Computed read-only property vs function in Swift

Question

In the Introduction to Swift WWDC session, a read-only property description is demonstrated:

class Vehicle {     var numberOfWheels = 0     var description: String {         return "\(numberOfWheels) wheels"     } }  let vehicle = Vehicle() println(vehicle.description) 

Are there any implications to choosing the above approach over using a method instead:

class Vehicle {     var numberOfWheels = 0     func description() -> String {         return "\(numberOfWheels) wheels"     } }  let vehicle = Vehicle() println(vehicle.description()) 

It seems to me that the most obvious reasons for choosing a read-only computed property are:

• Semantics - in this example it makes sense for description to be a property of the class, rather than an action it performs.
• Brevity/Clarity - prevents the need to use empty parentheses when getting the value.

Clearly the above example is overly simple, but are there other good reasons to choose one over the other? For example, are there some features of functions or properties that would guide your decision of which to use?

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N.B. At first glance this seems like quite a common OOP question, but I'm keen to know of any Swift-specific features that would guide best practice when using this language.

Answer 1

It seems to me that it's mostly a matter of style: I strongly prefer using properties for just that: properties; meaning simple values that you can get and/or set. I use functions (or methods) when actual work is being done. Maybe something has to be computed or read from disk or from a database: In this case I use a function, even when only a simple value is returned. That way I can easily see whether a call is cheap (properties) or possibly expensive (functions).

We will probably get more clarity when Apple publishes some Swift coding conventions.

Answer 2

Well, you can apply Kotlin 's advices https://kotlinlang.org/docs/reference/coding-conventions.html#functions-vs-properties.

In some cases functions with no arguments might be interchangeable with read-only properties. Although the semantics are similar, there are some stylistic conventions on when to prefer one to another.

Prefer a property over a function when the underlying algorithm:

• does not throw
• complexity is cheap to calculate (or caсhed on the first run)
• returns the same result over invocations