Re: [swift-users] strange property observer behavior

[adelzhang via swift-users]

Hi,

The following code works fine. The property `a` is stored twice. But
it don't enter infinite loop.

class Foo {
var a: Int = 0 {
 didSet {
 a = a + 1
 }
}
}

let foo = Foo()
foo.a = 2
print(foo.a) // output 3


Regards

--adel


在 Mon, 05 Sep 2016 00:27:16 +0800，Gerard Iglesias  
 写道:



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  
 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?





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Re: [swift-users] Data withUnsafeBytes

[Chris McIntyre via swift-users]

That makes sense. I think I've gotten so used to the type system magically 
figuring out what I want, combined with the error message pointing to the 
return type that this just went right over my head. 

--
Chris McIntyre

> On Sep 4, 2016, at 3:59 PM, Ole Begemann via swift-users 
>  wrote:
> 
> The compiler has no way of inferring the type of the generic ContentType 
> parameter because you're not using that parameter anywhere in the body of the 
> closure that could give the compiler a hint. So you have to provide the type 
> manually by explicitly annotating the type of the closure's parameter.
> 
> For example, if you want to treat `items` as a series of `UInt8` values:
> 
>let tryThis = items.withUnsafeBytes {
>(bytes: UnsafePointer) -> Int in
>return 1
>}
> 
> I agree the compiler diagnostic should probably be better, though.
> 
> Ole
> 
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Re: [swift-users] Data withUnsafeBytes

[Ole Begemann via swift-users]

The compiler has no way of inferring the type of the generic ContentType 
parameter because you're not using that parameter anywhere in the body 
of the closure that could give the compiler a hint. So you have to 
provide the type manually by explicitly annotating the type of the 
closure's parameter.


For example, if you want to treat `items` as a series of `UInt8` values:

let tryThis = items.withUnsafeBytes {
(bytes: UnsafePointer) -> Int in
return 1
}

I agree the compiler diagnostic should probably be better, though.

Ole

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Re: [swift-users] strange property observer behavior

[Jens Alfke via swift-users]

> On Sep 4, 2016, at 8:11 AM, adelzhang via swift-users  
> wrote:
> 
> 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:

I’m not a Swift guru, but that seems like a bug to me. The book explicitly says 
that it’s legal for a didSet block to set the property value, with no caveats 
about overridden properties.

You should be able to work around this by having the subclass override the 
`set` block instead, and pass the modified value to the superclass. (It’s 
probably slightly more efficient too.)

—Jens
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Re: [swift-users] strange property observer behavior

[adelzhang via swift-users]

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  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  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  
 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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Re: [swift-users] strange property observer behavior

[Zhao Xin via swift-users]

>
> 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@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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[swift-users] strange property observer behavior

[adelzhang via swift-users]

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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