How do you write a mock in Swift? - ios

Question

Where do I put the mocked code? Do I need to write everything again? Do I change the original code?

Answer 1

Short answer: using protocols.
If your injectable object is final, a struct, or an enum, it isn't even possible to override it to mock. Instead of using a concrete type as your dependency, use a protocol and conform your implementation to it. In addition to allowing "mocking" regardless of the real type (class, struct, enum), this lists the public interface all in one place, uninterrupted by the implementations. It also forces you to think about what needs to be part of the non-private interface.
With retroactive protocol conformance (i.e. in extensions), you can even use this to mock system classes like, e.g., CBCentralManager or CLLocationManager.
Example:
Not easily mockable:
struct Foo {
let id: Int
let data: Int
}
final class FooManager {
var x: Int
func getFoo(id: Int) -> Foo {
return Foo(id: id, data: x)
}
}
class FooUser {
let fooManager: FooManager
init(fooManager: FooManager) {
self.fooManager = fooManager
}
func getData() -> Int {
return fooManager.getFoo(id: 3).data
}
}
Trivially mockable:
struct Foo {
let id: Int
let data: Int
}
// Easy to see what's visible.
protocol FooManager {
func getFoo(id: Int) -> Foo
}
final class RealFooManager: FooManager {
private var x: Int
func getFoo(id: Int) -> Foo {
return Foo(id: id, data: x)
}
}
class FooUser {
let fooManager: FooManager
init(fooManager: FooManager) {
self.fooManager = fooManager
}
func getData() -> Int {
return fooManager.getFoo(id: 3).data
}
}
// In test target.
class MockFooManager: FooManager {
var requestedId: Int?
var data: Int = 17
func getFoo(id: Int) -> Foo {
requestedId = id
return Foo(id, data: data)
}
}
class FooUserTests {
func testFooUserGetData() {
let mock = MockFooManager()
let user = FooUser(fooManager: mock)
let data = user.getData()
XCTAssertEqual(data, mock.data)
XCTAssertEqual(mock.requestedId, 3)
}
}