How do I properly test a var that changes through a publisher in my viewModel in XCTestCase

The reason is that view model asynchronous but test is synchronous...

$username
  .receive(on: RunLoop.main)

... the .receive operator here makes final assignment of isValid on the next event cycle of RunLoop.main

but the test

model.username = "1234"
XCTAssertTrue(model.isValid) //<----- THIS FAILS HERE

expects that isValid will be changed immediately.

So there are following possible solutions:

1. remove .receive operator at all (in this case it is preferable, because it is UI workflow, which is anyway always on main runloop, so using scheduled receive is redundant.

   $username
       .removeDuplicates()
       .map { input in
           print("~~~> \(input.count >= 3)")
           return input.count >= 3
       }
   .assign(to: \.isValid, on: self)
   .store(in: &disposables)
   

Result:

model.username = "1234"
XCTAssertTrue(model.isValid) // << PASSED

2. make UT wait for one event and only then test isValid (in this case it should be documented that isValid has asynchronous nature by intention)

   model.username = "1234"
   RunLoop.main.run(mode: .default, before: .distantPast) // << wait one event
   XCTAssertTrue(model.isValid) // << PASSED
   

As @Asperi said: the reason of this mistake is that you receive values asynchronous. I searched a little and found Apple's tutorial about XCTestExpectation usage. So I tried to use it with your code and the tests passed successfully. The other way is to use Combine Expectations.

class StackoverFlowQuestionTests: XCTestCase {

    var model = UserViewModel()

    override func setUp() {
        model = UserViewModel()
    }

    func testIsValid() throws {

        let expectation = self.expectation(description: "waiting validation")

        let subscriber = model.$isValid.sink { _ in
            guard self.model.username != "" else { return }
            expectation.fulfill()
        }

        model.username = "1234"
        wait(for: [expectation], timeout: 1)
        XCTAssertTrue(model.isValid)

    }

    func testIsNotValid() {

        let expectation = self.expectation(description: "waiting validation")

        let subscriber = model.$isValid.sink { _ in
            guard self.model.username != "" else { return }
            expectation.fulfill()
        }

        model.username = "1"
        wait(for: [expectation], timeout: 1)
        XCTAssertFalse(model.isValid)

    }
}

UPDATE I add all the code and output for clarity. I changed testing validation like in your example (where you test both "1" and "1234" options). And you'll see, that I just copy-paste your model (except name and public for variables and init()). But still, I don't have this mistake:

Asynchronous wait failed: Exceeded timeout of 1 seconds, with unfulfilled expectations: "waiting validation".

// MARK: TestableCombineModel.swift file
import Foundation
import Combine

public class TestableModel: ObservableObject {

    @Published public var username = ""
    @Published public var isValid = false
    private var disposables = Set<AnyCancellable>()

    public init() {
        $username
            .receive(on: RunLoop.main) // as you see, I didn't delete it
            .removeDuplicates()
            .map { input in
                print("~~~> \(input.count >= 3)")
                return input.count >= 3
        }
        .assign(to: \.isValid, on: self)
        .store(in: &disposables)
    }

}

// MARK: stackoverflowanswerTests.swift file:
import XCTest
import stackoverflowanswer
import Combine

class stackoverflowanswerTests: XCTestCase {

    var model: TestableModel!

    override func setUp() {
        model = TestableModel()
    }

    func testValidation() throws {

        let expectationSuccessfulValidation = self.expectation(description: "waiting successful validation")
        let expectationFailedValidation = self.expectation(description: "waiting failed validation")

        let subscriber = model.$isValid.sink { _ in
            // look at the output. at the first time there will be "nothing"
            print(self.model.username == "" ? "nothing" : self.model.username)
            if self.model.username == "1234" {
                expectationSuccessfulValidation.fulfill()
            } else if self.model.username == "1" {
                expectationFailedValidation.fulfill()
            }

        }

        model.username = "1234"
        wait(for: [expectationSuccessfulValidation], timeout: 1)
        XCTAssertTrue(model.isValid)

        model.username = "1"
        wait(for: [expectationFailedValidation], timeout: 1)
        XCTAssertFalse(model.isValid)

    }

}

and here is the output

2020-01-14 09:16:41.207649+0600 stackoverflowanswer[1266:46298] Launching with XCTest injected. Preparing to run tests.
2020-01-14 09:16:41.389610+0600 stackoverflowanswer[1266:46298] Waiting to run tests until the app finishes launching.
Test Suite 'All tests' started at 2020-01-14 09:16:41.711
Test Suite 'stackoverflowanswerTests.xctest' started at 2020-01-14 09:16:41.712
Test Suite 'stackoverflowanswerTests' started at 2020-01-14 09:16:41.712
Test Case '-[stackoverflowanswerTests.stackoverflowanswerTests testValidation]' started.
nothing
~~~> true
1234
~~~> false
1
Test Case '-[stackoverflowanswerTests.stackoverflowanswerTests testValidation]' passed (0.004 seconds).
Test Suite 'stackoverflowanswerTests' passed at 2020-01-14 09:16:41.717.
     Executed 1 test, with 0 failures (0 unexpected) in 0.004 (0.005) seconds
Test Suite 'stackoverflowanswerTests.xctest' passed at 2020-01-14 09:16:41.717.
     Executed 1 test, with 0 failures (0 unexpected) in 0.004 (0.005) seconds
Test Suite 'All tests' passed at 2020-01-14 09:16:41.718.
     Executed 1 test, with 0 failures (0 unexpected) in 0.004 (0.006) seconds

UPDATE 2 Actually I do catch mistakes of "Asynchronous wait failed: ..." if I changed this line of code:

let subscriber = model.$isValid.sink { _ in

to this, as Xcode propose:

model.$isValid.sink { _ in // remove "let subscriber ="