feat(b-tabs): emit cancelable BvEvent before changing tabs via new `a…

…ctivate-tab` event (closes #4273) (#4274)

src/components/tabs/README.md

@@ -419,6 +419,23 @@ methods are `.activate()` and `.deactivate()`, respectively. If activation or de
 will remain active and the method will return `false`. You will need a reference to the `<b-tab>` in
 order to use these methods.
 
+## Preventing a `<b-tab>` from being activated
+
+To prevent a tab from activating, simply set the `disabled` prop on the `<b-tab>` component.
+
+Alternatively, you can listen for the `activate-tab` event, which provides an option to prevent the
+tab from activating. The `activate-tab` event is emitted with three arguments:
+
+- `newTabIndex`: The index of the tab that is going to be activated
+- `prevTabIndex`: The index of the currently active tab
+- `bvEvent`: The `BvEvent` object. Call `bvEvt.preventDefault()` to prevent `newTabIndex` from being
+  activated
+
+For accessibility reasons, when using the `activate-tab` event to prevent a tab from activating, you
+should provide some means of notification to the user as to why the tab is not able to be activated.
+It is recommended to use the `disabled` attribute on the `<b-tab>` component instead of using the
+`activate-tab` event (as `disabled` is more intuitive for screen reader users).
+
 ## Advanced examples
 
 ### External controls using `v-model`

src/components/tabs/package.json

@@ -93,6 +93,28 @@
               }
             ]
           },
+          {
+            "event": "activate-tab",
+            "version": "2.1.0",
+            "description": "Emitted just before a tab is shown/activated. Cancelable",
+            "args": [
+              {
+                "arg": "newTabIndex",
+                "type": "Number",
+                "description": "Tab being activated (0-based index)"
+              },
+              {
+                "arg": "prevTabIndex",
+                "type": "Number",
+                "description": "Tab that is currently active (0-based index). Will be -1 if no current active tab"
+              },
+              {
+                "arg": "bvEvt",
+                "type": "BvEvent",
+                "description": "BvEvent object. Call bvEvt.preventDefault() to cancel"
+              }
+            ]
+          },
           {
             "event": "changed",
             "description": "Emitted when a tab is added, removed, or tabs are re-ordered",

src/components/tabs/tabs.js

@@ -4,6 +4,7 @@ import looseEqual from '../../utils/loose-equal'
 import observeDom from '../../utils/observe-dom'
 import stableSort from '../../utils/stable-sort'
 import { arrayIncludes, concat } from '../../utils/array'
+import { BvEvent } from '../../utils/bv-event.class'
 import { requestAF, selectAll } from '../../utils/dom'
 import { isEvent } from '../../utils/inspect'
 import { omit } from '../../utils/object'
@@ -262,7 +263,7 @@ export const BTabs = /*#__PURE__*/ Vue.extend({
         old = parseInt(old, 10) || 0
         const tabs = this.tabs
         if (tabs[val] && !tabs[val].disabled) {
-          this.currentTab = val
+          this.activateTab(tabs[val])
         } else {
           // Try next or prev tabs
           if (val < old) {
@@ -481,14 +482,22 @@ export const BTabs = /*#__PURE__*/ Vue.extend({
       let result = false
       if (tab) {
         const index = this.tabs.indexOf(tab)
-        if (!tab.disabled && index > -1) {
-          result = true
-          this.currentTab = index
+        if (!tab.disabled && index > -1 && index !== this.currentTab) {
+          const tabEvt = new BvEvent('activate-tab', {
+            cancelable: true,
+            vueTarget: this,
+            componentId: this.safeId()
+          })
+          this.$emit(tabEvt.type, index, this.currentTab, tabEvt)
+          if (!tabEvt.defaultPrevented) {
+            result = true
+            this.currentTab = index
+          }
         }
       }
-      if (!result) {
-        // Couldn't set tab, so ensure v-model is set to `this.currentTab`
-        /* istanbul ignore next: should rarely happen */
+      // Couldn't set tab, so ensure v-model is set to `this.currentTab`
+      /* istanbul ignore next: should rarely happen */
+      if (!result && this.currentTab !== this.value) {
         this.$emit('input', this.currentTab)
       }
       return result
@@ -500,11 +509,9 @@ export const BTabs = /*#__PURE__*/ Vue.extend({
         // Find first non-disabled tab that isn't the one being deactivated
         // If no tabs are available, then don't deactivate current tab
         return this.activateTab(this.tabs.filter(t => t !== tab).find(notDisabled))
-      } else {
-        // No tab specified
-        /* istanbul ignore next: should never happen */
-        return false
       }
+      /* istanbul ignore next: should never/rarely happen */
+      return false
     },
     // Focus a tab button given it's <b-tab> instance
     focusButton(tab) {

