a matter of taste, as there is very little difference (any?) between class methods and global functions except that to use the class method you have to drag along the entire class.
Unlike global state, there isn't any danger in a global function. Sure, anyone can call it, but the same is true of class methods, and the global function can only operate on the arguments you pass to it.
For me, I use static or class methods to control class level properties or if I have to return customised instances of that particular class or struct. Like for example consider I have below struct.
struct Person {
let firstName: String
let lastName: String
}
Now if I am writing some test cases, where I need Person's instance initialized with a particular name John in many of my test classes, I can create a helper static method.
extension Person {
static func john() -> Person {
return Person(firstName: "John", lastName: "Appleseed")
}
}
let john = Person.john() // I could also create a static property instead, but it's a matter of personal choice and situation.
In above case, I could have made john as global function as well but for me, it will be very vague and not readable.
Another place I can think of where I prefer static method is returning count of cases for an enum.
enum Mood {
case happy
case angry
case lazy
case high
static func count() -> Int {
return 4
}
}
There are places, where I use global functions. I use global functions for logging.
func log(screenEvent name: String) {
let tracker = GAI.sharedInstance().defaultTracker
tracker.set(kGAIScreenName, value: screenName)
let builder = GAIDictionaryBuilder.createScreenView()
tracker.send(builder.build() as [NSObject : AnyObject])
}
Internally, the method is using a sharedInstance, creating a global method makes it easily accessible everywhere in the project just like a print function which logs output in console, but this is logging in some custom service.
Some other global functions which I usually include in my projects are GCD helpers.
func delay(delay:Double, closure: dispatch_block_t) {
dispatch_after(
dispatch_time(
DISPATCH_TIME_NOW,
Int64(delay * Double(NSEC_PER_SEC))
),
dispatch_get_main_queue(), closure)
}
func backgroundTask(closure: dispatch_block_t) {
dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), closure)
}
func mainThreadTask(closure: dispatch_block_t) {
dispatch_async(dispatch_get_main_queue(), closure)
}
These function don't need any information about a class so it makes sense to make them global instead of wrapping them inside a class.
Regarding instance methods, as answered by #Duncan C, they are called on instances, when you want to maintain a state. Below example shows usage of both static and instance methods.
enum TapType {
case water
case beer
}
struct Tap {
let tapType: TapType
//static method
static func unlimitedBeer() -> Tap {
let beer = Tap(tapType: .beer)
beer.turnOn(forDuration: Float.greatestFiniteMagnitude)
return beer
}
//instance method: will do operation on a particular instance of `Tap`.
func turnOn(forDuration duration: Float) {
//implementation
}
}
let unlimitedBeer = Tap.unlimitedBeer()
You can always use convenience initializer to initialize an object with custom behaviour, but again, it's a matter of choice. In above example, I couldn't think of any convenience initializer which would give me unlimited beer.
Related
I'm faced with a situation where I am defining a reusable base class in a module, and I want to provide certain functions that should be callable only by subclasses, not external users of that subclass.
I'm writing a framework and packaging it as a Swift module. Part of my framework includes a base class that can be subclassed to add functionality, but whereby the derived class also has a further external purpose as well. Imagine defining a new kind of view: it derives from UIView or NSView, then provides additional logic, and is then itself instantiated by another party.
In this case, I'm the one defining the UIView-like class that is intended to be subclassed, and along with it comes a lot of private UIView internal stuff, like measurement, arranging, who knows, internal stuff.
The point is, end users of this new view class don't want to see the internals of the architecture that supported the subclassing, those should be completely inside the black box of what the subclass represents.
And it strikes me that this is now impossible in Swift.
I really don't understand why Swift got rid of protected access control. According to Apple, the function that I want to expose only to subclasses "isn't really useful outside the subclass, so protection isn’t critical".
Am I missing something? Is this a whole class of design patterns that Swift simply cannot support?
One thought that occurs to me is I could perhaps split up the public-public and the private-public parts of my class into two parts, perhaps using protocols, whereby public-public users would only see the public protocol and "private" public users would see the "private" protocol as well. Alas this seems like a lot of engineering for something that used to be free.
