The logic is to clear an Array when it has a specified amount of elements. I could put the check outside of the Array but I was trying to see what if do it in Array's willSet event. The result is elements in Array stay still.
Here is the code
var MyArr=[String]() {
willSet{
print("now count is:\(MyArr.count)")
if MyArr.count>2 {
print("now remove all!")
MyArr.removeAll()
}
}
}
MyArr.append("hello")
MyArr.append(",world")
MyArr.append("!")
MyArr.append("too much.")
print("The conent is \(MyArr)")
MyArr was expected to have only one elements while actual result was four.
The behavior has nothing to do with value type / reference type
Please note the warning
Attempting to store to property 'MyArr' within its own willSet, which is about to be overwritten by the new value
which means that modifying the object in willSet
has no effect.
Citing the Language Guide - Properties - Property Observers [ emphasis mine]:
Property Observers
Property observers observe and respond to changes in a property's value.
...
You have the option to define either or both of these observers on a property:
willSet
is called just before the value is stored .didSet
is called immediately after the new value is stored .
As you are experimenting with the willSet
property observer, any mutation of the property you are observing within the willSet
block precedes the actual storing of the newValue
which follows immediately after the willSet
block. This means you are essentially attempting to mutate "the old copy" of myArr
prior to it being replaced with its new value.
Arguably this could be discussed as something illegal, as any mutation of myArr
should lead to the invocation of any property observers, thus mutation of a property within a property observer (mutation of the reference for reference types or the value for value types) could arguably lead to recursive calls to the property observer. This is not the case, however, and for the willSet
case, specifically, instead a warning is emitted, as pointed out in @vadian's answer . The fact that mutation of the property itself within a property observer will not trigger property observers is not really well-documented, but an example in the Language Guide - Properties - Type Properties points it out [ emphasis mine]:
Querying and Setting Type Properties
...
The
currentLevel
property has adidSet
property observer to check the value ofcurrentLevel
whenever it is set. ...NOTE
In the first of these two checks, the
didSet
observer setscurrentLevel
to a different value. This does not, however, cause the observer to be called again.
This is also a special case we can somewhat expect, as eg didSet
is an excellent place to incorporate eg bounds checks that clamps a given property value to some bounds; ie, over-writing the new value by a bounded one in case the former is out of bounds.
Now, if you change to mutate the property to after the new value has been stored, the mutation will take affect and, as covered above, not trigger any additional calls to the property observers. Applied to your example:
var myArr = [String]() {
didSet {
print("now count is: \(myArr.count)")
if myArr.count > 2 {
print("now remove all!")
myArr.removeAll()
}
}
}
myArr.append("hello")
myArr.append(",world")
myArr.append("!")
myArr.append("too much.")
print("The content is \(myArr)") // The content is ["too much."]
aAlan's comment on didSet
vs willSet
is interesting. I tried the same code but with didSet
and it did remove the elements from the array which seemed odd initially. I think it's by design. My reasoning is:
willSet
and made changes then everything would get overwritten. It makes all your changes nullified. Because you're doing this before you even read what's about to happen. Also (repeating what dfri has said) if you set it again inside willSet
well then you're going to trigger the property observer again and again and will create a feedback loop which will crash the compiler, hence the compiler behind the scene didSet
. You wait...read the value and then make your decision. The value of the property is known to you.
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