Generic viewability module: add new module (prebid#7643)

* - custom viewability core

* - IntersectionObserver implementation

* - support different type of trackers

* - viewability tests wip

* - increase test coverage for viewability

* - move viewability module js

* - remove uneccesary changes

* - allow uncomplete observers to be registered again

* - import explicitly from utils

* - add viewability.md

* - add module name to log messages

* update for higher legibility

modules/viewability.js

@@ -0,0 +1,177 @@
+import { logWarn, logInfo, isStr, isFn, triggerPixel, insertHtmlIntoIframe } from '../src/utils.js';
+import { getGlobal } from '../src/prebidGlobal.js';
+import find from 'core-js-pure/features/array/find.js';
+
+export const MODULE_NAME = 'viewability';
+
+export function init() {
+  (getGlobal()).viewability = {
+    startMeasurement: startMeasurement,
+    stopMeasurement: stopMeasurement,
+  };
+
+  listenMessagesFromCreative();
+}
+
+const observers = {};
+
+function isValid(vid, element, tracker, criteria) {
+  if (!element) {
+    logWarn(`${MODULE_NAME}: no html element provided`);
+    return false;
+  }
+
+  let validTracker = tracker &&
+    ((tracker.method === 'img' && isStr(tracker.value)) ||
+    (tracker.method === 'js' && isStr(tracker.value)) ||
+    (tracker.method === 'callback' && isFn(tracker.value)));
+
+  if (!validTracker) {
+    logWarn(`${MODULE_NAME}: invalid tracker`, tracker);
+    return false;
+  }
+
+  if (!criteria || !criteria.inViewThreshold || !criteria.timeInView) {
+    logWarn(`${MODULE_NAME}: missing criteria`, criteria);
+    return false;
+  }
+
+  if (!vid || observers[vid]) {
+    logWarn(`${MODULE_NAME}: must provide an unregistered vid`, vid);
+    return false;
+  }
+
+  return true;
+}
+
+function stopObserving(observer, vid, element) {
+  observer.unobserve(element);
+  observers[vid].done = true;
+}
+
+function fireViewabilityTracker(element, tracker) {
+  switch (tracker.method) {
+    case 'img':
+      triggerPixel(tracker.value, () => {
+        logInfo(`${MODULE_NAME}: viewability pixel fired`, tracker.value);
+      });
+      break;
+    case 'js':
+      insertHtmlIntoIframe(`<script src="${tracker.value}"></script>`);
+      break;
+    case 'callback':
+      tracker.value(element);
+      break;
+  }
+}
+
+function viewabilityCriteriaMet(observer, vid, element, tracker) {
+  stopObserving(observer, vid, element);
+  fireViewabilityTracker(element, tracker);
+}
+
+/**
+ * Start measuring viewability of an element
+ * @typedef {{ method: string='img','js','callback', value: string|function }} ViewabilityTracker { method: 'img', value: 'http://my.tracker/123' }
+ * @typedef {{ inViewThreshold: number, timeInView: number }} ViewabilityCriteria { inViewThreshold: 0.5, timeInView: 1000 }
+ * @param {string} vid unique viewability identifier
+ * @param {HTMLElement} element
+ * @param {ViewabilityTracker} tracker
+ * @param {ViewabilityCriteria} criteria
+ */
+export function startMeasurement(vid, element, tracker, criteria) {
+  if (!isValid(vid, element, tracker, criteria)) {
+    return;
+  }
+
+  const options = {
+    root: null,
+    rootMargin: '0px',
+    threshold: criteria.inViewThreshold,
+  };
+
+  let observer;
+  let viewable = false;
+  let stateChange = (entries) => {
+    viewable = entries[0].isIntersecting;
+
+    if (viewable) {
+      observers[vid].timeoutId = window.setTimeout(() => {
+        viewabilityCriteriaMet(observer, vid, element, tracker);
+      }, criteria.timeInView);
+    } else if (observers[vid].timeoutId) {
+      window.clearTimeout(observers[vid].timeoutId);
+    }
+  };
+
+  observer = new IntersectionObserver(stateChange, options);
+  observers[vid] = {
+    observer: observer,
+    element: element,
+    timeoutId: null,
+    done: false,
+  };
+
+  observer.observe(element);
+
+  logInfo(`${MODULE_NAME}: startMeasurement called with:`, arguments);
+}
+
+/**
+ * Stop measuring viewability of an element
+ * @param {string} vid unique viewability identifier
+ */
+export function stopMeasurement(vid) {
+  if (!vid || !observers[vid]) {
+    logWarn(`${MODULE_NAME}: must provide a registered vid`, vid);
+    return;
+  }
+
+  observers[vid].observer.unobserve(observers[vid].element);
+  if (observers[vid].timeoutId) {
+    window.clearTimeout(observers[vid].timeoutId);
+  }
+
+  // allow the observer under this vid to be created again
+  if (!observers[vid].done) {
+    delete observers[vid];
+  }
+}
+
+function listenMessagesFromCreative() {
+  window.addEventListener('message', receiveMessage, false);
+}
+
+/**
+ * Recieve messages from creatives
+ * @param {MessageEvent} evt
+ */
+export function receiveMessage(evt) {
+  var key = evt.message ? 'message' : 'data';
+  var data = {};
+  try {
+    data = JSON.parse(evt[key]);
+  } catch (e) {
+    return;
+  }
+
+  if (!data || data.message !== 'Prebid Viewability') {
+    return;
+  }
+
+  switch (data.action) {
+    case 'startMeasurement':
+      let element = data.elementId && document.getElementById(data.elementId);
+      if (!element) {
+        element = find(document.getElementsByTagName('IFRAME'), iframe => (iframe.contentWindow || iframe.contentDocument.defaultView) == evt.source);
+      }
+
+      startMeasurement(data.vid, element, data.tracker, data.criteria);
+      break;
+    case 'stopMeasurement':
+      stopMeasurement(data.vid);
+      break;
+  }
+}
+
+init();