FWIW — I've been continually asking for better access control in Swift (including protected) since before there was access control in Swift. Now, 3.5 years after we were told to give the Swift approach to access control a try, Swift has been my primary language for almost 3 of those years and I still think the access control paradigm is clumsy and unable to model concepts that are easy in almost all similar languages.
The largest mitigating factor for me is that Swift has steered me away from ever using inheritance and subclassing 95% of the time, which I think is a good thing. So this issue comes up less than it may have otherwise. But for situations exactly as you are describing, there isn't an equivalent way to accomplish what you are doing using only protocols and protocol extensions, so you are stuck either polluting a public API with possibly harmful internal details, or using some workaround (like the one that follows) which has the smallest possible public API exposure, and simulates what you want at the cost of boilerplate and awkwardness.
That said, the approach I take is somewhat inspired by Objective C, where there is also no real protected access control, but the convention is to declare a public API header (which client code will import and reference) and a special "+Subclassing" header which only subclasses will import in their implementation, giving them visibility into the not-for-public-consumption internals.
In Swift, this isn't directly possible either, but given a class like this:
open class SomeClass {
private var foo: String
private var bar: Data
public init(){
foo = "foo"
bar = Data()
}
private func doInternalThing() {
print(foo)
}
}
You can add a nested "Protected" wrapper via extension (has to be in the same file as your class declaration), which takes an instance of the class (or a subclass) and exposes the protected-level internals as a sort of proxy:
// Create a nested "Protected" type, which can accept an instance of SomeClass (or one of its subclasses) and expose the internal / protected members on it
public extension SomeClass {
public class Protected {
unowned private var someClass: SomeClass
public var foo: String {
get {
return someClass.foo
}
set {
someClass.foo = newValue
}
}
public init(_ someClass: SomeClass) {
self.someClass = someClass
}
public func doInternalThing() {
someClass.doInternalThing()
}
}
}
Outside of the framework, in the client application, the protected members are accessed in a subclass like this:
class SomeSubclass: SomeClass {
private lazy var protected: SomeClass.Protected = { SomeClass.Protected(self) }()
func doSomething() {
protected.foo = "newFoo" // Accesses the protected property foo and sets a new value "newFoo"
protected.doInternalThing() // Prints "newFoo" by calling the protected method doInternalThing which prints the foo property.
}
}
There are pros and cons for this approach. The cons are mainly the amount of boilerplate you need to write to map all your properties and functions from the Protected wrapper to the actual class instance as shown above. Also, there is no avoiding the fact that consumers will see SomeClass.Protected as a publicly visible type, but hopefully it's clear that it shouldn't be used and it's difficult enough to use it arbitrarily that it won't happen.
The pros are that there isn't a lot of boilerplate or pain for clients when creating subclasses, and its easy to declare a lazy "protected" var to get the desired API. It's pretty unlikely that non-subclass would stumble upon or use this API accidentally or unwittingly, and it's mostly hidden as desired. Instances of SomeSubclass will not show any extra protected API in code completion or to outside code at all.
I encourage anyone else who thinks access control — or really in this case, API visibility and organization — to be easier than it is in Swift today to let the Swift team know via the Swift forums, Twitter, or bugs.swift.org.
You can kinda, sorta work around it by separating out the for-subclasses stuff into a separate protocol, like this:
class Widget {
protocol SubclassStuff {
func foo()
func bar()
func baz()
}
func makeSubclassStuff() -> SubclassStuff {
// provide some kind of defaults, or throw a fatalError if this is
// an abstract superclass
}
private lazy var subclassStuff: SubclassStuff = {
return self.makeSubclassStuff()
}()
}
Then you can at least group the stuff that's not to be called in one place, to avoid it polluting the public interface any more than absolutely necessary and getting called by accident.
You can also reconsider whether you really need the subclass pattern here, and consider using a protocol instead. Unfortunately, since protocols can't nest types yet, this involves giving the subclass-specific protocol an Objective-C-style prefixed name:
protocol WidgetConcreteTypeStuff {
...
}
protocol Widget {
var concreteTypeStuff: WidgetConcreteTypeStuff { get }
}
Reading my own answer.
I fully understand why we need a weakSelf for members/properties. They could create memory cycles. But properties have a memory location.
Do functions also have memory locations?! I mean isn't a function something just happens on the go? If so is the memory location type any different from a property location?
