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WebSocket

Many of the new possibilities brought by the sketch package rely on the use of WebSocket. WebSocket is used to create a new graphics device for real-time plotting in R; it is also used to create an R-scriptable environment so that one can control a user-defined browser session with R commands.

To use WebSocket in your sketch application, you have to include #! load_library("websocket") in the header of the file, then follow the steps below.

1. Start a WebSocket server#

Start the WebSocket server with:

conn <- websocket$new()
conn$startServer()

2. Start your sketch application#

Start your sketch application with the source_r call. For illustration purposes, we will use a sample app provided by the new_app() method:

conn$new_app(debug = TRUE)

When the connection is established, a message will show at the R console.

Screenshot#

3a. Control the browser#

Now you can control the browser session using the conn$ws$send function. The message to be sent to the browser must be a JSON string (to be processed by your application). In the sample app, by default it can accept a string of the form {"type": "command", "message": "YOUR COMMAND"}. For example, to ask the browser to print the "Hello World!" string, use

conn$ws$send('{"type": "command", "message": "console.log(\'Hello World!\')"}')

Alternatively, it is more convenient to wrap the above into a function call.

send_command <- function(x) {
message <- list(type = "command", message = x)
conn$ws$send(jsonlite::toJSON(message, auto_unbox = TRUE))
}
send_command('console.log("Hello World!")')

Screenshot#

3b. Control the browser using sketch mode#

In the above, the command sent to the browser was written in JavaScript. Since sketch is an R-to-JavaScript transpiler, it is possible to write R commands instead. This functionality is provided by conn$sketch_mode() which transpiles R commands into JavaScript ones and sends them out to the browser session.

Screenshot#

4. Stop the WebSocket server#

When you finish with the session, use conn$stopServer() to close the connection.

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