问题
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"
回答1:
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.
回答2:
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"
回答3:
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)
来源：https://stackoverflow.com/questions/29221293/optional-chaining-used-in-left-side-of-assignment-in-swift