Added maximum promise queue lenght

src/pcf857x.ts

@@ -125,7 +125,7 @@ export abstract class PCF857x<PinNumber extends PCF8574.PinNumber | PCF8575.PinN
   private _currentlyPolling: boolean = false;
 
   /** PromiseQueue to handle requested polls in order. */
-  private _pollQueue: PromiseQueue = new PromiseQueue();
+  private _pollQueue: PromiseQueue = new PromiseQueue(1);
 
   /** Pin number of GPIO to detect interrupts, or null by default. */
   private _gpioPin: number | null = null;
@@ -315,6 +315,7 @@ export abstract class PCF857x<PinNumber extends PCF8574.PinNumber | PCF8575.PinN
    * If a change on an input is detected, an "input" Event will be emitted with a data object containing the "pin" and the new "value".
    * This have to be called frequently enough if you don't use a GPIO for interrupt detection.
    * If you poll again before the last poll was completed, the new poll will be queued up the be executed after the current poll.
+   * If you poll again while also a poll is queued, this will be rejected.
    * @return {Promise}
    */
   public doPoll (): Promise<void> {

src/promise-queue.ts

@@ -24,14 +24,36 @@ export class PromiseQueue {
    */
   private working: boolean = false;
 
+  /**
+   * The maximum length of the queue.
+   */
+  private maxQueueLength: number | undefined = undefined;
+
+  /**
+   * Create a Promise Queue.
+   * @param maxQueueLength Optional maximum allowed length of the queue.
+   */
+  constructor (maxQueueLength?: number) {
+    if (typeof maxQueueLength === 'number') {
+      this.maxQueueLength = maxQueueLength;
+    }
+  }
+
   /**
    * Enqueue a Promise.
    * This adds the given Promise to the queue. If the queue was empty the Promise
    * will be started immediately.
+   * If the PromiseQueue was initialized with a maximum length and the Promise to
+   * enqueue would exceed the limit, a Promise rejection will be returned instant.
    * @param promise Function which returns the Promise.
-   * @returns A Promise which will be resolves (or rejected) if the queued promise is done.
+   * @returns A Promise which will be resolved (or rejected) if the queued promise is done. Or an instant Promise rejection if the maximum allowed queue length is exceeded.
    */
   public enqueue<T = void> (promise: () => Promise<T>): Promise<T> {
+    // check the maximum queue length
+    if (this.maxQueueLength !== undefined && this.queue.length >= this.maxQueueLength) {
+      return Promise.reject('Maximum queue length exceeded');
+    }
+
     return new Promise((resolve, reject) => {
       this.queue.push({
         promise,