[swift-users] strange property observer behavior

[Gerard Iglesias]

Hi,

didSet is called as soon as the property is stored… Excepted when the value is stored in the initialiser code.

For me it is completely predictable that your code enter an infinite loop

Regards


> On 4 Sep 2016, at 17:11, adelzhang via swift-users <swift-users at swift.org> wrote:
> 
> Thanks for reply.
> 
> How does Swift choose *rules* as you said?
> 
> Swfit encourage to override the property observer. But when we change the own property in Child class's `didSet` observer, that would cause infinite loop:
> 
>     class Base { 
>         var a: Int = 0 
>     } 
> 
>     class Child : Base { 
>         override var a: Int { 
>             didSet { 
>                  a = a + 1 
>             } 
>         } 
>      }
>     
>      let child = Child() 
>      child.a = 3
> 
> Any differcen with situation 1? 
> 
> 
> 在 Sun, 04 Sep 2016 20:12:42 +0800，Zhao Xin <owenzx at gmail.com> 写道:
> 
> 1) when `didSet` observer will call?
> 
> ​For me, it is more like Swift developer tries to override some beginner's flaw. 
> 
> Above is incorrect. You can change property's value in `didSet`, that won't cause didSet called again as it is intended to give you the opportunity to do that. 
> 
> ​2) infinite loop
> 
> This can't apply the above rule as they set each other, causing the infinite loops. 
> 
> Zhaoxin
> 
> On Sun, Sep 4, 2016 at 7:41 PM, Zhao Xin <owenzx at gmail.com <mailto:owenzx at gmail.com>> wrote:
> 1) when `didSet` observer will call?
> 
> ​For me, it is more like Swift developer tries to override some beginner's flaw. 
> 
> ​2) infinite loop
> 
> ​If you intended to do things bad, things ​went bad.
> 
> 3) override property observer
>  
> ​You mentioned "TSPL(The Swift Programming Language) ​", and it says in it:
> 
> “NOTE
> 
> The willSet and didSet observers of superclass properties are called when a property is set in a subclass initializer, after the superclass initializer has been called. They are not called while a class is setting its own properties, before the superclass initializer has been called.
> 
> For more information about initializer delegation, see Initializer Delegation for Value Types and Initializer Delegation for Class Types.”
> 
> From: Apple Inc. “The Swift Programming Language (Swift 3 Beta)”。 iBooks. https://itun.es/us/k5SW7.l <https://itun.es/us/k5SW7.l>
> 
> You didn't provide a `init()`, but since you properties were already set. There was a hidden `init()` when you called `Child()`.
> 
> Last, 
> 
>  let base = child as Base
>  base.a  = 4 // still output "base didset" and "child didset"
> 
> In Swift, as or as! won't change the instance's dynamic type. So it does nothing. `type(of:base)` is still `Child`.
> 
> Zhaoxin
> 
> 
> 
> On Sun, Sep 4, 2016 at 6:25 PM, adelzhang via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> Hi all
> 
> It sounds convenient to monitor change in property's value using property observer.
> But TSPL(The Swift Programming Language) talk little about property observer. There
> are some questions abouts property observer.
> 
> 1) when `didSet` observer will call?
> 
> I assume it's fine that changing property's value in `didSet` observer.
> 
>     class Foo {
>         var a: Int = 0 {
>             didSet {
>                 print("didset")
>                 a = a + 1
>             }
>         }
>     }
> 
>     let foo = Foo()
>     foo.a = 4  // only output "didset" once
> 
> Why it don't cause infinite loop?
> 
> 2) infinite loop
> 
>     // this code snippet cause inifinite loop
>     class Foo {
>         var a: Int = 0 {
>             didSet {
>                 b = a + 1
>             }
>         }
> 
>         var b: Int = 1 {
>             didSet {
>                 a = b - 1
>             }
>         }
>     }
> 
>     let foo = Foo()
>     foo.a = 2
> 
> 3) override property observer
> 
>     class Base {
>         var a: Int = 0 {
>             didSet {
>                 print("base didset")
>             }
>         }
>     }
> 
>     class Child : Base {
>         override var a : Int {
>             didSet {
>                 print("child didset")
>             }
>         }
>     }
> 
>     let child = Child()
>     child.a = 2 // output "base didset" and "child didset"
>     let base = child as Base
>     base.a  = 4 // still output "base didset" and "child didset"
> 
> Why overriding property observer still call parent's `didSet` observer?
> 
> --
> Adel
> 
> 
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