R: Create an R-level task callback manager

taskCallbackManager {base} | R Documentation

## Create an R-level task callback manager

### Usage

    taskCallbackManager(handlers = list(), registered = FALSE, verbose = FALSE)

### Arguments

`handlers` | this can be a list of callbacks in which each element is a list with an element named `"f"` which is a callback function, and an optional element named `"data"` which is the 5-th argument to be supplied to the callback when it is invoked. Typically this argument is not specified, and one uses `add` to register callbacks after the manager is created.
`registered` | a logical value indicating whether the `evaluate` function has already been registered with the internal task callback mechanism. This is usually `FALSE` and the first time a callback is added via the `add` function, the `evaluate` function is automatically registered. One can control when the function is registered by specifying `TRUE` for this argument and calling `addTaskCallback` manually.
`verbose` | a logical value, which if `TRUE`, causes information to be printed to the console about certain activities this dispatch manager performs. This is useful for debugging callbacks and the handler itself.

### Value

A list containing 6 functions:

`add` | register a callback with this manager, giving the function, an optional 5-th argument, an optional name by which the the callback is stored in the list, and a `register` argument which controls whether the `evaluate` function is registered with the internal C-level dispatch mechanism if necessary.
`remove` | remove an element from the manager's collection of callbacks, either by name or position/index.
`evaluate` | the `real' callback function that is registered with the C-level dispatch mechanism and which invokes each of the R-leve callbacks within this manager's control.
`suspend` | a function to set the suspend state of the manager. If it is suspended, none of the callbacks will be invoked when a task is completed. One sets the state by specifying a logical value for the `status` argument.
`register` | a function to register the `evaluate` function with the internal C-level dispatch mechanism. This is done automatically by the `add` function, but can be called manually.
`callbacks` | returns the list of callbacks being maintained by this manager.

### Note

This is an experimental feature and the interface may be changed in the future.

### See Also

`addTaskCallback` `removeTaskCallback` `getTaskCallbackNames` http://developer.r-project.org/TaskHandlers.pdf

### Examples

       # create the manager
      h <- taskCallbackManager()

       # add a callback
      h$add(function(expr, value, ok, visible) {
                             cat("In handler\n")
                             return(TRUE)
                           }, name = "simpleHandler")

       # look at the internal callbacks.
      getTaskCallbackNames()

       # look at the R-level callbacks
      names(h$callback())

       #
      getTaskCallbackNames()
      removeTaskCallback("R-taskCallbackManager")

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