Lazy property

[wnagrodzki]

It should be used only to defer property costly initialization. Before using it check if the following rules are met:

• The property initialization is noticable by the user and making it lazy would fix the problem.
• Property initialization outcome should be independent from the state of the owning instance at the time of the initialization
• The property initialization should not rely on other lazy propertes.
• Lazy property should not be used as workaround for using self in init() method before all properties are initialized (in that case use implicityly unwrapped optional).

To avoid cluttering the interface with lazy property initialization closure, extract it's content to a private method.

lazy var data: Data = self.provideFileData()

private func provideFileData() -> Data {
  // load data from disk
}

[pwetrifork]

The initialized property state should be independent from the state of the instance. The initialized property state should be independent from the time of it's initalization.

Consider a simpler phrasing for the above (I think that independence from the time of initialization is in fact independence from the state of the instance, which changes over time):

Property initialization outcome should be independent from the state of the owning instance at the time of the initialization

Then, what is the reasoning behind the rule about initialization not relying on other lazy properties?

[Moskacz]

What about a situation where initialising a property is costly but is not noticeable by the user? How should we measure this?

[wnagrodzki]

@Moskacz If it is not noticeable by the user we do not optimise it