I have an application which talks to a device over a BLE UART connection.
When data is received, a didReceiveData() delegate is called.
Here I need determine who called the data and trigger the corresponding method (of another delegate).
I was planning on creating a connections dictionary keyed by a connectionID string created when the connection is established, along with a selector to the callback (may not always be supplied).
class Connection: NSObject {
var selectr: Selector
var dataString: String?
init(selectR:Selector,dString:String) {
selectr = selectR
dataString = dString
}
}
connections[String:Connection]()
func sendData(dataString:String,callbackSelector:Selector){
con = Connection(selectR: callbackSelector, dString:"")
connections[cid] = con
}
...
When calling:
let sel = Selector(anotherDelegate?.didReceiveLocation(""))
self.sendData("sendMe",Selector(anotherDelegate?.didReceiveLocation))
I get a few errors doing this, first a Type NSData does not conform to protocol StringLiteralConvertible. NSData referring to the argument of didReceiveLocation.
The second is on the self.sendData line: Cannot invoke 'init' with an argument list of type (StringLiteralConvertible,Selector).
Does this approach make sense? How can I store the callback method of another delegate in a dictionary or other struct to make it accessible from the didReceiveData delegate method?
Selectors are so Objective-C... why not just use a closure?
To declare a dictionary with String as keys and functions that take a String as a parameter and return nothing:
connections = [String:String->()]()
To declare a function that takes a closure with a single String argument and no return value, use something like:
func sendData(dataString:String, callback:String->()) {
...
connections[cid] = callback
...
}
To invoke that closure later, you can call it as a subroutine (in this case after performing a dictionary lookup, using optional chaining in case it hasn't been assigned yet):
func didReceiveData(...) {
...
connections[cid]?(response)
}
Then to call the sendData routine declared above and pass in a closure using one of the several abbreviated syntaxes:
self.sendData("sendMe") { response in
...
}
Note that this is actually short-hand for:
self.sendData("sendMe", callback:{ response:String in
...
})
Note that this mechanism can also be used by an Objective-C caller, since closures map more or less directly to Objective-C blocks.
For more information on closures, their definition and invocation, I'd strongly recommend downloading the free Swift book from the iTunes book store.
Related
TL;DR
I have a class with no public initializers or instances that passes an instance of itself to a closure in another class. It does this through a mirror of the other class. When I go to access that instance from within the closure, I'm getting a EXC_BAD_ACCESS error, but other parameters passed to the closure are clearly accessible and do not result in a bad access error. I have no idea why. See code below for replication in a new project or a playground.
Detailed Explanation
I've been trying to figure out a way to implement class-specific access control, where multiple specific classes have sole access to another class containing variables and functions to be shared between them. All other classes would not have such access. Kind of like a static class, or a Singleton pattern, but with specific, class-named access control.
I thought I had something that would actually work in pure swift, (which is nice for me since I don't know Objective-C, and only started on swift about 16 months ago.) It's done in an almost anti-swift manner, so just bear with me - my goal is to start with something functional and move it towards elegance and beauty from there.
Even though I'm reasonably confident it should all work, I'm encountering a EXC_BAD_ACCESS error in a very unexpected place.
The "class-specific private" class that you are not allowed to access an instance of unless you are on its "okay" list, we can call the Restricted class.
The class(es) that is(are) allowed access to the Restricted class we can call the Accessor class(es).
The programmer must tell the Restricted class to call a function from the Accessor, and "drop in" an instance of the Restricted class by passing it as a parameter to that function. You do this by passing in the name of the function to be called, an instance of the Accessor class on which to call said function, and any parameters that the function would need in addition to the Restricted class instance.
I could make an enormous switch in the Restricted class, each case of which properly calls each function indicated on each of the Accessor classes...but to get around that excessive overhead/setup, I have the name of the function to be called on the Accessor classes passed in as a string, and accessed through a mirror. Since mirrors only reflect properties and not functions, the function must be a property with an assigned closure, instead of a traditional function.
We can call these closures DropClosures, since their purpose is to have the shared, Restricted class dropped into them. In fact we could call this whole pattern the "DropClosure Pattern". (Or maybe anti-pattern, I know it's kind of gruesome as-is.)
The properties of the "shared" instance of the Restricted class are stored internally as a private static dict (as json, basically). To generate an actual instance of itself, the Restricted class uses a private initializer that accepts that dict as a parameter. After a DropClosure runs with said initialized instance, the Restricted class uses a Mirror of that instance to store any changes back in the "shared" dict, and the instance will go out of scope unless a reference is made to it. So after each DropClosure completes its run, the instance passed to it is more or less useless as a representation of the "shared" aspect of the class, intentionally so.
