How to capture stdout & stderr in Swift XCTest unit tests? - swift

Question

I’m writing a command-line tool in Swift. In my unit tests, I want to test the code’s output to stdout (and, secondarily, to stderr).
The examples I’ve found of using a Pipe() to copy or capture stdout output mostly rely on spawning a sub-process and assigning its stdout—and I don’t know where to begin figuring out how to deal with that in XCTest. But I have managed to pull together something that sort of begins to work without spawning a separate process.
What I’ve come up with so far is compiling and running, and with an ugly hack does sort of what I need—but just for one use (if I try to use it across multiple tests in the same run, all but the first fail). The hack is that, after the calls that write to stdout, I have to add a call to write to stderr to have the data already sent to stdout then be written to my capturedStdout variable.
It seems there ought to be a more appropriate way to get this to work.
Here’s what I’ve got so far:
import XCTest
final class StdoutCaptureTests: XCTestCase {
func testCapturePrint() {
let pipe = Pipe()
var capturedStdout = ""
// Setup to copy data going to stdout into capturedStdout.
setvbuf(stdout, nil, _IONBF, 0)
// Assign stdout’s pointer to the pipe:
dup2(pipe.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
// A closure for receiving the data sent to stdout:
pipe.fileHandleForReading.readabilityHandler = { handle in
if let str = String(data: handle.availableData, encoding: .utf8) {
capturedStdout += str
}
}
// The subject of the test outputs to stdout:
// (In this simplified example, just print "test" to stdout.)
print("test")
// Here’s where it gets weird...
// If none of the following lines are uncommented,
// the XCTAssertEqual test below will fail
// (nothing gets written to capturedStdout).
// Uncommenting the following line makes the XCTAssertEqual work:
//XCTAssert(false) // ⚠️
// (but that means the test is considered failed.)
// And this has no effect:
//XCTAssert(true) // 🛑
// fflush has no effect (when it gets nil as argument here,
// the man(ual) page says it “flushes all open streams.”)
//fflush(nil) // 🛑
// But this works and lets the test pass:
//FileHandle.standardError.write("".data(using: .utf8)!) // ✅
XCTAssertEqual(capturedStdout, "test\n")
// When the above commented lines are still commented:
// 🛑 XCTAssertEqual failed: ("") is not equal to ("test")
}
}
Given that it fails when used in subsequent test methods in the same run, I’m thinking there’s probably some cleanup I should be doing at the end of each test to restore stdout before the next test method is run.
Some of the references I’ve found:
using pipe() in Swift App to redirect stdout into a textView (only runs in simulator, not native)
Redirect Process stdout to Apple System Log Facility in Swift
https://www.hackingwithswift.com/forums/ios/redirecting-output-to-a-log-file-when-not-attached-to-debugger/5766