Swift Fallthrough: An Introduction
If you're not used to Swift's fallthrough statement for switch cases, then this brief summary will give you a sense of how they work.
Join the DZone community and get the full member experience.
Join For FreeSwift switch
statements are extremely useful and powerful conditionals. They allow you to slim down your code by avoiding repetitive if-else statements or nested ifs, and their pattern matching capabilities are extremely robust. But, while Swift’s switch
works much the same as C’s, there are some variations. One such variation is the fact that it is unnecessary to break
out of a case in a Swift switch
; the switch
statement breaks out of the first case that is matched. This means there is no “implicit fallthrough” in Swift switch
statements; it makes it harder to accidentally execute on multiple cases.
If you want your switch
statement to fall through cases, Swift offers the fallthrough
statement. This statement forces the execution of the next case’s code block. It’s important to note that the condition of the next case does not matter; the code in that case will execute whether the condition has been met or not. Consider the example:
var whatever = 100
switch whatever {
case 1...1000:
print("Within 1-1000")
fallthrough
case 1...100:
print("Within 1-100")
fallthrough
case 1...10:
print("Within 1-10")
fallthrough
default:
print("Out of range")
}
Here, whatever
, with a value of 100, matches the first range case of 1-1000, causing the case 1...1000:
block to execute. The fallthrough then causes the next block to execute. In this case, the condition would still match. But the fallthrough here causes the next block to execute, even though this condition is not a match. The final fallthrough executes the default case’s code block, so for an output we get:
Within 1-1000
Within 1-100
Within 1-10
Out of range
So you can’t use fallthrough to try to match multiple cases, but you can use it to create cascading “default” cases based on the matched value. For example:
var whatever = 3
switch whatever {
case 3:
print("Three.")
fallthrough
case 2:
print("Two.")
fallthrough
case 1:
print("One.")
fallthrough
default:
print("Done.")
}
This switch
statement will match case 3
and cascade through the rest of the cases, producing:
Three.
Two.
One.
Done.
But matching case 1
will skip the first two matches, producing:
One.
Done.
Anything outside of the 1-3 range will just produce the default output Done.
Be careful of your switch
statement construction; coming from a C or Objective-C background can take a bit of adjustment from continuously breaking from switch
cases, while not having that experience can create an issue if you aren’t aware of what fallthrough
can and cannot do.
Related Refcard:
Opinions expressed by DZone contributors are their own.
Comments