protocols + variables + type issue

Question

eblu OP

Created Jun ’20

Replies 3

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I have a protocol, in which I define a variable.

public protocol CNEnvelopment {
    var pan : CGPoint {get set}
    var zoom : Float {get set}
    
    var dagObjs : [CNDagObj] {get set} // this one.

I use that protocol type coerce another type, to shield myself from having to test the object type, and write about 30 lines of code for the same behavior in a series of classes:

(theParent as! CNEnvelopment).dagObjs.insert(contentsOf: droppedItems, at: theIndex + 1)

but I get an error when I do that:

Cannot use mutating member on immutable value of type '[CNDagObj]'

do I need to just throw the protocol away and use a class? Because it will work properly if I make CNEnvelopment a class.

is this a limitation of protocols?

Answered by eblu in 423032022

var.

if var envelope = theParent as? CNEnvelopment{

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Answer 1

OOPer OP

Jun ’20

Please show more context.

As far as I tried, this compiles:

var theParent: CNEnvelopment = MyEnvelopment()
theParent.dagObjs.insert(contentsOf: droppedItems, at: theIndex + 1)

So, the problem is `as!`-casting. Why do you need it? How have declared `theParent`?

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Answer 2

eblu OP

Jun ’20

there is a parent class to the classes that are CNEnvelopment.

so the child classes are something like :

public class CNNode : CNDagObj, CNIdentity, CNProperties, CNEnvelopment, CNStackObj, CNDagConnection, BKObserver, CNRenderable, CNOutputsObj {

CNDagObj is the parent class. Not all CNDagObjects are CNEnvelopment.

the Property in which we find the CNEnvelopment, is of the class CNDag.

public var parent : CNDagObj? = nil

all objects that will be a parent, will also be CNEnvelopment, But as I recall... the compiler doesn't like it if I do this instead :

public var parent : CNEnvelopment? = nil

so I'm stuck with the next lowest common denominator class (CNDag)

i suppose i should :

if let envelope = theParent as? CNEnvelopment{

?

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Answer 3

eblu OP

Jun ’20

Accepted Answer

var.

if var envelope = theParent as? CNEnvelopment{

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