This page describes how to define contextors in the contextors library.
Contextors are constructed using createContextor. They require an array of
sources, each of which is a Contextor or a Contexto.Context object,
and a combining function, which returns data based on the current values
associated with those sources.
Typically your contextors would be defined at the module level, similar to React contexts.
A very simple contextor might depend on the value of a single context:
const BookCount =
createContextor(
[BooksContext], // A context defined somewhere
(books) => books.length // Operates on the local value of the context
);
More complex contextors can depend on the values of multiple contexts:
const CurrentBook =
createContextor(
[CurrentBookIdContext, BooksContext],
(currentBookId, books) =>
books.filter(book => book.id === currentBookId)[0]
);
Contextors can also depend on the values of other contextors:
const BookSummary =
createContextor(
[CurrentBook, AuthorsContext],
(book, authors) => ({
title: book.title,
author: authors.filter(book.authorId === author.id)
})
);