I only do this because there is no way to require that all references to a certain weak reference also be weak. I don't want a class with access to the weak reference to assign a strong reference to the same instance and keep it in memory, that would defeat the access control goal by allowing the instance to be shared outside of its access scope. Since I can't force the instance to expire once the closure has completed, the next best thing is to remove the motivation for doing so by making the object no longer connected to the shared source after the closure completes.
This all theoretically works, and will compile, and will not throw any swift exceptions when run.
The Accessor (or any class that has an instance of an Accessor) calls RestrictedClass.run(), the run code validates the Accessor instance, finds the DropClosure in that instance, and passes in an instance of the Restricted class to that closure.
However, whenever I try to access that instance from within the DropClosure, it gives me the aforementioned bad access error, seemingly on a C or Objective-C level.
As far as I can tell, the instance should be accessible at this point, and none of the variables being used should be dropping out of scope yet.
At this point I'm totally spitballing - is it possible that there is something in the background that prevents a class with no public initializers from being passed through a mirror? Does it have to do with passing it into a closure called from that mirror? Is there some kind of hidden weak reference that's making the instance get ARC'd?
Please note that I've tried discarding the "weak" wrapper object and only passing in the Restricted instance to the closure, and I get the same bad access error. The error is independent of the instance being weakly referenced.
Code:
import Foundation
typealias DropClosureVoid<T: AnyObject & AccessRestricted> = (_ weaklyConnectedInterface: WeaklyConnectedInterface<T>, _ usingParameters: Any?)->Void
typealias DropClosureAny<T: AnyObject & AccessRestricted> = (_ weaklyConnectedInterface: WeaklyConnectedInterface<T>, _ usingParameters: Any?)->Any?
enum AccessError : Error {
case InvalidFunction
case InvalidAccessClass
}
protocol AccessRestricted {
static func run<T:AnyObject>(_ closureName:String, in classObject: T, with parameters:Any?) throws
static func runAndReturn<T:AnyObject>(_ closureName:String, in classObject: T, with parameters:Any?) throws -> Any?
}
///This class contains an instance that should be expected to only temporarily represent the original, even if a strong reference is made that keeps the value in scope.
class WeaklyConnectedInterface<T:AnyObject> {
weak var value:T?
init(_ value: T) {
self.value = value
}
}
class Accessor {
let restrictedClassPassable:DropClosureVoid<RestrictedAccessClass> = { weaklyConnectedInterface, parameters in
print(weaklyConnectedInterface) // **EXC_BAD_ACCESS error here**
//note that the error above happens even if I pass in the instance directly, without the WeaklyConnectedInterface wrapper.
//It's clearly an issue that occurs when trying to access the instance, whether the instance is wrapped in a the class that makes a weak reference to it or not, which means that it is inaccessible even when strongly referenced.
if let parameterDict = parameters as? [String:String] {
print(parameterDict["paramkey"] ?? "nil")
print(weaklyConnectedInterface)
weaklyConnectedInterface.value?.restrictedVariable = "I've changed the restricted variable"
}
}
let anotherRestrictedClassPassable:DropClosureAny<RestrictedAccessClass> = { weaklyConnectedInterface, parameters in
if let parameterDict = parameters as? [String:String] {
print(parameterDict["paramkey"] ?? "nil")
print(weaklyConnectedInterface.value?.restrictedVariable as Any)
return weaklyConnectedInterface.value?.restrictedVariable
}
return nil
}
func runRestrictedClassPassable() throws {
let functionName = "restrictedClassPassable"
print("trying validateClosureName(functionName)")
try validateClosureName(functionName)//this is in case you refactor/change the function name and the "constant" above is no longer valid
print("trying RestrictedAccessClass.run")
try RestrictedAccessClass.run(functionName, in: self, with: ["paramkey":"paramvalue"])
let returningFunctionName = "anotherRestrictedClassPassable"
print("trying validateClosureName(returningFunctionName)")
try validateClosureName(returningFunctionName)
print("trying RestrictedAccessClass.runAndReturn")
let result = (try RestrictedAccessClass.runAndReturn(returningFunctionName, in: self, with: ["paramkey":"ParamValueChanged"]) as! String?) ?? "NIL, something went wrong"
print("result is \(result)")
}
func validateClosureName(_ name:String) throws {
let mirror = Mirror(reflecting: self)
var functionNameIsPresent = false
for child in mirror.children {
if child.label != nil && child.label! == name {
functionNameIsPresent = true
break
}
}
guard functionNameIsPresent else {
print("invalid function")
throw AccessError.InvalidFunction
}
}
}
extension Mirror {
func getChildrenDict() -> [String:Any]
{
var dict = [String:Any]()
for child in children
{
if let name = child.label
{
dict[name] = child.value
}
}
return dict
}
}
class RestrictedAccessClass:AccessRestricted {
private static var shared:[String:Any] = [
"restrictedVariable" : "You can't access me!"
