Test Delegate

import XCTest

class MyViewModel {
    weak var delegate: MyDelegateProtocol?
    func viewModelFunc() {
        delegate?.myFunc()
    }
}

/// Protocol defined in our app.
protocol MyDelegateProtocol: class {
    func myFunc()
}

/// Class, implementing our protocol, defined in the test case.
class TestMyDelegate: MyDelegateProtocol {
    var myFuncHelper: Bool = false
    func myFunc() {
        myFuncHelper = true
    }
}

/// ViewModel we want to test.
class TestMyViewModel: XCTestCase {
    func testDelegate() {
        let viewModel = MyViewModel()
        let delegate = TestMyDelegate()
        viewModel.delegate = delegate

        XCTAssertFalse(delegate.myFuncHelper)
        viewModel.viewModelFunc()
        XCTAssertTrue(delegate.myFuncHelper)
    }
}