src/components/tabs/tabs.spec.js

@@ -311,6 +311,72 @@ describe('tabs', () => {
     wrapper.destroy()
   })
 
+  it('`activate-tab` event works', async () => {
+    const App = Vue.extend({
+      methods: {
+        preventTab(next, prev, bvEvt) {
+          // Prevent 3rd tab (index === 2) from activating
+          if (next === 2) {
+            bvEvt.preventDefault()
+          }
+        }
+      },
+      render(h) {
+        return h(BTabs, { props: { value: 0 }, on: { 'activate-tab': this.preventTab } }, [
+          h(BTab, { props: {} }, 'tab 0'),
+          h(BTab, { props: {} }, 'tab 1'),
+          h(BTab, { props: {} }, 'tab 2')
+        ])
+      }
+    })
+    const wrapper = mount(App)
+    expect(wrapper).toBeDefined()
+
+    await waitNT(wrapper.vm)
+    await waitRAF()
+    const tabs = wrapper.find(BTabs)
+    expect(tabs).toBeDefined()
+    expect(tabs.findAll(BTab).length).toBe(3)
+
+    // Expect 1st tab (index 0) to be active
+    expect(tabs.vm.currentTab).toBe(0)
+    expect(tabs.vm.tabs[0].localActive).toBe(true)
+    expect(tabs.emitted('input')).not.toBeDefined()
+    expect(tabs.emitted('activate-tab')).not.toBeDefined()
+
+    // Set 2nd BTab to be active
+    tabs.setProps({ value: 1 })
+    await waitNT(wrapper.vm)
+    await waitRAF()
+    expect(tabs.vm.currentTab).toBe(1)
+    expect(tabs.emitted('input')).toBeDefined()
+    expect(tabs.emitted('input').length).toBe(1)
+    expect(tabs.emitted('input')[0][0]).toBe(1)
+    expect(tabs.emitted('activate-tab')).toBeDefined()
+    expect(tabs.emitted('activate-tab').length).toBe(1)
+    expect(tabs.emitted('activate-tab')[0][0]).toBe(1)
+    expect(tabs.emitted('activate-tab')[0][1]).toBe(0)
+    expect(tabs.emitted('activate-tab')[0][2]).toBeDefined()
+    expect(tabs.emitted('activate-tab')[0][2].vueTarget).toBe(tabs.vm)
+
+    // Attempt to set 3rd BTab to be active
+    tabs.setProps({ value: 2 })
+    await waitNT(wrapper.vm)
+    await waitRAF()
+    expect(tabs.vm.currentTab).toBe(1)
+    expect(tabs.emitted('input')).toBeDefined()
+    expect(tabs.emitted('input').length).toBe(2)
+    expect(tabs.emitted('input')[1][0]).toBe(1)
+    expect(tabs.emitted('activate-tab').length).toBe(2)
+    expect(tabs.emitted('activate-tab')[1][0]).toBe(2)
+    expect(tabs.emitted('activate-tab')[1][1]).toBe(1)
+    expect(tabs.emitted('activate-tab')[1][2]).toBeDefined()
+    expect(tabs.emitted('activate-tab')[1][2].vueTarget).toBe(tabs.vm)
+    expect(tabs.emitted('activate-tab')[1][2].defaultPrevented).toBe(true)
+
+    wrapper.destroy()
+  })
+
   it('clicking on tab activates the tab, and tab emits click event', async () => {
     const App = Vue.extend({
       render(h) {