Property change notification in go

Question

How can you signal 'property' changes to multiple receivers in go?

Similar to how you would define a property in Qt with a notify signal.

Eg if you imagine having some value that needs to be shown in multiple ways, like a progress value that could be shown both as a progress bar and as textual %, where both would need to update when the underlying value changes.

Answer 1

One way could to be to utilize channels .

Your central code which manages/changes the property or variable that needs to be listened may provide a GetChan() function which returns a channel on which modifications (eg new values) will be broadcasted:

// The variable or property that is listened:
var i int

// Slice of all listeners
var listeners []chan int

func GetChan() chan int {
    listener := make(chan int, 5)
    listeners = append(listeners, listener)
    return listener
}

Whenever you change the variable/property, you need to broadcast the change:

func Set(newi int) {
    i = newi
    for _, ch := range listeners {
        ch <- i
    }
}

And listeners need to "listen" for change events, which can be done by a for range loop on the channel returned by GetChan() :

func Background(name string, ch chan int, done chan int) {
    for v := range ch {
        fmt.Printf("[%s] value changed: %d\n", name, v)
    }
    done <- 0
}

Here is the main program:

func main() {
    l1 := GetChan()
    l2 := GetChan()

    done := make(chan int)

    go Background("B1", l1, done)
    go Background("B2", l2, done)

    Set(3)
    time.Sleep(time.Second) // Wait a little
    Set(5)

    // Close all listeners:
    for _, listener := range listeners {
        close(listener)
    }

    // Wait 2 background threads to finish:
    <-done
    <-done
}

And its output:

[B1] value changed: 3
[B2] value changed: 3
[B1] value changed: 5
[B2] value changed: 5

You can try the complete program on the Go Playground .

You may also implement a "broker" which realizes a subscriber model and allows broadcasting messages. See How to broadcast message using channel .