How to unit test asynchronous APIs?

Question

I have installed Google Toolbox for Mac into Xcode and followed the instructions to set up unit testing found here.

It all works great, and I can test my synchronous

Answer 1

I appreciate that this question was asked and answered almost a year ago, but I can't help but disagree with the given answers. Testing asynchronous operations, particularly network operations, is a very common requirement, and is important to get right. In the given example, if you depend on actual network responses you lose some of the important value of your tests. Specifically, your tests become dependent on the availability and functional correctness of the server you're communicating with; this dependency makes your tests

• more fragile (what happens if the server goes down?)
• less comprehensive (how do you consistently test a failure response, or network error?)
• significantly slower imagine testing this:

Unit tests should run in fractions of a second. If you have to wait for a multi-second network response each time you run your tests then you're less likely to run them frequently.

Unit testing is largely about encapsulating dependencies; from the point of view of your code under test, two things happen:

1. Your method initiates a network request, probably by instantiating an NSURLConnection.
2. The delegate you specified receives a response via certain method calls.

Your delegate doesn't, or shouldn't, care where the response came from, whether from an actual response from a remote server or from your test code. You can take advantage of this to test asynchronous operations by simply generating the responses yourself. Your tests will run much faster, and you can reliably test success or failure responses.

This isn't to say you shouldn't run tests against the real web service you're working with, but those are integration tests and belong in their own test suite. Failures in that suite may mean the web service has changes, or is simply down. Since they're more fragile, automating them tends to have less value than automating your unit tests.

Regarding how exactly to go about testing asynchronous responses to a network request, you have a couple options. You could simply test the delegate in isolation by calling the methods directly (e.g. [someDelegate connection:connection didReceiveResponse:someResponse]). This will work somewhat, but is slightly wrong. The delegate your object provides may be just one of multiple objects in the delegate chain for a specific NSURLConnection object; if you call your delegate's methods directly you may be missing some key piece of functionality provided by another delegate further up the chain. As a better alternative, you can stub the NSURLConnection object you create and have it send the response messages to its entire delegate chain. There are libraries that will reopen NSURLConnection (amongst other classes) and do this for you. For example: https://github.com/pivotal/PivotalCoreKit/blob/master/SpecHelperLib/Extensions/NSURLConnection%2BSpec.m