]
private static func validateType<T>(of classObject:T) throws {
switch classObject {
case is Accessor:
return
default:
print("Invalid access class")
throw AccessError.InvalidAccessClass
}
}
var restrictedVariable:String
private init() {
restrictedVariable = "You can't access me!"
}
private init(from json:[String:Any]) {
restrictedVariable = json["restrictedVariable"] as! String
}
static func run<T:AnyObject>(_ closureName:String, in classObject: T, with parameters:Any?) throws {
print("trying validateType(of: classObject) in run")
try validateType(of: classObject)
for child in Mirror(reflecting: classObject).children {
if let childName = child.label {
if childName == closureName {
let dropClosure = child.value as! DropClosureVoid<RestrictedAccessClass>
let selfInstance = RestrictedAccessClass(from:shared)
let interface = WeaklyConnectedInterface(selfInstance)
dropClosure(interface, parameters)
runCleanup(on: selfInstance)//parses any data changed by the end of the drop closure back into the dict for use in future instances. This means you mustn't try using the instance in an async closure. The correct way to do this would be to call run inside of an async closure, rather than putting an anync closure inside of the drop closure.
_ = interface.value
return
}
}
}
}
static func runAndReturn<T:AnyObject>(_ closureName:String, in classObject: T, with parameters:Any?) throws -> Any? {
print("trying validateType(of: classObject) in runAndReturn")
try validateType(of: classObject)
for child in Mirror(reflecting: classObject).children {
if let childName = child.label {
if childName == closureName {
let dropClosure = child.value as! DropClosureAny<RestrictedAccessClass>
let selfInstance = RestrictedAccessClass(from:shared)
let interface = WeaklyConnectedInterface(selfInstance)
let result = dropClosure(interface, parameters)
runCleanup(on: selfInstance)//parses any data changed by the end of the drop closure back into the dict for use in future instances. This means you mustn't try using the instance in an async closure. The correct way to do this would be to call run inside of an async closure, rather than putting an anync closure inside of the drop closure.
_ = interface.value
return result
}
}
}
return nil
}
private static func runCleanup(on instance:RestrictedAccessClass) {
shared = Mirror(reflecting:instance).getChildrenDict()
//once this function goes out of scope(or shortly thereafter), the instance passed will become useless as a shared resource
}
}
Code to encounter error:
I just put this in a new project's AppDelegate.application(didFinishLaunching). You can put all of the code above and below, in order, in a playground and it will break in the same spot, but not as clearly.
let accessor = Accessor()
do {
try accessor.runRestrictedClassPassable()
}
catch {
print(error.localizedDescription)
}
Updates
Whether zombie objects are turned on or off, I'm getting the same error message from Xcode: Thread 1: EXC_BAD_ACCESS (code=1, address=0x1a1ebac696e)
Running an analysis with Command+Shift+B reveals no warnings.
Running with all of the malloc options enabled reveals the following error:
Thread 1: signal SIGABRT, objc[somenumber]: Attempt to use unknown class 0xSomevalue
This just got weird...
Apparently, the "unknown class" is the project. I found this out by selecting the (i) bubble on the inline object inspector for the Restricted instance that was causing the crash. It gives me the following message:
Printing description of weaklyConnectedInterface:
expression produced error: error:
/var/folders/zq/_x931v493_vbyhrfc25z1yd80000gn/T/expr52-223aa0..swift:1:65:
error: use of undeclared type 'TestProject'
Swift._DebuggerSupport.stringForPrintObject(Swift.UnsafePointer<TestProject.RestrictedAccessClass>(bitPattern: 0x103450690)!.pointee)
^~~~~~~~~~~
I thought that maybe this would happen for other classes, so I tested, and it's able to access other project-level classes just fine. Only for this specific instance is the project "namespace" undefined.
Please find below required modifications (not many)... Tested with Xcode 11.2 / iOS 13.2.
