override.go

ackage main

import "fmt"
// assume this is the original implementation of the callback:
/*
// by default the callback say hello only to jane
PrintCallback(name string) bool {
	if name == "jane" {
		return true
	}
	return false
}
*/
// and this function then use the callback
func PrintHello(name string) {
	if PrintCallback(name) {
		fmt.Println("hello world: " + name)
	} else {
		fmt.Println("not going to say hello to: " + name)
	}
}

// now, use a closure instead of a regular function to allow override
var PrintCallback = func(name string) bool {
	if name == "jane" {
		return true
	} 
	return false
}

// this is added for testing purposes
// this callback never says hello
func MockPrintCallback(name string) bool {
	return false
}

func main() {
	fmt.Println("say hello to jane")
	PrintHello("jane")
	fmt.Println("don't say hello to john")
	PrintHello("john")
	// now inject the test method
	PrintCallback = MockPrintCallback
	fmt.Println("won't say hello to jane either")
	PrintHello("jane")
}