Added Blob Edit Content component

The component is used to render single-file Editor when editing a
snippet or a blob

app/assets/javascripts/blob/components/blob_edit_content.vue

+<script>
+import { initEditorLite } from '~/blob/utils';
+
+export default {
+  props: {
+    value: {
+      type: String,
+      required: true,
+    },
+    fileName: {
+      type: String,
+      required: false,
+      default: '',
+    },
+  },
+  data() {
+    return {
+      content: this.value,
+      editor: null,
+    };
+  },
+  watch: {
+    fileName(newVal) {
+      this.editor.updateModelLanguage(newVal);
+    },
+  },
+  mounted() {
+    this.editor = initEditorLite({
+      el: this.$refs.editor,
+      blobPath: this.fileName,
+      blobContent: this.content,
+    });
+  },
+  methods: {
+    triggerFileChange() {
+      const val = this.editor.getValue();
+      this.content = val;
+      this.$emit('input', val);
+    },
+  },
+};
+</script>
+<template>
+  <div class="file-content code">
+    <pre id="editor" ref="editor" data-editor-loading @focusout="triggerFileChange">{{
+      content
+    }}</pre>
+  </div>
+</template>

spec/frontend/blob/components/__snapshots__/blob_edit_content_spec.js.snap

+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Blob Header Editing rendering matches the snapshot 1`] = `
+<div
+  class="file-content code"
+>
+  <pre
+    data-editor-loading=""
+    id="editor"
+  >
+    Lorem ipsum dolor sit amet, consectetur adipiscing elit.
+  </pre>
+</div>
+`;

spec/frontend/blob/components/blob_edit_content_spec.js

+import { shallowMount } from '@vue/test-utils';
+import BlobEditContent from '~/blob/components/blob_edit_content.vue';
+import { initEditorLite } from '~/blob/utils';
+import { nextTick } from 'vue';
+
+jest.mock('~/blob/utils', () => ({
+  initEditorLite: jest.fn(),
+}));
+
+describe('Blob Header Editing', () => {
+  let wrapper;
+  const value = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.';
+  const fileName = 'lorem.txt';
+
+  function createComponent() {
+    wrapper = shallowMount(BlobEditContent, {
+      propsData: {
+        value,
+        fileName,
+      },
+    });
+  }
+
+  beforeEach(() => {
+    createComponent();
+  });
+
+  afterEach(() => {
+    wrapper.destroy();
+  });
+
+  describe('rendering', () => {
+    it('matches the snapshot', () => {
+      expect(wrapper.element).toMatchSnapshot();
+    });
+
+    it('renders content', () => {
+      expect(wrapper.text()).toContain(value);
+    });
+  });
+
+  describe('functionality', () => {
+    it('initialises Editor Lite', () => {
+      const el = wrapper.find({ ref: 'editor' }).element;
+      expect(initEditorLite).toHaveBeenCalledWith({
+        el,
+        blobPath: fileName,
+        blobContent: value,
+      });
+    });
+
+    it('reacts to the changes in fileName', () => {
+      wrapper.vm.editor = {
+        updateModelLanguage: jest.fn(),
+      };
+
+      const newFileName = 'ipsum.txt';
+
+      wrapper.setProps({
+        fileName: newFileName,
+      });
+
+      return nextTick().then(() => {
+        expect(wrapper.vm.editor.updateModelLanguage).toHaveBeenCalledWith(newFileName);
+      });
+    });
+
+    it('emits input event when the blob content is changed', () => {
+      const editorEl = wrapper.find({ ref: 'editor' });
+      wrapper.vm.editor = {
+        getValue: jest.fn().mockReturnValue(value),
+      };
+
+      editorEl.trigger('focusout');
+
+      return nextTick().then(() => {
+        expect(wrapper.emitted().input[0]).toEqual([value]);
+      });
+    });
+  });
+});