1) made interface inout to pass it as-is original, otherwise it somehow copied loosing type information
typealias DropClosureVoid<T: AnyObject & AccessRestricted> =
(_ weaklyConnectedInterface: inout WeaklyConnectedInterface<T>, _ usingParameters: Any?)->Void
typealias DropClosureAny<T: AnyObject & AccessRestricted> =
(_ weaklyConnectedInterface: inout WeaklyConnectedInterface<T>, _ usingParameters: Any?)->Any?
2) fix places of usage (same in two places)
var interface = WeaklyConnectedInterface(selfInstance) // made var
dropClosure(&interface, parameters) // << copy closure args here was a reason of crash
3) ... and that's it - build & run & output
Note: I would recommend to avoid force unwrap and use the following
if let dropClosure = child.value as? DropClosureVoid<RestrictedAccessClass> {
dropClosure(&interface, parameters)
}
I am working in an iOS application called ConnectApp and I am using a framework called Connector. Now, Connector framework completes actual connection task with BLE devices and let my caller app (i.e. ConnectApp) know the connection request results through ConnectionDelegate. Let's see example code,
ConnectApp - host app
class ConnectionService: ConnectionDelegate {
func connect(){
var connector = Connector()
connector.setDelegate(self)
connector.connect()
}
func onConnected(result: ConnectionResult) {
//connection result
}
}
Connector Framework
public class ConnectionResult {
// many complicated custom variables
}
public protocol ConnectionDelegate {
func onConnected(result: ConnectionResult)
}
public class Connector {
var delegate: ConnectionDelegate?
func setDelegate(delegate: ConnectionDelegate) {
self.delegate = delegate
}
func connect() {
//…..
// result = prepared from framework
delegate?.onConnected(result)
}
}
Problem
Sometimes developers have no BLE device and we need to mock the Connector layer of framework. In case of simple classes (i.e. with simpler methods) we could have used inheritance and mock the Connector with a MockConnector which might override the lower tasks and return status from MockConnector class. But when I need to deal with a ConnectionDelegate which returns complicated object. How can I resolve this issue?
Note that framework does not provide interfaces of the classes rather we need to find way around for concrete objects like, Connector, ConnectionDelegate etc.
Update 1:
Trying to apply Skwiggs's answer so I created protocol like,
protocol ConnectorProtocol: Connector {
associatedType MockResult: ConnectionResult
}
And then injecting real/mock using strategy pattern like,
class ConnectionService: ConnectionDelegate {
var connector: ConnectorProtocol? // Getting compiler error
init(conn: ConnectorProtocol){
connector = conn
}
func connect(){
connector.setDelegate(self)
connector.connect()
}
func onConnected(result: ConnectionResult) {
//connection result
}
}
Now I am getting compiler error,
Protocol 'ConnectorProtocol' can only be used as a generic constraint because it has Self or associated type requirements
What am I doing wrong?
In Swift, the cleanest way to create a Seam (a separation that allows us to substitute different implementations) is to define a protocol. This requires changing the production code to talk to the protocol, instead of a hard-coded dependency like Connector().
First, create the protocol. Swift lets us attach new protocols to existing types.
protocol ConnectorProtocol {}
extension Connector: ConnectorProtocol {}
This defines a protocol, initially empty. And it says that Connector conforms to this protocol.
What belongs in the protocol? You can discover this by changing the type of var connector from the implicit Connector to an explicit ConnectorProtocol:
var connector: ConnectorProtocol = Connector()
Xcode will complain about unknown methods. Satisfy it by copying the signature of each method it needs into the protocol. Judging from your code sample, it may be:
protocol ConnectorProtocol {
func setDelegate(delegate: ConnectionDelegate)
func connect()
}
Because Connector already implements these methods, the protocol extension is satisfied.
Next, we need a way for the production code to use Connector, but for test code to substitute a different implementation of the protocol. Since ConnectionService creates a new instance when connect() is called, we can use a closure as a simple Factory Method. The production code can supply a default closure (creating a Connector) like with a closure property:
private let makeConnector: () -> ConnectorProtocol
Set its value by passing an argument to the initializer. The initializer can specify a default value, so that it makes a real Connector unless told otherwise:
init(makeConnector: (() -> ConnectorProtocol) = { Connector() }) {
self.makeConnector = makeConnector
super.init()
}
In connect(), call makeConnector() instead of Connector(). Since we don't have unit tests for this change, do a manual test to confirm we didn't break anything.
Now our Seam is in place, so we can begin writing tests. There are two types of tests to write:
Are we calling Connector correctly?
