Context <- setRefClass(
  'Context',
  fields = list(
    id = 'character',
    .invalidated = 'logical',
    .callbacks = 'list'
  ),
  methods = list(
    initialize = function() {
      id <<- .getReactiveEnvironment()$nextId()
      .invalidated <<- F
      .callbacks <<- list()
    },
    run = function(func) {
      "Run the provided function under this context."
      env <- .getReactiveEnvironment()
      env$runWith(.self, func)
    },
    invalidate = function() {
      "Schedule this context for invalidation. It will not actually be
        invalidated until the next call to \\code{\\link{flushReact}}."
      if (.invalidated)
        return()
      .invalidated <<- T
      .getReactiveEnvironment()$addPendingInvalidate(.self)
      NULL
    },
    onInvalidate = function(func) {
      "Register a function to be called when this context is invalidated.
        If this context is already invalidated, the function is called
        immediately."
      if (.invalidated)
        func()
      else
        .callbacks <<- c(.callbacks, func)
      NULL
    },
    executeCallbacks = function() {
      "For internal use only."
      lapply(.callbacks, function(func) {
        tryCatch({
          func()
        }, warning = function(e) {
          # TODO: Callbacks in app
          print(e)
        }, error = function(e) {
          # TODO: Callbacks in app
          print(e)
        })
      })
    }
  )
)

ReactiveEnvironment <- setRefClass(
  'ReactiveEnvironment',
  fields = c('.currentContext', '.nextId', '.pendingInvalidate'),
  methods = list(
    initialize = function() {
      .currentContext <<- NULL
      .nextId <<- 0L
      .pendingInvalidate <<- list()
    },
    nextId = function() {
      .nextId <<- .nextId + 1L
      return(as.character(.nextId))
    },
    currentContext = function() {
      if (is.null(.currentContext))
        stop('No reactive context is active')
      return(.currentContext)
    },
    runWith = function(ctx, func) {
      old.ctx <- .currentContext
      .currentContext <<- ctx
      on.exit(.currentContext <<- old.ctx)
      func()
    },
    addPendingInvalidate = function(ctx) {
      .pendingInvalidate <<- c(.pendingInvalidate, ctx)
    },
    flush = function() {
      while (length(.pendingInvalidate) > 0) {
        contexts <- .pendingInvalidate
        .pendingInvalidate <<- list()
        lapply(contexts, function(ctx) {
          ctx$executeCallbacks()
          NULL
        })
      }
    }
  )
)

.getReactiveEnvironment <- function() {
  if (!exists('.ReactiveEnvironment', envir=.GlobalEnv, inherits=F)) {
    assign('.ReactiveEnvironment', ReactiveEnvironment$new(), envir=.GlobalEnv)
  }
  get('.ReactiveEnvironment', envir=.GlobalEnv, inherits=F)
}

#' Causes any pending invalidations to run.
flushReact <- function() {
  .getReactiveEnvironment()$flush()
}

#' Retrieves the current reactive context, or errors if there is no reactive
#' context active at the moment.
getCurrentContext <- function() {
  .getReactiveEnvironment()$currentContext()
}