[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.

​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
>
>
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