react-native-epson-escposprinter/lib/module/asyncIterator.js

A Turbo Module wrapper for the Epson ePOS Printer SDK.

"use strict";

import { Queue } from "./Queue.js";

/** Indicates the iterator is still in active state. */
const returnSymbol = Symbol.for("AsyncIterator.return");

/**
 * Async iterator with manual resolvers exposed.
 *
 * The Promise.withResolvers counterpart for async iterators.
 */

/**
 * Options for async iterator with resolvers.
 */

/**
 * Create an async iterator for deferred iterator.
 */
export const asyncIteratorWithResolvers = options => {
  const queue = new Queue();
  let frontPressure = 0;
  let returnPromise;
  return {
    async [Symbol.asyncDispose]() {
      await this.return();
      await options?.dispose?.();
    },
    async return(value, force = true) {
      if (returnPromise === undefined) {
        returnPromise = Promise.resolve(value);

        // Release pending consumers
        while (queue.locked) {
          queue.push(returnSymbol);
        }

        // Clear pending values
        while (queue.size > 0) {
          if (!force) {
            await new Promise(resolve => setTimeout(resolve, 100));
          } else {
            await queue.pop();
          }
        }

        // compat for runtimes without `await using`
        // 1. Users must call return() manually
        // 2. 100ms should be enough for multiple macrotasks
        if (options?.dispose && !("asyncDispose" in Symbol)) {
          setTimeout(() => options?.dispose?.(), 100);
        }
      }
      return {
        done: true,
        value: await returnPromise
      };
    },
    async next() {
      if (returnPromise !== undefined && queue.size === 0) {
        return {
          done: true,
          value: await returnPromise
        };
      }
      frontPressure++;
      const value = await queue.pop();
      frontPressure--;
      if (value === returnSymbol) {
        return {
          done: true,
          value: await returnPromise
        };
      }
      return {
        done: false,
        value: value
      };
    },
    push(value) {
      if (value instanceof Promise) {
        queue.push(value);
      } else {
        queue.push(Promise.resolve(value));
      }
    },
    get active() {
      return returnPromise === undefined;
    },
    get backPressure() {
      return queue.size;
    },
    get frontPressure() {
      return frontPressure;
    }
  };
};

/**
 * Async iterable iterator with manual resolvers exposed.
 */

/**
 * Create an async iterator for deferred iterable.
 *
 * Manually push values to the iterator and closes it when done.
 */
export const asyncIterableIteratorWithResolvers = options => {
  if (Symbol.asyncIterator === undefined) {
    throw new Error(`AsyncIterator is not supported.`);
  }
  const iterator = asyncIteratorWithResolvers(options);
  return {
    ...iterator,
    [Symbol.asyncIterator]() {
      return this;
    }
  };
};
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