Inside the "present" tool

Andrew Gerrand
Google
@enneff
adg@golang.org
https://go.dev


* The Playground API

The API used by the Playground (and the Tour) is a simple HTTP POST request
that returns a JSON-encoded response.

Request:

	POST /compile HTTP/1.1
	Host:play.golang.org
	Content-Length:113
	Content-Type:application/x-www-form-urlencoded; charset=UTF-8
	
	body=package+main%0A%0Aimport+%22fmt%22%0A%0Afunc+main()+%7B%0A%09fmt.Println(%22Hello%2C+playground%22)%0A%7D%0A

Response body:

	{"compile_errors":"","output":"Hello, playground\n"}


* Playground drawbacks

The compile service has no concept of time. (Necessary to limit resource use.)

The API reflects this; output is sent in one blob, not streamed.

Even when running locally, the API is bad for demonstrating code that uses time.

Rob needed to use time in his _Go_Concurrency_Patterns_ talk.


* Enter WebSockets

WebSockets are a bi-directional communication channel between a JavaScript program running in a web browser and a web server. They are part of HTML 5. 

The `websocket` package in Go's `go.net` sub-repository provides a WebSocket client and server.

I thought I could use WebSockets to stream program output to a running
presentation.

And thus the `present` tool was born.


* Hello, WebSocket

.code insidepresent/websocket.js
.play insidepresent/websocket.go


* Messages

The client (browser) and server (present) communicate with JSON-encoded messages.

.code insidepresent/socket.go /Message is/,/^}/

Go's `encoding/json` format can convert these `Message` values to and from JSON.

Go:

	Message{Id: "0", Kind: "run", Body: `package main; func main() { print("hello"); }`}

JSON:

	{"Id":"0","Kind":"run","Body":"package main; func main() { print(\"hello\"); }"}


* On the wire

.play insidepresent/hello.go

.html insidepresent/wire.html


* Implementation

* socketHandler (1/3)

First, register the handler with the `net/http` package:

	http.Handle("/socket", websocket.Handler(socketHandler))

Implementation:

.code insidepresent/socket.go /func socketHandler/,/errc/

* socketHandler (2/3)

.code insidepresent/socket.go /Decode messages/,/END/

* socketHandler (3/3)

.code insidepresent/socket-simple.go /Start and kill/,/^}/


* Process

.code insidepresent/socket.go /Process represents/,/^}/

* StartProcess

.code insidepresent/socket.go /StartProcess builds/,/^}/

* Process.start (1/2)

.code insidepresent/socket.go /start builds/,/END/

* Process.start (2/2)

.code insidepresent/socket.go /build x\.go/,/^}/

* Process.cmd

.code insidepresent/socket.go /cmd builds/,/^}/
.code insidepresent/socket.go /messageWriter is/,/END/

* Process.wait and Process.end

.code insidepresent/socket.go /wait waits/,/^}/
.code insidepresent/socket.go /end sends/,/^}/

* Process.Kill

.code insidepresent/socket.go /Kill stops/,/^}/


* One more thing

* Limiting output (1/2)

.code insidepresent/socket.go /switch m\.Kind/,/^			}/

* Limiting output (2/2)

.code insidepresent/socket.go /limiter returns/,/^}/