If I don't refer using self I get this error.
Call to method 'alertFunc' in closure requires explicit 'self.' to
make capture semantics explicit
which is slightly different from:
Reference to property 'window' in closure requires explicit 'self.' to
make capture semantics explicit
My code is as follows:
let another = UIAlertAction(title: "Log", style:UIAlertActionStyle.Default){action in
logAction ()
}
private func logAction () {
print("health")
}
credit to iOS nerds I know from a meetup
tl;dr you need to use self for instance methods, but don't need it for class methods. A class may have many instances, but it can only have one declaration of itself (which also contains its type/class/static functions).
Why error occurs 1: Bare in mind the alertFunc could have had a reference to a property.
It could have been :
private func logAction () {
print("someProperty = \(self.someProperty)")
}
So, in this case it's obvious that you are eventually referencing a property.
Why error occurs 2: Even if you don't have a reference to self within your function, still because you wrote it as an instance method, the self is implicitly passed on. But, we don't see!
An instance method is really just syntactic sugar for a function that takes an instance for the first parameter, which is self passed automatically.
Under the hood it may look something like this :
private func alertFunc (_ instance: MyType) {
print("someProperty = \(instance.someProperty)")
}
// and actually the call to it *may* look something like this:
MyType.someFunc(self)
We never see the self being passed! It's a syntactic sugar that deceives us.
So, if your method does not use self internally (i.e. doesn't rely on the state of the instance in any way) then it's actually probably best to make it a static/type/class method or free function.
Alternative1: use a free function.
class someClass {
...
}
func freeFunc {
print("Hi this is a free function, written outside the scope of any class...")
}
And then within your closure you call it using freeFunc()
Alternative2: Use a class function.
class someClass {
...
class private func alertFunc() {
print("Hi this was a class function, see the class at the beginning of the func definition")
}
}
And then within your closure you call it using yourClassName.alertFunc()
But, why is it that class functions don't create memory cycles but instance functions do?
I'm glad you asked:
For instance mehtods, for each instance that you reach out to, there is a new memory location and generate a new memory cycle which would persist deallocations.
For class methods, for each time you reach out to the class/type methods you reach out to the same class/type method, and while you persist that class method, you won't be creating it over and over, it is only created once and done!
In objective-C (and C++) type methods:
When the app starts up, the system can fairly safely just pre-allocate ALL the instructions
for those type methods into memory along with their class pointers
so there’s little to no overhead calling those over and over and over again.
I imagine swift is doing the same thing
When you write logAction(), it implicitly means self.logAction(). (Methods are called on some instance; when you don't specify, it defaults to self.) So you are using self inside the closure, which means the closure captures self, and whether it captures a strong or weak reference has memory management implications.
updated Clarifying question to make clear this is an issue with a protocol that has a typealias, causing the general error of can only be used as a generic constraint.
I have the following class/protocol pattern:
protocol Storage { /* ... */ }
protocol StorageView {
typealias StorageType: Storage
/* ... */
}
class StorageColumnView<StorageType:Storage>: StorageView { /* ... */ }
class SomeStorage: Storage { /* ... */ }
and I want to define a class that combines my Storage class with View class. Ideally, it would look something like:
class MyClass<S:StorageType> {
var view:ViewType<S>
}
This won't compile because you can't specify a variable's type based on a protocol. After searching around, the general answer I found was to use type-erasure and make a AnyView class. However, such an approach seems cumbersome for a single variable (in theory this is the only place I'll use it) and difficult because StorageView has enough functionality to make wrapping each variable time consuming. Additionally, the methods of the view may get called a decent amount (yes, premature optimization is the root of all evil, but its subscripts will be called in loops), so I'm worried about the overhead.
Three alternative methods I'm currently investigating are:
(1) Declaring view as AnyObject, and then casting it to the correct type:
var view:AnyObject
// ...
view = StorageColumnView(/*...*/)
// ...
if let v = view as? StorageView {
// operate on v
}
(2) Treating view as a function, and letting the type be defined using a closure:
var view: () -> StorageView
// ...
view = { return StorageColumnView(self) }
/// ...
view().doX()
(3) Parameterizing MyClass by the ViewType rather than Storage:
class MyClass<V:ViewType> {
typealias StorageType = ViewType.StorageType
}
I'm trying to evaluate which of the 3 options is best (in terms of Swift style as well as speed), or if there is another alternative I'm not aware of (or I really should just use type-erasure to solve this problem -- though it seems like AnyObject is essentially just that).
