A stub replaces a dependency with a small implementation that returns predetermined results. Use one when the code under test needs input or a service, but the real dependency is unavailable or would make a unit test cumbersome. section 6.5 section 6.5.5
Supply player input without a player
A game’s `ask` method reads from a `bufio.Reader`. Passing `strings.NewReader(tc.input)` to the game constructor supplies test input instead of waiting for someone at the terminal. Test cases include five-rune guesses in several scripts and an invalid guess followed by a valid one. section 5.1.2
For a service dependency, define the smallest interface the caller needs, then implement it in the test. A currency converter needs only `FetchExchangeRate`; its stub can return a chosen rate or error. This also avoids passing `nil`, which causes a panic when the converter calls the dependency. section 6.5.5
Listing 6.52 convert_test.go: Stubbing the dependency
// stubRate is a very simple stub for the exchangeRates.
type stubRate struct {
rate money.ExchangeRate
err error
}
// FetchExchangeRate implements the interface exchangeRates with the same
// signature but fields are unused for tests purposes.
func (m stubRate) FetchExchangeRate(
_, _ money.Currency, #1
) (money.ExchangeRate, error) {
return m.rate, m.err
}
The exchange-rate stub stores a rate and an error, then returns them from its interface method; the currency arguments are unused. section 6.5.5
Keep an interface narrow at an HTTP handler boundary, too. A game-creation handler needs only a repository method that adds a game. A one-method stub lets its test call the handler without connecting to an external database. section 8.4.3
Listing 8.31 newgame/handler_test.go: Stubbing the repository
func TestHandle(t *testing.T) {
handleFunc := Handler(gameAdderStub{}) #1
req, err := ...
// ...
handleFunc(recorder, req) #2
assert...
}
type gameAdderStub struct { #3
err error
}
func (g gameAdderStub) Add(_ session.Game) error {
return g.err
}
The handler test passes `gameAdderStub{}` to `Handler`; the stub’s `Add` method returns its configured error. section 8.4.3
A stub supplies behavior but does not verify how it was called. When a test must check call counts or arguments, use a mock instead; generated mocks are an option when hand-written test doubles become less convenient. section 6.5.5 section 10.4
