/**
 * @license
 * Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at
 * http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at
 * http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at
 * http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at
 * http://polymer.github.io/PATENTS.txt
 */

import {directive, Directive, NodePart} from '../lit-html.js';

/**
 * A directive that renders the items of an async iterable[1], replacing
 * previous values with new values, so that only one value is ever rendered
 * at a time.
 *
 * Async iterables are objects with a [Symbol.asyncIterator] method, which
 * returns an iterator who's `next()` method returns a Promise. When a new
 * value is available, the Promise resolves and the value is rendered to the
 * Part controlled by the directive. If another value other than this
 * directive has been set on the Part, the iterable will no longer be listened
 * to and new values won't be written to the Part.
 *
 * [1]: https://github.com/tc39/proposal-async-iteration
 *
 * @param value An async iterable
 * @param mapper An optional function that maps from (value, index) to another
 *     value. Useful for generating templates for each item in the iterable.
 */
export const asyncReplace =
    <T>(value: AsyncIterable<T>, mapper?: (v: T, index?: number) => any):
        Directive<NodePart> => directive(async (part: NodePart) => {
          // If we've already set up this particular iterable, we don't need
          // to do anything.
          if (value === part.value) {
            return;
          }

          // We nest a new part to keep track of previous item values separately
          // of the iterable as a value itself.
          const itemPart = new NodePart(part.templateFactory);
          part.value = value;

          let i = 0;

          for await (let v of value) {
            // When we get the first value, clear the part. This let's the
            // previous value display until we can replace it.
            if (i === 0) {
              part.clear();
              itemPart.appendIntoPart(part);
            }

            // Check to make sure that value is the still the current value of
            // the part, and if not bail because a new value owns this part
            if (part.value !== value) {
              break;
            }

            // As a convenience, because functional-programming-style
            // transforms of iterables and async iterables requires a library,
            // we accept a mapper function. This is especially convenient for
            // rendering a template for each item.
            if (mapper !== undefined) {
              v = mapper(v, i);
            }

            itemPart.setValue(v);
            itemPart.commit();
            i++;
          }
        });