Use cookie consent in event tracker

- update module to have a delayed start if consent has not been given
- if consent is given, the cookie-consent event fires and the module is immediately started (no need for page reload)

app/assets/javascripts/govuk_publishing_components/analytics-ga4/ga4-event-tracker.js

@@ -11,6 +11,18 @@ window.GOVUK.Modules = window.GOVUK.Modules || {};
   }
 
   Ga4EventTracker.prototype.init = function () {
+    var consentCookie = window.GOVUK.getConsentCookie()
+
+    if (consentCookie && consentCookie.settings) {
+      this.startModule()
+    } else {
+      this.startModule = this.startModule.bind(this)
+      window.addEventListener('cookie-consent', this.startModule)
+    }
+  }
+
+  // triggered by cookie-consent event, which happens when users consent to cookies
+  Ga4EventTracker.prototype.startModule = function () {
     if (window.dataLayer) {
       this.module.addEventListener('click', this.trackClick.bind(this), true) // useCapture must be true
     }

spec/javascripts/govuk_publishing_components/analytics-ga4/ga4-event-tracker.spec.js

@@ -5,15 +5,65 @@ describe('Google Analytics event tracking', function () {
   var element
   var expected
 
+  function agreeToCookies () {
+    GOVUK.setCookie('cookies_policy', '{"essential":true,"settings":true,"usage":true,"campaigns":true}')
+  }
+
+  function denyCookies () {
+    GOVUK.setCookie('cookies_policy', '{"essential":false,"settings":false,"usage":false,"campaigns":false}')
+  }
+
   beforeEach(function () {
     window.dataLayer = []
     element = document.createElement('div')
+    agreeToCookies()
   })
 
   afterEach(function () {
     document.body.removeChild(element)
   })
 
+  describe('when the user has a cookie consent choice', function () {
+    it('starts the module if consent has already been given', function () {
+      agreeToCookies()
+      document.body.appendChild(element)
+      var tracker = new GOVUK.Modules.Ga4EventTracker(element)
+      spyOn(tracker, 'trackClick')
+      tracker.init()
+
+      element.click()
+      expect(tracker.trackClick).toHaveBeenCalled()
+    })
+
+    it('starts the module on the same page as cookie consent is given', function () {
+      denyCookies()
+      document.body.appendChild(element)
+      var tracker = new GOVUK.Modules.Ga4EventTracker(element)
+      spyOn(tracker, 'trackClick')
+      tracker.init()
+
+      element.click()
+      expect(tracker.trackClick).not.toHaveBeenCalled()
+
+      // page has not been reloaded, user consents to cookies
+      window.GOVUK.triggerEvent(window, 'cookie-consent')
+
+      element.click()
+      expect(tracker.trackClick).toHaveBeenCalled()
+    })
+
+    it('does not do anything if consent is not given', function () {
+      denyCookies()
+      document.body.appendChild(element)
+      var tracker = new GOVUK.Modules.Ga4EventTracker(element)
+      spyOn(tracker, 'trackClick')
+      tracker.init()
+
+      element.click()
+      expect(tracker.trackClick).not.toHaveBeenCalled()
+    })
+  })
+
   describe('configuring tracking without any data', function () {
     beforeEach(function () {
       element.setAttribute('data-ga4', '')