What happens when the delegate method is called?
Let's make a Mock Object to check the first part. It's important that we call setDelegate(delegate:) before calling connect(), so let's have the mock record all calls in an array. The array gives us a way to check the call order. Instead of having the test code examine the array of calls (acting as a Test Spy which just records stuff), your test will be cleaner if we make this a full-fledged Mock Object — meaning it will do its own verification.
final class MockConnector: ConnectorProtocol {
private enum Methods {
case setDelegate(ConnectionDelegate)
case connect
}
private var calls: [Methods] = []
func setDelegate(delegate: ConnectionDelegate) {
calls.append(.setDelegate(delegate))
}
func connect() {
calls.append(.connect)
}
func verifySetDelegateThenConnect(
expectedDelegate: ConnectionDelegate,
file: StaticString = #file,
line: UInt = #line
) {
if calls.count != 2 {
fail(file: file, line: line)
return
}
guard case let .setDelegate(delegate) = calls[0] else {
fail(file: file, line: line)
return
}
guard case .connect = calls[1] else {
fail(file: file, line: line)
return
}
if expectedDelegate !== delegate {
XCTFail(
"Expected setDelegate(delegate:) with \(expectedDelegate), but was \(delegate)",
file: file,
line: line
)
}
}
private func fail(file: StaticString, line: UInt) {
XCTFail("Expected setDelegate(delegate:) followed by connect(), but was \(calls)", file: file, line: line)
}
}
(That business with passing around file and line? This makes it so that any test failure will report the line that calls verifySetDelegateThenConnect(expectedDelegate:), instead of the line that calls XCTFail(_).)
Here's how you'd use this in ConnectionServiceTests:
func test_connect_shouldMakeConnectorSettingSelfAsDelegateThenConnecting() {
let mockConnector = MockConnector()
let service = ConnectionService(makeConnector: { mockConnector })
service.connect()
mockConnector.verifySetDelegateThenConnect(expectedDelegate: service)
}
That takes care of the first type of test. For the second type, there's no need to test that Connector calls the delegate. You know it does, and it's outside your control. Instead, write a test to call the delegate method directly. (You'll still want it to make a MockConnector to prevent any calls to the real Connector).
func test_onConnected_withCertainResult_shouldDoSomething() {
let service = ConnectionService(makeConnector: { MockConnector() })
let result = ConnectionResult(…) // Whatever you need
service.onConnected(result: result)
// Whatever you want to verify
}
You could try
protocol MockConnector: Connector {
associatedType MockResult: ConnectionResult
}
Then, for each connector you need to mock, define a concrete class that conforms to this mock connector
class SomeMockConnector: MockConnector {
struct MockResult: ConnectionResult {
// Any mocked variables for this connection result here
}
// implement any further requirements from the Connector class
var delegate: ConnectionDelegate?
func connect() {
// initialise your mock result with any specific data
let mockResult = MockResult()
delegate?.onConnected(mockResult)
}
}
I'm reasonably proficient with Groovy insofar as my job requires, but not having a background in OOP means that some things still elude me, so apologies if some of the wording is a little off here (feel free to edit if you can make the question clearer).
I'm trying to create an overloaded method where the signature (ideally) differs only in the return type of the single Closure parameter. The Closure contains a method call that returns either an ItemResponse or ListResponse object, both of which could contain an object/objects of any type (which is the type I would like to infer).
The following code is a simplified version of what I'm trying to implement - an error handling method which takes a reference to a service call, safely attempts to resolve it, and returns the item/items from the response as appropriate:
public <T> T testMethod(Closure<ItemResponse<T>> testCall) {
testCall.call().item as T
}
public <T> List<T> testMethod(Closure<ListResponse<T>> testCall) {
testCall.call().items as T
}
Obviously this doesn't work, but is there any alternate approach/workaround that would achieve the desired outcome?
I'm trying to create an overloaded method where the signature
(ideally) differs only in the return type of the single Closure
parameter.
You cannot do that because the return type is not part of the method signature. For example, the following is not valid:
class Demo {
int doit() {}
String doit() {}
}
As mentioned by yourself and #jeffscottbrown, you can't have two methods with the same parameters but different return value. The workaround I can see here is to use a call-back closure. The return value of your testMethod would default to Object and you would provide an "unwrapper" that would the bit after the closure call (extract item or items). Try this out in your GroovyConsole:
class ValueHolder <T> {
T value
}
Closure<List<Integer>> c = {
[1]
}
Closure<ValueHolder<String>> d = {
new ValueHolder(value:'hello world')
}
Closure liu = {List l ->
l.first()
}
Closure vhsu = {ValueHolder vh ->
vh.value
}
// this is the generic method
public <T> Object testMethod(Closure<T> testCall, Closure<T> unwrapper) {
unwrapper(testCall.call()) as T
}
println testMethod(c, liu)
println testMethod(d, vhsu)
It works with both a list or a value holder.
