Closure forwarding without evaluating - swift

Question

I'm writing a logging function that also calls a CocoaLumberjack function (yes, I know about custom loggers and chose not to use it). My function is using a forward, but I have a question about how the 'forwarding' works.
Here's my functions:
public func MyLogDebug(_ message: #autoclosure () -> String) {
// some code
MyLogMessage(message(), .debug)
}
public func MyLogMessage(_ message: #autoclosure () -> String, flag: DDLogFlag) {
// some code
if(myLogLevel.rawValue & flag.rawValue != 0) {
DDLogDebug(message())
}
}
My question is about the MyLogMessage(message(), .debug) function call. I know that avoiding string concatenation is really helpful for performance with logs, and I can see that inside MyLogMessage the string closure is only ran if the log level is passed. However, it look like in MyLogDebug the closure is also being evaluated.
I don't want to run the closure in MyLogDebug, so I tried to change it to MyLogMessage(message, .debug), but xcode gives me an error: Add () to forward #autoclosure parameter.
Will my code above evaluate the closure inside MyLogDebug? If so, is there another way to forward the closure parameter without evaluating it?

Answer 1

Will my code above evaluate the closure in MyLogDebug?
No. The call message() will be wrapped in a closure by the #autoclosure in MyLogMessage and only evaluated when message() is subsequently called in MyLogMessage.
Here is a little standalone example to play with:
func DDLogDebug(_ message: String) {
print("in DDLogDebug")
print(message)
print("leaving DDLogDebug")
}
public func MyLogDebug(_ message: #autoclosure () -> String) {
// some code
print("in MyLogDebug")
MyLogMessage(message(), flag: true)
print("leaving MyLogDebug")
}
public func MyLogMessage(_ message: #autoclosure () -> String, flag: Bool) {
// some code
print("in MyLogMessage")
if (flag) {
DDLogDebug(message())
}
print("leaving MyLogMessage")
}
MyLogDebug({ print("evaluated"); return "error message" }())
Output:
in MyLogDebug
in MyLogMessage
evaluated
in DDLogDebug
error message
leaving DDLogDebug
leaving MyLogMessage
leaving MyLogDebug
Note that the initial closure passed to MyLogDebug isn't evaluated until the DDLogDebug(message()) in MyLogMessage().
If you change the flag to false in the call to MyLogMessage(), the initial closure will never be evaluated.