I have such class setup. In my methods I want to work with specific array depending on what parameter is passed. My question is : is "array" variable a copy of selected array or a reference to it? If it is a copy, how does one pass a reference to an array? I don't want to copy it becuase it is quite long.
I heard that in times of Swift 1 arrays were copied only when needed (compiler decides when). How things are now in Swift 2?
class ... {
private var currentVertexes = [CCVertex]()
private var mainVertexes : [CCVertex]!
private var leftVertexes : [CCVertex]!
private var rightVertexes : [CCVertex]!
private var topVertexes : [CCVertex]!
private var bottomVertexes : [CCVertex]!
...
internal func method(var factor: Float) {
let array = factor < 0.0 ? leftVertexes : rightVertexes
...
}
Depends on whether CCVertex is a struct or a class. If it's a struct it will be copied, not if it's a class.
From Apple's documentation (and with good examples too):
Copying an array also copies all of the elements of that array that are value types. This means that changing one of the elements of an array does not change the elements of any of the copies of the array
If the elements in an array are instances of classes, changing the class does affect other copies, because classes have reference semantics
Arrays are value types, but use copy-on-write to prevent unnecessary copies when you merely access them in a read-only fashion (see SwiftDocs)
So if the rest of your method only reads from the array, then you don't need to worry about copies (irrespective of whether CVVertex is a struct or a class).
Related
I'm trying to create a survey using SwiftUI, where the survey can have an arbitrary number of questions.
I'm trying to capture what values the user inputted through state variables such as:
#State var answer: String = ""
ForEach(survey) { surveyQuestion in
Text(surveyQuestion.question)
TextField(surveyQuestion.placeholder, text: $answer)
}
However, since I don't know how many questions are going to be in the survey beforehand, I don't know how many of these state variables to store the answers to make. I could create the variables on the fly inside the ForEach loop but then the variables would be out of scope when I actually go to submit the survey (since the submitting would happen outside of the ForEach loop).
How do I create an arbitrary number of state variables to capture the user's answers to the survey?
EDIT: I had tried making my answers variable a dictionary, where the keys are the IDs to the questions. My code looked like:
#State var answers: [String:String] = [:]
ForEach(survey) { surveyQuestion in
Text(surveyQuestion.question)
TextField(surveyQuestion.placeholder, text: $answers[surveyQuestion.id!])
}
However, I kept getting the error:
Cannot convert value of type 'Binding<String?>' to expected argument type 'Binding<String>'
So then I tried replacing $answers[surveyQuestion.id!] with $(answers[surveyQuestion.id!]!) but then the system gets confused and responds with:
'$' is not an identifier; use backticks to escape it
I had also tried adjusting my question model so that there's a field for an answer in the same struct. My code looked like this:
TextField(surveyQuestion.placeholder, text: $surveyQuestion.answer)
I kept getting the error:
Cannot find '$surveyQuestion' in scope
Using the strategy in the edit that you added, with the Dictionary, you could provide a custom Binding, like this:
func bindingForID(id: String) -> Binding<String> {
.init {
answers[id] ?? ""
} set: { newValue in
answers[id] = newValue
}
}
And you could use it like this:
TextField(surveyQuestion.placeholder, text: bindingForID(id: surveyQuestion.id))
In terms of adding this data to Firestore, you could trigger Firestore updates in the set closure from the custom binding. However, I'd probably recommend moving this logic to a #Published property on a view model (ObservableObject) where you could use Combine to do things like Debouncing before you send the data to Firestore (probably a little beyond the scope of this question).
Create a struct that holds id, question, and answer. Your #State var should be an array of those.
struct QA: Identifiable {
id: String
question: String
answer: String
}
…
#State private var myQAs: [QA] = myQAs() // or populate them in init, onAppear(if asynchronous) or however you see fit
…
ForEach(myQAs) { qa in
Text(qa.question)
TextField(placeholder, text: $qa.answer)
}
I'm working on a KMM app. The shared module has a helper class, that relies on different native libraries for the android part and for the iOS part. This is implemented with the already know "expected/actual" pattern.
As said, the iOS actual class makes use of an iOS framework, that performs some calculations, and returns an array of objects. The ios framework that creates the list of objects works correctly (it was tested with unit tests). A simplified example follows below.