So:
Are there any major penalties for the first approach? Is this closer to c++'s static_cast or dynamic_cast?
Is there a way to make the closure approach a little more user-friendly (i.e. I rather not require the user to actually pass in a closure, but rather the type). Maybe create a helper function that is a generic that then returns the type?
While the last solution is potentially the cleanest in amount of extra code required, it also requires a design that is against what I'm trying to do. The ViewType is really supposed to act like a delegate, and be fungible. Instead, I'm not creating a specific type of MyClass based on the ViewType.
Any and all opinions welcome on the best design pattern! I'm a little surprised that making a delegate-type pattern is so difficult (assuming I'm doing things correctly), considering that is primarily how Objective-C is used in Cocoa.
Also, does anyone know the rationale for not letting a variable to be defined as a protocol type that has a typealias? What's the underlying difference between a protocol with and without a Self?
I'm new to coding, apologies for dumb question.
Am following a tutorial to build a note taking app using Swift in Xcode.
Within a class definition I have been defining methods using the keyword func myMethod etc. At one point the instructor decides to define a Class method (within the existing class) using class func myMethod.
Why would you do this?
Thanks in advance for any feedback.
By defining a class method it means that you don't need an instance of that class to use the method. So instead of:
var myInstance: MyClass = MyClass()
myInstance.myMethod()
You can simply use:
MyClass.myMethod()
The static (class) function is callable without needing an instance of the class available; it may be called without having to instantiate an object.
This can be useful for encapsulation (avoiding placing the function in the global namespace), or for operations that apply to all objects of a given class, such as tracking the total number of objects currently instantiated.
Static functions can be used to define a namespaces collection of related utility functions:
aDate = Utils.getDate()
aTime = Utils.getTime()
Another common use is for the singleton pattern, where a static function is used to provide access to an object that is limited to being instantiate only once:
obj = MySingleton.getInstance()
obj.whatever()
One answer is namespacing. If a function is only relevant to a certain class there is no need to declare the function globally.
This is Swift's take on static methods:
Static methods are meant to be relevant to all the instances of a class (or no instances) rather than to any specific instance.
An example of these are the animation functions in UIView, or the canSendMail function from MFMailComposeViewController.
Looking at:
Using a dispatch_once singleton model in Swift
I see a very generic pattern for creating a shared singleton instance of my class. What I'd like to be able to do is create an extension for "all classes" that implements this sharedInstance method with generics.
I don't see any syntax for doing this; anybody want to take a crack at it?
As others have pointed out, Swift offers a simpler way to create singletons.
As an example: let's say we have a class called Model, and we want to have a single instance, visible throughout our app. All we need to write in the global scope is:
let modelSingleton = Model()
This will create an instance of the Model class, visible everywhere, and that cannot be replaced by another instance (Hmmm, that's pretty much what one would expect from a singleton, isn't it?).
Now, this way of doing would, on the other hand, still allow you to create other Model instances, apart from the singleton. While this departs from the official definition of singletons in other languages, this approach would have the interesting advantage of allowing the creation of other instances for testing purposes (singletons have bad press in the unit testing world :) ).
Swift will soon offer everything needed to create real Singleton<T> classes (it's hard now because class vars aren't allowed yet). But that being said, the approach described above will probably be more than enough for many Swift programmers.
I don't think this is possible.
Even if it was possible to extend Any / AnyObject, every object would share the same implementation of the sharedInstance singleton getter, and therefore the same static instance variable. Thus, instance would get set to an instance of the first class on which sharedInstance was called.
extension Any {
class var sharedInstance:TPScopeManager {
get {
struct Static {
static var instance : TPScopeManager? = nil
}
if !Static.instance {
Static.instance = TPScopeManager()
}
return Static.instance!
}
}
}
...
NSString.sharedInstance() // Returns an NSString
NSArray.sharedInstance() // Returns the same NSString object!
In Swift you can make an extension to NSObject, but you can't extend Any/AnyObject