What does it mean when optional chaining is used in the left side of the assignment statement? Will the app crash if the optional variable is nil?
e.g.
// cell is a UITableViewCell
cell.textLabel?.text = "Test"
You specify optional chaining by placing a question mark ( ? ) after the optional value on which you wish to call a property, method or subscript if the optional is non- nil . This is very similar to placing an exclamation point ( ! ) after an optional value to force the unwrapping of its value.
Optional chaining (?.) The optional chaining ( ?. ) operator accesses an object's property or calls a function. If the object is undefined or null , it returns undefined instead of throwing an error.
Optional binding allows an entire block of logic to happen the same way every time. Multiple lines of optional chaining can make it unclear exactly what is going to happen and could potentially be subject to race-conditions causing unexpected behavior.
An optional in Swift is basically a constant or variable that can hold a value OR no value. The value can or cannot be nil. It is denoted by appending a “?” after the type declaration.
Sort of like a short-circuiting &&
operator that stops when it reaches the first false value, optional chaining will stop when it hits the first nil value.
So in an extreme case like container?.cell?.textLabel?.text = "foo"
, any of container, cell, or textLabel could be nil. If any of them are, that statement is effectively a no-op. Only if the whole chain is non-nil will the assignment happen.
For the sake of completeness, in addition to @gregheo answer:
The unwrapped value of an optional-chaining expression can be modified, either by mutating the value itself, or by assigning to one of the value’s members. If the value of the optional-chaining expression is
nil
, the expression on the right hand side of the assignment operator is not evaluated.
Quoted from "Optional-Chaining Expression"
It will just act as an optional, you can set it as a string or nil. Neither of them will crash the app unless you unwrap your variable without catching if it is nil, when it is nil. If anything in your chain is an optional, your whole chain is (or at least everything after the optional)
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