This is the class of the objects that are inside of the array:
public class ChildNodeIos: NSObject{
public let content:String
public let isTextNode:Bool
public init(content:String,isTextNode:Bool=false){
self.content=content
self.isTextNode=isTextNode
}
}
The helper class on the iOS side that returns the list of objects would be something like that:
#objc public class IOSCoolHelper: NSObject {
#objc public func getChildNodes(message: String) -> [ChildNodeIos] {
//build the array of child nodes here and return them
}
}
In the kotlin shared module, inside the iOS expected class, the function is called like the following:
#Serializable
data class ChildNodeKN(val content :String,val isTextNode :Boolean=false)
import com.mydomain.iosframeworks.IosCoolHelper
actual class CoolHelper actual constructor(private val someStuff: String) : ICoolHelper {
actual override fun getChildNodes(message: String): List<ChildNodeKN> {
val iosHelper= IOSCoolHelper()
val swiftarray:List<ChildNodeIos> = iosHelper.getChildNodes(message)
//I was expecting to do something like that but it does not work (the content of "array is always empty"):
val kotlinList:List<ChildNodeKN> = swiftarray as List<ChildNodeIos>
return kotlinList
}
}
}
Or maybe if the list of swift objects can not be direct be casted to the equivalent kotlin object list, I was expecting to be able to iterate over the swift list and convert it to the kotlin list, something like that:
val kotlinList=mutableListOf<ChildNodeKN>()
swiftArray.foreach{
kotlinList.add(ChildNodeKN(it.content,it.isTextNode))
}
But again, the content of the swift Array is empty. Doing a lot of tests (I can no reproduce them now), I managed to access something inside the array, but it was not an object of type ChildNodeIos, nor something I could read on the kotlin side.
Well, the question is, how to receive on the kotlin side, a list with more or less complex objects inside, that was generated on the iOS side?
I have to say, that this swift helper class has many other functions that return primitive values (strings, booleans, or int), and that is working very well.
I suppose a workaround would be instead an array with objects, to return an array with primitive types and two dimensions from the Swift side, but I would like to work with an array of objects if it is possible.
Thank you for your help
I managed to find the solution by myself. The problem was the declaration of the Swift class of the object contained in the list. I forgot the #objc declaration for the properties of the class, because if that I was not able to read the objects inside the returned array.
public class ChildNodeIos: NSObject{
#objc public let content:String
#objc public let isTextNode:Bool
public init(content:String,isTextNode:Bool=false){
self.content=content
self.isTextNode=isTextNode
}
}
And then, on the Kotlin side, I did not achieve to cast it directly to a list, but with a foreach loop it is very easy to write the iOS objects in Kotlin objects:
I'm trying to write a simple app in Swift, backed with Realm. When I write vars within classes, I keep getting errors such as "Property cannot be implicitly #objc because its type cannot be represented in Objective-C". Sometimes these errors (which seem to involve Objective-C, which I'm not using) appear and sometimes they don't--See screenshots below.
I'm an admitted Newbie to Swift, but I thought I understood enough to get started. Specifically:
1) Should I add "#objc" before the class declaration?
2) Should I add "#objc" before the dynamic var declaration?
3) It seems that a Float var must be initialized, but not so for String?
4) Why does the compiler complain about a declaration on one line, but not about identical syntax in the next line?
Would sure appreciate some guidance, or pointers to specific instructions for creating Realm object classes!
I reckon the error comes from calling #objc instead of #objcMembers.
Try change
#objc class Transaction: Object
to
#objcMembers class Transaction: Object
And remove all #objc in front of your dynamic variables. So your final code should look like:
import Foundation
import RealmSwift
#objcMembers class Transaction: Object {
dynamic var picPath: String?
dynamic var transAmount: Float = 0.0
}
#objcMembers class Transaction: Object {
dynamic var currentBalance: Float = 0.0
dynamic var highWaterBalance: Float = 0.0
dynamic var acctName: String?
}
Our fellow Paul Hudson has described this attribute on his site hackingwithswift.
Does Swift support reflection? e.g. is there something like valueForKeyPath: and setValue:forKeyPath: for Swift objects?
Actually does it even have a dynamic type system, something like obj.class in Objective-C?
Looks like there's the start of some reflection support:
class Fruit {
var name="Apple"
}
reflect(Fruit()).count // 1
reflect(Fruit())[0].0 // "name"
reflect(Fruit())[0].1.summary // "Apple"
From mchambers gist, here:
https://gist.github.com/mchambers/fb9da554898dae3e54f2
If a class extends NSObject, then all of Objective-C's introspection and dynamism works. This includes:
The ability to ask a class about its methods and properties, and to invoke methods or set properties.
The ability to exchange method implementations. (add functionality to all instances).
The ability to generate and assign a new sub-class on the fly. (add functionality to a given instance)
One shortcoming of this functionality is support for Swift optional value types. For example Int properties can be enumerated and modified but Int? properties cannot. Optional types can be enumerated partially using reflect/MirrorType, but still not modified.
If a class does not extend NSObject, then only the new, very limited (and in progress?) reflection works (see reflect/MirrorType), which adds limited ability to ask a instance about its class and properties, but none of the additional features above.
