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Simple AJAX system for Shiny apps (JS -> R -> JS communication)

Sometimes it's useful to be able to call an R function from JavaScript and use the return value from R back in JavaScript. This sort of communication is usually done with AJAX in JavaScript. This app shows how to implement a simple and ultra lightweight AJAX-like system in Shiny, to be able to call functions in R.

Using this system is easy:
1. Include api.js in your Shiny app's UI
2. Include api.R in your Shiny app's server
3. In your R code: define the functions that you want to be able to call from JavaScript. Each function must have a name beginning with api. and take a single argument which is a list of the parameters passed from JavaScript. This function should return a list (can be an empty list), which will be passed to the callback function in JavaScript (if a callback was provided).
4. In your JavaScript code: make a call to, where params is an object that contains all the parameters as key-value pairs. You must specify the _method parameter -- this is the R function that will get called. For example, if in R you defined a function api.myfunc then you can call it in JavaScript with'_method' : 'myfunc') (notice that the prefix api. is ommitted). You can optionally provide _callback and _failureCallback functions which can be called when the R function completes or when it throws an error. You can also add any other arbitrary parameters, and they will be available to you in the R function.

This may sound a bit complicated, but if you take a look at the source code of the sample app you'll see it's actually pretty simple to use.