modules/viewability.md

@@ -0,0 +1,87 @@
+# Overview
+
+Module Name: Viewability
+
+Purpose: Track when a given HTML element becomes viewable
+
+Maintainer: atrajkovic@magnite.com
+
+# Configuration
+
+Module does not need any configuration, as long as you include it in your PBJS bundle.
+Viewability module has only two functions `startMeasurement` and `stopMeasurement` which can be used to enable more complex viewability measurements. Since it allows tracking from within creative (possibly inside a safe frame) this module registers a message listener, for messages with a format that is described bellow.
+
+## `startMeasurement`
+
+| startMeasurement Arg Object | Scope | Type | Description | Example |
+| --------------------- | -------- | ------------ | -------------------------------------------------------------------------------- | --------- |
+| vid | Required | String | Unique viewability identifier, used to reference particular observer | `"ae0f9"` |
+| element | Required | HTMLElement |  Reference to an HTML element that needs to be tracked | `document.getElementById('test_div')` |
+| tracker | Required | ViewabilityTracker | How viewaility event is communicated back to the parties of interest | `{ method: 'img', value: 'http://my.tracker/123' }` |
+| criteria | Required | ViewabilityCriteria| Defines custom viewability criteria using the threshold and duration provided  | `{ inViewThreshold: 0.5, timeInView: 1000 }` |
+
+| ViewabilityTracker | Scope | Type | Description | Example |
+| --------------------- | -------- | ------------ | -------------------------------------------------------------------------------- | --------- |
+| method | Required | String | Type of method for Tracker | `'img' OR 'js' OR 'callback'` |
+| value | Required | String | URL string for 'img' and 'js' Trackers, or a function for 'callback' Tracker | `'http://my.tracker/123'` |
+
+| ViewabilityCriteria | Scope | Type | Description | Example |
+| --------------------- | -------- | ------------ | -------------------------------------------------------------------------------- | --------- |
+| inViewThreshold | Required | Number | Represents a percentage threshold for the Element to be registered as in view | `0.5` |
+| timeInView | Required | Number | Number of milliseconds that a given element needs to be in view continuously, above the threshold | `1000` |
+
+## Please Note:
+- `vid` allows for multiple trackers, with different criteria to be registered for a given HTML element, independently. It's not autogenerated by `startMeasurement()`, it needs to be provided by the caller so that it doesn't have to be posted back to the source iframe (in case viewability is started from within the creative).
+- In case of 'callback' method, HTML element is being passed back to the callback function.
+- When a tracker needs to be started, without direct access to pbjs, postMessage mechanism can be used to invoke `startMeasurement`, with a following payload: `vid`, `tracker` and `criteria` as described above, but also with `message: 'Prebid Viewability'` and `action: 'startMeasurement'`. Optionally payload can provide `elementId`, if available at that time (for ad servers where name of the iframe is known, or adservers that render outside an iframe). If `elementId` is not provided, viewability module will try to find the iframe that corresponds to the message source. 
+
+
+## `stopMeasurement`
+
+| stopMeasurement Arg Object | Scope | Type | Description | Example |
+| --------------------- | -------- | ------------ | -------------------------------------------------------------------------------- | --------- |
+| vid | Required | String | Unique viewability identifier, referencing an already started viewability tracker. | `"ae0f9"` |
+
+## Please Note:
+- When a tracker needs to be stopped, without direct access to pbjs, postMessage mechanism can be used here as well. To invoke `stopMeasurement`, you provide the payload with `vid`, `message: 'Prebid Viewability'` and `action: 'stopMeasurement`. Check the example bellow.
+
+# Examples
+
+## Example of starting a viewability measurement, when you have direct access to pbjs
+```
+pbjs.viewability.startMeasurement(
+    'ae0f9', 
+    document.getElementById('test_div'),
+    { method: 'img', value: 'http://my.tracker/123' },
+    { inViewThreshold: 0.5, timeInView: 1000 }
+);
+```
+
+## Example of starting a viewability measurement from within a rendered creative
+```
+let viewabilityRecord = {
+    vid: 'ae0f9',
+    tracker: { method: 'img', value: 'http://my.tracker/123'},
+    criteria: { inViewThreshold: 0.5, timeInView: 1000 },
+    message: 'Prebid Viewability',
+    action: 'startMeasurement'
+}
+
+window.parent.postMessage(JSON.stringify(viewabilityRecord), '*');
+```
+
+## Example of stopping the viewability measurement, when you have direct access to pbjs
+```
+pbjs.viewability.stopMeasurement('ae0f9');
+```
+
+## Example of stopping the viewability measurement from within a rendered creative
+```
+let viewabilityRecord = {
+    vid: 'ae0f9',
+    message: 'Prebid Viewability',
+    action: 'stopMeasurement'
+}
+
+window.parent.postMessage(JSON.stringify(viewabilityRecord), '*');
+```