Add loading button

Encompasses designs from
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/23206#note_43355199
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/29985#note_30417407

app/assets/javascripts/vue_shared/components/loading_button.vue

+<script>
+
+/* This is a re-usable vue component for rendering a button
+   that will probably be sending off ajax requests and need
+   to show the loading status by setting the `loading` option.
+   This can also be used for initial page load when you don't
+   know the action of the button yet by setting
+   `loading: true, label: undefined`.
+
+  Sample configuration:
+
+  <loading-button
+    :loading="true"
+    :label="Hello"
+    @click="..."
+  />
+
+*/
+
+import loadingIcon from './loading_icon.vue';
+
+export default {
+  props: {
+    loading: {
+      type: Boolean,
+      required: false,
+      default: false,
+    },
+    label: {
+      type: String,
+      required: false,
+    },
+  },
+  components: {
+    loadingIcon,
+  },
+  methods: {
+    onClick(e) {
+      this.$emit('click', e);
+    },
+  },
+};
+</script>
+
+<template>
+  <button
+    class="btn btn-align-content"
+    @click="onClick"
+    type="button"
+    :disabled="loading"
+  >
+      <transition name="fade">
+        <loading-icon
+          v-if="loading"
+          :inline="true"
+          class="js-loading-button-icon"
+          :class="{
+              'append-right-5': label
+          }"
+        />
+      </transition>
+      <transition name="fade">
+        <span
+          v-if="label"
+          class="js-loading-button-label"
+        >
+          {{ label }}
+        </span>
+      </transition>
+  </button>
+</template>

app/assets/stylesheets/framework.scss

@@ -5,6 +5,7 @@
 @import "framework/layout";
 @import "framework/animations";
+@import "framework/vue_transitions";
 @import "framework/avatar";
 @import "framework/asciidoctor";
 @import "framework/banner";

app/assets/stylesheets/framework/buttons.scss

@@ -292,6 +292,11 @@
   }
 }
+.btn-align-content {
+  display: flex;
+  align-items: center;
+}
+
 .btn-group {
   &.btn-grouped {
     @include btn-with-margin;

app/assets/stylesheets/framework/vue_transitions.scss

+.fade-enter-active,
+.fade-leave-active {
+  transition: opacity $sidebar-transition-duration $general-hover-transition-curve;
+}
+
+.fade-enter,
+.fade-leave-to {
+  opacity: 0;
+}

app/assets/stylesheets/pages/repo.scss

-.fade-enter-active,
-.fade-leave-active {
-  transition: opacity $sidebar-transition-duration;
-}
-
 .monaco-loader {
   position: absolute;
   top: 0;

changelogs/unreleased/add-shared-vue-loading-button.yml

+---
+title: Add loading button for new UX paradigm
+merge_request: 14883
+author:
+type: added

spec/javascripts/vue_shared/components/loading_button_spec.js

+import Vue from 'vue';
+import loadingButton from '~/vue_shared/components/loading_button.vue';
+import mountComponent from '../../helpers/vue_mount_component_helper';
+
+const LABEL = 'Hello';
+
+describe('LoadingButton', function () {
+  let vm;
+  let LoadingButton;
+
+  beforeEach(() => {
+    LoadingButton = Vue.extend(loadingButton);
+  });
+
+  afterEach(() => {
+    vm.$destroy();
+  });
+
+  describe('loading spinner', () => {
+    it('shown when loading', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: true,
+      });
+
+      expect(vm.$el.querySelector('.js-loading-button-icon')).toBeDefined();
+    });
+  });
+
+  describe('disabled state', () => {
+    it('disabled when loading', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: true,
+      });
+
+      expect(vm.$el.disabled).toEqual(true);
+    });
+
+    it('not disabled when normal', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: false,
+      });
+
+      expect(vm.$el.disabled).toEqual(false);
+    });
+  });
+
+  describe('label', () => {
+    it('shown when normal', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: false,
+        label: LABEL,
+      });
+      const label = vm.$el.querySelector('.js-loading-button-label');
+
+      expect(label.textContent.trim()).toEqual(LABEL);
+    });
+
+    it('shown when loading', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: true,
+        label: LABEL,
+      });
+      const label = vm.$el.querySelector('.js-loading-button-label');
+
+      expect(label.textContent.trim()).toEqual(LABEL);
+    });
+  });
+
+  describe('click callback prop', () => {
+    it('calls given callback when normal', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: false,
+      });
+      spyOn(vm, '$emit');
+
+      vm.$el.click();
+
+      expect(vm.$emit).toHaveBeenCalledWith('click', jasmine.any(Object));
+    });
+
+    it('does not call given callback when disabled because of loading', () => {
+      vm = mountComponent(LoadingButton, {
+        loading: true,
+        indeterminate: true,
+      });
+      spyOn(vm, '$emit');
+
+      vm.$el.click();
+
+      expect(vm.$emit).not.toHaveBeenCalled();
+    });
+  });
+});