[swift-evolution] The great renaming and the state of new Unified Logging in Swift

Ben Rimmington me at benrimmington.com
Sat Sep 3 13:06:18 CDT 2016

> On 3 Sep 2016, at 04:24, Brandon Knope wrote:
> I am unsure where to post this…but iOS 10 is introducing a new API for activity tracing and logging.
> The API has been “Swift-ified” a little, but it still is a little awkward to use:
> os_log("Sender: %{public}@", log: ViewController.ui_log, type: .debug, sender)
> (This is taken from: https://developer.apple.com/library/prerelease/content/samplecode/Logging/Introduction/Intro.html#//apple_ref/doc/uid/TP40017510 <https://developer.apple.com/library/prerelease/content/samplecode/Logging/Introduction/Intro.html#//apple_ref/doc/uid/TP40017510>)
> Note: the {public} modifier above does not work in swift but does in Objective-C. I have filed a radar for this.

Activity Tracing (2014) and Unified Logging (2016) APIs are complex preprocessor macros, which are not imported into Swift.



The os_log_with_type macro in <os/log.h> uses __builtin_os_log_format to encode its arguments into a byte array. The overlay for Swift instead uses its _os_log_encode C++ function, which only recognizes the {private} modifier.



> A few things:
> • This looks like how the Dispatch APIs use to look: very C-like which was made to be much better in The Great Renaming
> • Cannot use Swift’s string interpolation, resulting in the arguments being passed in at the end
> For reference, this is what it looks like in Objective-C, showing that SOME renaming has occurred: 
> os_log_debug(ui_log, "Sender: %{public}@", sender);
> This might look more Swift-like:
> uiLog.log("Sender: %{public}\(sender)”, type: .debug)
> * Makes “log” a method on an OSLog object 
> • Uses string interpolation

The format string of os_log APIs is required to be constant. The overlay uses a StaticString, which isn't ExpressibleByStringInterpolation. SE-0137 also deprecated the protocol, so that it can be redesigned.


An instance method of OSLog might be better, except when using the `default` object.


Similarly, the Swift.print(_:separator:terminator:to:) function might be better as a TextOutputStream method.

-- Ben

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