I am new to Swift and I'm wondering what self is used for and why.
I have seen it in classes and structures but I really don't find them essential nor necessary to even mention them in my code. What are they used for and why? In what situations it's necessary to use it?
I have been reading lots of questions and answers for this question but none of them fully answers my questions and they always tend to compare it with this as in Java, with which I'm not familiar whatsoever.
Yes it is the same as this in Java and self in Objective-C, but with Swift, self is only required when you call a property or method from a closure or to differentiate property names inside your code, such as initializers. So you can use almost all of your class components safely without using self unless you are making the call from a closure.
“The self Property Every instance of a type has an implicit property
called self, which is exactly equivalent to the instance itself. You
use the self property to refer to the current instance within its
own instance methods.
The increment() method in the example above could have been written
like this:
func increment() {
self.count += 1
}
In practice, you don’t need to write self in your code very often.
If you don’t explicitly write self, Swift assumes that you are
referring to a property or method of the current instance whenever you
use a known property or method name within a method. This assumption
is demonstrated by the use of count (rather than self.count)
inside the three instance methods for Counter.
The main exception to this rule occurs when a parameter name for an
instance method has the same name as a property of that instance. In
this situation, the parameter name takes precedence, and it becomes
necessary to refer to the property in a more qualified way. You use
the self property to distinguish between the parameter name and the
property name.
Here, self disambiguates between a method parameter called x and
an instance property that is also called x:”
Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).”
This is how Ray Wenderlich recommends the use of self in Swift for their tutorials:
Use of Self
For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.
Use self when required to differentiate between property names and arguments in initializers, and when referencing properties in closure expressions as required by the compiler:
class BoardLocation {
let row: Int, column: Int
init(row: Int, column: Int) {
self.row = row
self.column = column
let closure = {
println(self.row)
}
}
}
And this is GitHub's recommendations on self for their applications:
Only explicitly refer to self when required
When accessing properties or methods on self, leave the reference to self implicit by default:
private class History {
var events: [Event]
func rewrite() {
events = []
}
}
Only include the explicit keyword when required by the language — for example, in a closure, or when parameter names conflict:
extension History {
init(events: [Event]) {
self.events = events
}
var whenVictorious: () -> () {
return {
self.rewrite()
}
}
}
Rationale: This makes the capturing semantics of self stand out more in closures, and avoids verbosity elsewhere.
You will also use self a lot when creating your extensions, example:
extension Int {
func square() -> Int {
return self * self
}
// note: when adding mutating in front of it we don't need to specify the return type
// and instead of "return " whatever
// we have to use "self = " whatever
mutating func squareMe() {
self = self * self
}
}
let x = 3
let y = x.square()
println(x) // 3
printlx(y) // 9
now let's say you want to change the var result itself
you have to use the mutating func to make change itself
var z = 3
println(z) // 3
now let's mutate it
z.squareMe()
println(z) // 9
// now let's see another example using strings :
extension String {
func x(times:Int) -> String {
var result = ""
if times > 0 {
for index in 1...times{
result += self
}
return result
}
return ""
}
// note: when adding mutating in front of it we don't need to specify the return type
// and instead of "return " whatever
// we have to use "self = " whatever
mutating func replicateMe(times:Int){
if times > 1 {
let myString = self
for index in 1...times-1{
self = self + myString
}
} else {
if times != 1 {
self = ""
}
}
}
}
var myString1 = "Abc"
let myString2 = myString1.x(2)
println(myString1) // "Abc"
println(myString2) // "AbcAbc"
now let's change myString1
myString1.replicateMe(3)
println(myString1) // "AbcAbcAbc"
In what situations it's necessary to use it
It is necessary to use it only when the name of a local variable overshadows the name of a property.
However, as a matter of style (and readability), I always use it:
I use it with property names, because otherwise I am left wondering what this variable is (since it is neither locally declared nor an incoming parameter).
I use it as the receiver of function (method) calls, in order to differentiate such methods from top-level or local functions.
This is why we need self.
When we define a class, like:
class MyClass {
func myMethod()
}
We are creating a "Class Object". Yes, Class is an object too.
