[swift-users] strange property observer behavior

[adelzhang at qq.com]

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> 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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