CODEWITHSUNDEEP
swiftproperties
Pisan
Pisan

Reputation: 69

Advantage of computed properties (gettable ones only) vs. stored properties

I would like to clarify if I understand the following concept correctly.

Assume that my goal is to store the String "Good morning, Mike" into the variable var sayGoodMorningToUser.

The String is composed of two variables, namely

var greeting = "Good morning, "
var username = "Mike"

What difference does it make if I use stored properties vs. computed properties, in other words:

var sayGoodMorningToUserStored = greeting + username

vs.

var sayGoodMorningToUserComputed:String {
     return greeting + username
}

The only difference I see between these two approaches is that anyone could change the value of sayGoodMorningToUserStored easily and directly, e.g. by writing

var sayGoodMorningToUserStored = "myNewChangedValue"

whereas the variable sayGoodMorningToUserComputed cannot be modified directly, because it cannot simply be set it to a new String value:

var sayGoodMorningToUserComputed = "Hallo" //this would cause an error

Otherwise I cannot understand why people compute the variable instead of simply writing 

var sayGoodMorningToUserStored = greeting + username.

Can anyone explain if I understood it correctly? Or are there also other advantages of computed variables vs. stored ones?

I would like to limit my question to gettable variables only, because discussing settable variables here would go beyond the scope.

Upvotes: 0

Views: 1234

Answers (2)

GetSwifty
GetSwifty

Reputation: 7746

The difference in your example is this: 

var sayGoodMorningToUserStored = "myNewChangedValue"

and 

var sayGoodMorningToUserStored = greeting + username

are set when your class is initialized, whereas this:

var sayGoodMorningToUserComputed:String {
     return greeting + username
}

is evaluated every time the property is accessed.

The simple example is a class that has a firstName and lastName, but also want fullName. Using a normal property, every time you update firstName or lastName you will also have to update fullName so it will match. Using a computed property, every time you access fullName you will get the up to date information.

Upvotes: 3

Alexander
Alexander

Reputation: 63157

Computed Properties

var sayGoodMorningToUserComputed: String {
     return greeting + username
}

sayGoodMorningToUserComputed acts just like a function. If a change has been made to greeting or username, then sayGoodMorningToUserComputed will return an up-to-date result that will be the concatenation of the current values.

You would want to use this if you want to ensure your returned value is computed off the latest values of its dependencies (greeting and username).

In the case that both dependencies are final, then it's very likely that the compiler would optimise this computed property into a stored property, because it knows the dependencies can't change

Stored properties

var sayGoodMorningToUserStored = greeting + username

sayGoodMorningToUserStored is just a variable, with nothing special going on. However, it's only set once, whenever the containing scope is initialized. It's computed once, stored and remains constant until it is overwritten by an external source. As such, if greeting or username changes, there will be no effect on sayGoodMorningToUserStored, because it's been computed from the old values, and stored.

You would want to use this if you want to improve performance by caching the result of a computation whose dependencies are constant.

Upvotes: 1

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