Then no matter how many instances are created using the class, all instances will have a reference pointer to its Class Object.
You can imagine that all instance methods defined by the Class are in the Class Object, and there will be only one copy of them.
That means all instances created using the Class are sharing the same method.
Now imagine you are the myMethod in the Class Object, and because you are shared for all instances, you must have a way to tell which instance you are working on.
When someone says instance1.myMethod(), it means "Hi! myMethod, please do your work and instance1 is the object you are working on".
To reference the object that the caller sent to you, use self.
“In practice, you don’t need to write self in your code very often. If
you don’t explicitly write self, Swift assumes that you are referring
to a property or method of the current instance whenever you use a
known property or method name within a method.”
Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks.
https://itun.es/tw/jEUH0.l
The reserved word self in Swift is similar to this but it's not the same as in Java or JavaScript.
As #Dave Gomez correctly quoted:
Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself.
Here lies one of the main differences, because:
"Every instance" in Swift (at least for now) is almost every-thing.
In Java, for example, you can only use the word this inside an instance scope, in Swift you can use it almost every-where.
Here are a few examples:
//Example 1:
var x="foo"
x.self="bar".self//compiles and run
//Example 2:
print.self(x);//compiles and run
//Example 3:
func myOther(self otherSelf:Person){}
myOther(self: personObject);//compiles and run
//Example 4:
class Foo{
var bar=""
init(){
self.addSome()//this would be the same in Java
}
func addSome(){
//But definitely not this:
self.self.bar.self.self="some".self.self
}
}
//Guess what - also compiles and run...
let f=Foo()
print(f.bar)
See : Why 'self.self' compiles and run in swift for more information.
I arrived at this question while searching for self as a class function, which looks like this: Int.self, String.self, or YourClass.self
Previously, as near as I can tell, only Dmitri Pavlutin's answer touches on this, when he said:
When self is accessed in a type method (static func or class func), it refers to the actual type (rather than an instance).
When self is used this way, it actually returns what in Swift is called a Metatype. You can read the Swift documentation page on Types for more information.
There is also an article with more details about using and understanding metatypes called "What's .self, .Type and .Protocol? Understanding Swift Metatypes" on swiftrocks.com.
"How to use correctly 'self' keyword in Swift" explains self in detail.
self is a property on the instance that refers to itself. It's used to access class, structure and enumeration instance within methods.
When self is accessed in a type method like static func or class func, it refers to the actual type rather than an instance.
Swift allows omitting self when you want to access instances properties.
When a method parameter has the same name as an instance property, you have to explicitly use self.myVariable = myVariable to make a distinction.
Notice that method parameters have a priority over instance properties.
I'm a total noob at coding in general - and whilst these answers are great, from the perspective of a total noob who just wants it answered as practically as possible without all these difficult words and concepts, here's my Super Noob dumbed down version:
'self' is used because the coding application doesn't know which variable to use if you type the variable in the scope of your functions. It's all about the scope and making it clear which variable you're using if some other variable has the same name. Scope is the area inside the curly {} brackets. so for example:
{ scope1 {scope2} }
Here you don't need to use self:
class example {
private var exampleVar = “this is the class scope variable”
func x() {
//You don't have to use self here
print(exampleVar)
}
}
Here you do need to use self:
class example {
private var exampleVar = “this is the class scope variable”
func x(_ exampleVar: String) {
//It would be confusing which exampleVar is used here so you should use self
print(exampleVar)
print(self.exampleVar)
}
}
There's also this situation:
class example {
private var exampleVar = “this is the class scope variable”
func x() {
randomMethod { _ in
//This is not the class scope here, so we need to use self here.
//You will be flagged during build time of this if you don't use self.
print(self.exampleValue)
}
}
}
self is a property on the instance that refers to itself. It's used to access the class, structure and enumeration instance within methods.
When a method parameter has the same name as an instance property, you have to explicitly use self.myVariable = myVariable to make a distinction.
Notice that method parameters have a priority over instance properties.
struct Weather {
let windSpeed: Int
let chanceOfRain: Int
init(windSpeed: Int, chanceOfRain: Int) {
self.windSpeed = windSpeed
self.chanceOfRain = chanceOfRain
}
func isDayForWalk() -> Bool {
let comfortableWindSpeed = 5
let acceptableChanceOfRain = 30
return self.windSpeed <= comfortableWindSpeed
&& self.chanceOfRain <= acceptableChanceOfRain
}
}
// A nice day for a walk
let niceWeather = Weather(windSpeed: 4, chanceOfRain: 25)
print(niceWeather.isDayForWalk()) // => true
I have an elegant case for using self in Swift. I use it in blocks like this:
class MyBase {
private var baseValue: Int = 100
var block: ((Int)->())? = nil
func baseMethod(anotherValue: Int) {
guard let b = block else { return }
b(baseValue + anotherValue)
}
...
