Swift version: 5.10
The @autoclosure
attribute can be applied to a closure parameter for a function, and automatically creates a closure from an expression you pass in. When you call a function that uses this attribute, the code you write isn't a closure, but it becomes a closure, which can be a bit confusing – even the official Swift reference guide warns that overusing autoclosures makes your code harder to understand.
To help you understand how it works, here's a trivial example:
func printTest1(_ result: () -> Void) {
print("Before")
result()
print("After")
}
printTest1({ print("Hello") })
That code creates a printTest()
method, which accepts a closure and calls it. As you can see, the print("Hello")
is inside a closure that gets called between "Before" and "After", so the final output is "Before", "Hello", "After".
If we used @autoclosure
instead, it would allow us to rewrite the printTest()
call so that it doesn't need braces, like this:
func printTest2(_ result: @autoclosure () -> Void) {
print("Before")
result()
print("After")
}
printTest2(print("Hello"))
These two pieces of code produce identical results thanks to @autoclosure
. In the second code example, the print("Hello")
won't be executed immediately because it gets wrapped inside a closure for execution later.
The @autoclosure
attribute is used inside Swift wherever code needs to be passed in and executed only if conditions are right. For example, the &&
operator uses @autoclosure
to allow short-circuit evaluation, and the assert()
function uses it so that the assertion isn’t checked outside of debug mode.
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This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
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