promise() creates a new promise. A promise is a placeholder object for the
eventual result (or error) of an asynchronous operation. This function is not
generally needed to carry out asynchronous programming tasks; instead, it is
intended to be used mostly by package authors who want to write asynchronous
functions that return promises.
promise(action)A function with signature function(resolve, reject), or a
one-sided formula. See Details.
A promise object (see then).
The action function should be a piece of code that returns quickly, but
initiates a potentially long-running, asynchronous task. If/when the task
successfully completes, call resolve(value) where value is the result of
the computation (like the return value). If the task fails, call
reject(reason), where reason is either an error object, or a character
string.
It's important that asynchronous tasks kicked off from action be coded very
carefully--in particular, all errors must be caught and passed to reject().
Failure to do so will cause those errors to be lost, at best; and the caller
of the asynchronous task will never receive a response (the asynchronous
equivalent of a function call that never returns, i.e. hangs).
The return value of action will be ignored.
# Create a promise that resolves to a random value after 2 secs
p1 <- promise(function(resolve, reject) {
later::later(~resolve(runif(1)), delay = 2)
})
p1 %...>% print()
# Create a promise that errors immediately
p2 <- promise(~{
reject("An error has occurred")
})
then(p2,
onFulfilled = ~message("Success"),
onRejected = ~message("Failure")
)