}
class MyClass {
init()
{
// Define base class block.
// This may hold self in block causing memory leaks
// if self is a strong reference
block = {
// indicate self is a weak reference
[weak self] (para) in
// guaranty self existence
guard let this = self else {return}
let value = this.value1 + para
// this call passes value to other obj's method
this.obj.method(value)
return
}
}
func callBaseBlock(otherValue: Int) {
baseMethod(otherValue)
}
private var value1: Int = 1 // ini value
private var obj: OtherClass // another class with method defined
...
...
}
This way, I guarantee that there will be no strong references to self in the block. The syntax is very clear. I use this way to prevent memory leaks.
I want to call a c function from objective-c and pass objective-c function as a callback
the problem is this function has a callback as parameter, so I have to pass objective-c function as a call back to c function
here is the header of the c function
struct mg_context *mg_start(const struct mg_callbacks *callbacks,
void *user_data,
const char **configuration_options);
here is where I try to call it
- (void)serverstarted
{
NSLog(#"server started");
}
- (IBAction)startserver:(id)sender {
NSLog(#"server should start");
const char *options[] =
{
"document_root", "www",
"listening_ports", "8080",
NULL
};
mg_start(serverstarted(), NULL, options);
}
I have tried several ways to do it and searched the web to just get a clue how to do it but with not luck
here is the library I am incuding in my code
https://github.com/valenok/mongoose
Your chief problem is the first parameter to mg_start(), which is described in the declaration as const struct mg_callbacks *callbacks. You are trying pass a pointer to a function. (Actually you are trying to pass the result of a call to that function, which is even further from the mark.) That isn't what it says: it says a pointer to a struct (in particular, an mg_callbacks struct).
The example code at https://github.com/valenok/mongoose/blob/master/examples/hello.c shows you how to configure this struct. You have to create the struct and put the pointer to the callback function inside it. Then you pass the address of that struct.
Other problems with your code: your callback function itself is all wrong:
- (void)serverstarted
{
NSLog(#"server started");
}
What's wanted here is a C function declared like this: int begin_request_handler(struct mg_connection *conn), that is, it takes as parameter a pointer to an mg_connection struct. Your serverstarted not only doesn't take that parameter, it isn't even a C function! It's an Objective-C method, a totally different animal. Your use of the term "Objective-C function" in your title and your question is misleading; C has functions, Objective-C has methods. No Objective-C is going to be used in the code you'll be writing here.
What I suggest you do here is to copy the hello.c example slavishly at first. Then modify the content / names of things slowly and bit by bit to evolve it to your own code. Of course learning C would also help, but you can probably get by just by copying carefully.
As matt already said, you cannot pass an Objective-C method as callback where a C function
is expected. Objective-C methods are special functions, in particular the receiver ("self")
is implicitly passed as first argument to the function.
Therefore, to use an Objective-C method as request handler, you need an (intermediate) C function as handler and you have to pass self to that function, using the user_data argument. The C function can then call the Objective-C method:
// This is the Objective-C request handler method:
- (int)beginRequest:(struct mg_connection *)conn
{
// Your request handler ...
return 1;
}
// This is the intermediate C function:
static int begin_request_handler(struct mg_connection *conn) {
const struct mg_request_info *request_info = mg_get_request_info(conn);
// Cast the "user_data" back to an instance pointer of your class:
YourClass *mySelf = (__bridge YourClass *)request_info->user_data;
// Call instance method:
return [mySelf beginRequest:conn];
}
- (IBAction)startserver:(id)sender
{
struct mg_callbacks callbacks;
memset(&callbacks, 0, sizeof(callbacks));
callbacks.begin_request = begin_request_handler;
const char *options[] =
{
"document_root", "www",
"listening_ports", "8080",
NULL
};
// Pass "self" as "user_data" argument:
mg_start(&callbacks, (__bridge void *)self, options);
}
Remarks:
If you don't use ARC (automatic reference counting) then you can omit the (__bridge ...)
casts.
You must ensure that the instance of your class ("self")
is not deallocated while the server is running. Otherwise the YourClass *mySelf
would be invalid when the request handler is called.