github.com/larrybotha/learn-go-with-tests/09-mocking

Dependency Injection

Table of Contents

• Takeaways
  • Go
  • Tests
• Resources

Takeaways

Go

• functions that return multiple values define those values in parenthesis:

  func MyFunc() (n int, s string, xs []int) {}
  

• to implement an interface on a struct, the method must be implemented with a receiver:

  type MyStruct struct {}
  
  // MyStruct implements Write on the io.Writer interface
  func (m MyStruct) Write(p byte[]) (n int, err error) {}
  

• like sleep in bash, Go has a time.Sleep method which accepts a value of type time.Duration:

  var sleepTime time.Duration = 5 * time.Second
  time.Sleep(sleepTime)
  

• one can use a struct to encapsulate configurations:

  type CustomSleeper struct {
    duration time.Duration
    sleep func(time.Duration)
  }
  
  func (c CustomSleeper) sleep() {
    c.sleep(c.duration)
  }
  
  func main() {
    sleeper := CustomSleeper{duration: 5, sleep: time.Sleep}
  }
  

• to implement an interface method, one requires a struct on which to implement the method

• an interface method can't be defined on an anonymous struct, as interface methods have to be defined using receivers, while functions on structs are equivalent to assigning a function to a variable

• one can either explicitly pass a value by its address to a function, or initialise the variable with the address:

  buffer := bytes.Buffer{}
  fmt.Fprint(&buffer, "foo")
  
  // or
  bufferAddress = &bytes.Buffer{}
  fmt.Fprint(bufferAddress, "foo")
  

• a single struct may implement multiple interfaces

Tests

• a simple mock can be implemented with a struct that mutates an internal property each time the interface it implemnts is called:

  type MySpy struct {
    Calls []string
  }
  
  // create a mock for time.Sleep
  func (m *MySpy) Sleep(duration time.Duration) {
    m.Calls = append(m.Calls, "called")
  }
  

Resources

• Uncle Bob's "When to mock"
• The little mocker
  • A conversation covering the differences between mocks, stubs, spies, and other test doubles