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The Link Callback

Link callbacks let you present your users with embedded URLs they can click on. I use them to present my users with multiple-choice questions like this:

The callback function looks like this:

cb := broker.LinkCallback(`option1`)
cb := broker.LinkCallback(`option2`)

The formal definition is here in httpserver.go

Link callbacks are a lot different from the other types, and you have to know a little bit more about how lazlo works to use them effectively. The first thing you need to know, is that lazlo has a built-in HTTP server. This is implemented in lib/httpserver.go.

The next thing you need to know is the built-in HTTP server uses two environment variables to configure itself. The first of these is PORT, and the second is LAZLO_URL

The PORT environment variable dictates the TCP port lazlo listens on. Paas services like Heroku set this variable for you automatically when they run your app in a dyno, but if you run Lazlo on localhost, you need to set this or lazlo won't run.

The LAZLO_URL variable is an optional variable you need to set if you're using link callbacks. It specifies the public address of the server you're running Lazlo on. The linkCallback uses this value to create a clickable URL that lazlo will be able to see.

Now you know what you need to know

Ok, lets say you're running lazlo on an AWS instance, and you set PORT to 5000, and LAZLO_URL to 54.87.22.100, and then you create a link callback like so:

cb := broker.LinkCallback(`option1`)

Lazlo is going to create a local API endpoint at /option1, and give you back a struct of type lazlo.LinkCallback. It looks like this:

type LinkCallback struct {
   ID      string
   Path    string // the computed URL
   URL     string
   Handler func(res http.ResponseWriter, req *http.Request)
   Chan    chan *http.Request
}

ID uniquely identifies your callback (as you're probably used to by now)

PATH is the URL Lazlo intends for you to present your user. In our current example, PATH would equal this: http://54.87.22.100:5000/option1

URL is the URL you passed in to broker.LinkCallback()

Handler if you're familiar with pat, you can pass in your own http handler function as an optional second argument to broker.LinkCallback(). If that sounded like greek to you, no worries; ignore it.

Chan here's the magic part. Whenver a user visits the link specified by your callback's Path attribute, Lazlo will hand your module an http.Request that represents their click.

Wait, what?

;tldr, if you properly set up the PORT and LAZLO_URL environment variables, and Lazlo is reachable from your user's browser, then you can do this:

option1 := broker.LinkCallback(`option1`)
option2 := broker.LinkCallback(`option2`)

And lazlo will wire up http://localhost/option1 and http://localhost/option2 to your callback's Chan channel.

You can present those links to your users by wrapping them in slack compatible markdown like this:

options := fmt.Sprintf("choose <%s|option1> or <%s|option2>", option1.URL, option2.URL)
broker.Say(options)

Then you can block waiting for the user to click on an answer:

for {
	select {
	case choice := <-option1.Chan:
		broker.Say('you chose option1')
	case choice := <-option2.Chan:
		broker.Say('you chose option2')
	}
}