NSURLSessionTaskMetrics with custom NSURLProtocol

Question

Hi,

I would like to create a custom network library by implementing and regestering a custom NSURLProtocol. The library should be able to populate NSURLSessionTaskMetrics and return it to the client using the standard URLSession:task:didFinishCollectingMetrics: callback method of the NSURLSessionTaskDelegate class. I would expect NSURLProtocolClient to have a method that would allow passing the metrics to the client; however I wasn't able to find one. What would be the best approach to implement that kind of functionallity?

Thank you!

Network

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Posted by

Andrei G.

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Answer 1

I would expect NSURLProtocolClient to have a method that would allow passing the metrics to the client

Indeed.

however I wasn't able to find one.

That’s correct.

What would be the best approach to implement that kind of functionallity?

I don’t think you’ll be able to do this. CFNetwork’s support for custom protocols (that is, subclasses of

NSURLProtocol

) is rather lacking. It’s not completely useless — there are scenarios where a custom protocol is the best way to get something done — but there’s no way that you can make a custom protocol that does all the stuff that the built-in HTTP[S] protocol does.

You should feel free to file an enhancement request for the features you need, making sure to describe your use case. However, I have to set expectations here.

NSURLProtocolClient

has not been updated to account for features that we added in Mac OS X 10.6 (!) [1], so it’s clearly not a high priority.

If you do file an ER, please post your bug number here, just for the record.

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

[1] Namely, an entry point that that’s equivalent to the

-connection:canAuthenticateAgainstProtectionSpace:

delegate callback.

Posted by

eskimo

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Answer 2

I would expect NSURLProtocolClient to have a method that would allow passing the metrics to the client

Indeed.

however I wasn't able to find one.

That’s correct.

What would be the best approach to implement that kind of functionallity?

I don’t think you’ll be able to do this. CFNetwork’s support for custom protocols (that is, subclasses of

NSURLProtocol

) is rather lacking. It’s not completely useless — there are scenarios where a custom protocol is the best way to get something done — but there’s no way that you can make a custom protocol that does all the stuff that the built-in HTTP[S] protocol does.

You should feel free to file an enhancement request for the features you need, making sure to describe your use case. However, I have to set expectations here.

NSURLProtocolClient

has not been updated to account for features that we added in Mac OS X 10.6 (!) [1], so it’s clearly not a high priority.

If you do file an ER, please post your bug number here, just for the record.

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

[1] Namely, an entry point that that’s equivalent to the

-connection:canAuthenticateAgainstProtectionSpace:

delegate callback.

Posted by

eskimo

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Answer 3

@eskimo, thanks for the answer. I have filed an enhancement request 35620971.

Would it be possible to implement a work-around using method-swizzling by swizzling [NSURLSession sessionWithConfiguration:delegate:delegateQueue:] method and replacing the |delegate| parameter with a proxy that would forward all method invocations to the original delegate without any changes, except for didFinishCollectingMetrics that would substitute |metrics| with the one created by NSURLProtocol.

Posted by

Andrei G.

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Answer 4

I have filed an enhancement request 35620971.

Thanks for that.

Would it be possible … ?

Possible, maybe. A good idea, definitely not. From a binary compatibility standpoint you should stay away from swizzling methods on classes that you don’t own. This is especially true for

NSURLSession

, which already plays games like this internally (notice how, when you

po

an

NSURLSession

instance, the actual type is printed as

__NSURLSessionLocal

or something else that’s not visible to you).

Why not have your protocol provide its own API that allows clients to register for a task metrics callback?

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Posted by

eskimo

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NSURLSessionTaskMetrics with custom NSURLProtocol

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