/**
 * @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
 */
var __asyncValues = (this && this.__asyncValues) || function (o) {
    if (!Symbol.asyncIterator) throw new TypeError("Symbol.asyncIterator is not defined.");
    var m = o[Symbol.asyncIterator], i;
    return m ? m.call(o) : (o = typeof __values === "function" ? __values(o) : o[Symbol.iterator](), i = {}, verb("next"), verb("throw"), verb("return"), i[Symbol.asyncIterator] = function () { return this; }, i);
    function verb(n) { i[n] = o[n] && function (v) { return new Promise(function (resolve, reject) { v = o[n](v), settle(resolve, reject, v.done, v.value); }); }; }
    function settle(resolve, reject, d, v) { Promise.resolve(v).then(function(v) { resolve({ value: v, done: d }); }, reject); }
};
import { 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 = directive((value, mapper) => async (part) => {
    var e_1, _a;
    if (!(part instanceof NodePart)) {
        throw new Error('asyncReplace can only be used in text bindings');
    }
    // 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.options);
    part.value = value;
    let i = 0;
    try {
        for (var value_1 = __asyncValues(value), value_1_1; value_1_1 = await value_1.next(), !value_1_1.done;) {
            let v = value_1_1.value;
            // 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;
            }
            // 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);
            }
            // 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) {
                // This is safe because T must otherwise be treated as unknown by
                // the rest of the system.
                v = mapper(v, i);
            }
            itemPart.setValue(v);
            itemPart.commit();
            i++;
        }
    }
    catch (e_1_1) { e_1 = { error: e_1_1 }; }
    finally {
        try {
            if (value_1_1 && !value_1_1.done && (_a = value_1.return)) await _a.call(value_1);
        }
        finally { if (e_1) throw e_1.error; }
    }
});