When not extending NSObject, or using the '#objc' directive, Swift defaults to static- and vtable-based dispatch. This is faster, however, in the absence of a virtual machine does not allow runtime method interception. This interception is a fundamental part of Cocoa and is required for the following types of features:
Cocoa's elegant property observers. (Property observers are baked right in to the Swift language).
Non-invasively applying cross-cutting concerns like logging, transaction management (i.e Aspect Oriented Programming).
Proxies, message forwarding, etc.
Therefore its recommended that clases in Cocoa/CocoaTouch applications implemented with Swift:
Extend from NSObject. The new class dialog in Xcode steers in this direction.
Where the overhead of of a dynamic dispatch leads to performance issues, then static dispatch can be used - in tight loops with calls to methods with very small bodies, for example.
Summary:
Swift can behave like C++, with fast static/vtable dispatch and limited reflection. This makes it suitable for lower level or performance intensive applications, but without the complexity, learning curve or risk of error associated with C++
While Swift is a compiled language, the messaging style of method invocation adds the introspection and dynamism found in modern languages like Ruby and Python, just like Objective-C, but without Objective-C's legacy syntax.
Reference data: Execution overhead for method invocations:
static : < 1.1ns
vtable : ~ 1.1ns
dynamic : ~4.9ns
(actual performance depends on hardware, but the ratios will remain similar).
Also, the dynamic attribute allows us to explicitly instruct Swift that a method should use dynamic dispatch, and will therefore support interception.
public dynamic func foobar() -> AnyObject {
}
The documentation speaks about a dynamic type system, mainly about
Type and dynamicType
See Metatype Type (in Language Reference)
Example:
var clazz = TestObject.self
var instance: TestObject = clazz()
var type = instance.dynamicType
println("Type: \(type)") //Unfortunately this prints only "Type: Metatype"
Now assuming TestObject extends NSObject
var clazz: NSObject.Type = TestObject.self
var instance : NSObject = clazz()
if let testObject = instance as? TestObject {
println("yes!") //prints "yes!"
}
Currently, there is no reflection implemented.
EDIT: I was apparently wrong, see stevex's answer. There is some simple readonly reflection for properties build in, probably to allow IDEs to inspect object contents.
No reflect keyword in Swift 5, now you can use
struct Person {
var name="name"
var age = 15
}
var me = Person()
var mirror = Mirror(reflecting: me)
for case let (label?, value) in mirror.children {
print (label, value)
}
It seems that a Swift reflection API is not a high priority for Apple at the moment. But besides #stevex answer there is another function in the standard library that helps.
As of beta 6 _stdlib_getTypeName gets the mangled type name of a variable. Paste this into an empty playground:
import Foundation
class PureSwiftClass {
}
var myvar0 = NSString() // Objective-C class
var myvar1 = PureSwiftClass()
var myvar2 = 42
var myvar3 = "Hans"
println( "TypeName0 = \(_stdlib_getTypeName(myvar0))")
println( "TypeName1 = \(_stdlib_getTypeName(myvar1))")
println( "TypeName2 = \(_stdlib_getTypeName(myvar2))")
println( "TypeName3 = \(_stdlib_getTypeName(myvar3))")
The output is:
TypeName0 = NSString
TypeName1 = _TtC13__lldb_expr_014PureSwiftClass
TypeName2 = _TtSi
TypeName3 = _TtSS
Ewan Swick's blog entry helps to decipher these strings:
e.g. _TtSi stands for Swift's internal Int type.
Mike Ash has a great blog entry covering the same topic.
You might want to consider using toString() instead. It is public and works just the same as _stdlib_getTypeName() with the difference that it also works on AnyClass, e.g. in a Playground enter
class MyClass {}
toString(MyClass.self) // evaluates to "__lldb_expr_49.MyClass"
So I want to extend the dictionary class. Everything works so far except that in some of my methods that need to reference the dictionary's content I make a call like:
this[ key ]
It doesn't like that. It just tells me that there's no property 'key'. Is there a way to way to access the data within this class?
Also, I'm using an integer for the key.
Edit: I've found that the same behavior happens when you extend Array.
var myArray : Array = new Array();
trace( myArray[ 0 ] );
var myArrayExtender : ArrayExtender = new ArrayExtender();
trace( myArrayExtender[ 0 ] );
Where in this case, myArray returns "undefined" and myArrayExtender throws error 1069. ArrayExtender is an empty class that extends Array and calls super() in the constructor.
from what you say, i am quite sure you did not declare ArrayExtender as dynamic
in ECMAscript array access and property access are semantically equivalent ... #1069 happens, if you access an undefined property, on a sealed class, because thes do not allow adding properties at runtime ...
same thing for the Dictionary ...
greetz
back2dos