[swift-users] strange property observer behavior

[Zhao Xin]

>
> 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> 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
>
> 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> 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
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>
>
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