make title/tabbar's close icon customable.

packages/widgets/src/tabbar.ts

@@ -1752,7 +1752,11 @@ export namespace TabBar {
      * @returns A virtual element representing the tab close icon.
      */
     renderCloseIcon(data: IRenderData<any>): VirtualElement {
-      return h.div({ className: 'lm-TabBar-tabCloseIcon' });
+      const { title } = data;
+      let className = this.createCloseIconClass(data);
+
+      // If title.closeIcon is undefined, it will be ignored.
+      return h.div({ className }, title.closeIcon!);
     }
 
     /**
@@ -1847,6 +1851,19 @@ export namespace TabBar {
       return extra ? `${name} ${extra}` : name;
     }
 
+    /**
+     * Create the class name for the tab closeIcon.
+     *
+     * @param data - The data to use for the tab.
+     *
+     * @returns The full class name for the tab closeIcon.
+     */
+    createCloseIconClass(data: IRenderData<any>): string {
+      let name = 'lm-TabBar-tabCloseIcon';
+      let extra = data.title.closeIconClass;
+      return extra ? `${name} ${extra}` : name;
+    }
+
     private static _nInstance = 0;
     private readonly _uuid: number;
     private _tabID = 0;

packages/widgets/src/title.ts

@@ -40,7 +40,6 @@ export class Title<T> implements IDisposable {
     if (options.icon !== undefined) {
       this._icon = options.icon;
     }
-
     if (options.iconClass !== undefined) {
       this._iconClass = options.iconClass;
     }
@@ -56,6 +55,12 @@ export class Title<T> implements IDisposable {
     if (options.closable !== undefined) {
       this._closable = options.closable;
     }
+    if (options.closeIcon !== undefined) {
+      this._closeIcon = options.closeIcon;
+    }
+    if (options.closeIconClass !== undefined) {
+      this._closeIconClass = options.closeIconClass;
+    }
     this._dataset = options.dataset || {};
   }
 
@@ -254,6 +259,55 @@ export class Title<T> implements IDisposable {
     this._changed.emit(undefined);
   }
 
+  /**
+   * Get the closeIcon renderer for the title.
+   *
+   * #### Notes
+   * The default value is undefined.
+   */
+  get closeIcon() {
+    return this._closeIcon;
+  }
+
+  /**
+   * Set the closeIcon renderer for the title.
+   *
+   * #### Notes
+   * A renderer is an object that supplies a render and unrender function.
+   */
+  set closeIcon(value) {
+    if (this._closeIcon === value) {
+      return;
+    }
+    this._closeIcon = value;
+    this._changed.emit(undefined);
+  }
+
+  /**
+   * Get the closeIcon class name for the title.
+   *
+   * #### Notes
+   * The default value is an empty string.
+   */
+
+  get closeIconClass() {
+    return this._closeIconClass;
+  }
+
+  /**
+   * Set the closeIcon class name for the title.
+   *
+   * #### Notes
+   * Multiple class names can be separated with whitespace.
+   */
+  set closeIconClass(value) {
+    if (this._closeIconClass === value) {
+      return;
+    }
+    this._closeIconClass = value;
+    this._changed.emit(undefined);
+  }
+
   /**
    * Get the dataset for the title.
    *
@@ -308,6 +362,8 @@ export class Title<T> implements IDisposable {
   private _iconLabel = '';
   private _className = '';
   private _closable = false;
+  private _closeIcon: VirtualElement.IRenderer | undefined = undefined;
+  private _closeIconClass = '';
   private _dataset: Title.Dataset;
   private _changed = new Signal<this, void>(this);
   private _isDisposed = false;
@@ -371,6 +427,16 @@ export namespace Title {
      */
     closable?: boolean;
 
+    /**
+     * The closeIcon renderer for the title.
+     */
+    closeIcon?: VirtualElement.IRenderer;
+
+    /**
+     * The closeIcon class name for the title.
+     */
+    closeIconClass?: string;
+
     /**
      * The dataset for the title.
      */

packages/widgets/tests/src/tabbar.spec.ts

@@ -2067,6 +2067,11 @@ describe('@lumino/widgets', () => {
             'lm-TabBar-tabIcon'
           )[0] as HTMLElement;
           expect(icon.classList.contains(title.iconClass)).to.equal(true);
+
+          let closeIcon = node.getElementsByClassName(
+            'lm-TabBar-tabCloseIcon'
+          )[0] as HTMLElement;
+          expect(closeIcon.classList.contains(title.closeIconClass)).to.equal(true);
         });
       });
 
@@ -2080,6 +2085,16 @@ describe('@lumino/widgets', () => {
         });
       });
 
+      describe('#renderCloseIcon()', () => {
+        it('should render the closeIcon element for a tab', () => {
+          let renderer = new TabBar.Renderer();
+          let vNode = renderer.renderCloseIcon({ title, current: true, zIndex: 1 });
+          let node = VirtualDOM.realize(vNode as VirtualElement);
+          expect(node.className).to.contain('lm-TabBar-tabCloseIcon');
+          expect(node.classList.contains(title.closeIconClass)).to.equal(true);
+        });
+      });
+
       describe('#renderLabel()', () => {
         it('should render the label element for a tab', () => {
           let renderer = new TabBar.Renderer();
@@ -2154,6 +2169,19 @@ describe('@lumino/widgets', () => {
           expect(className).to.contain(title.iconClass);
         });
       });
+
+      describe('#createCloseIconClass()', () => {
+        it('should create class name for the tab close icon', () => {
+          let renderer = new TabBar.Renderer();
+          let className = renderer.createCloseIconClass({
+            title,
+            current: true,
+            zIndex: 1
+          });
+          expect(className).to.contain('lm-TabBar-tabCloseIcon');
+          expect(className).to.contain(title.closeIconClass);
+        });
+      });
     });
 
     describe('.defaultRenderer', () => {

packages/widgets/tests/src/title.spec.ts

@@ -187,6 +187,55 @@ describe('@lumino/widgets', () => {
       });
     });
 
+    describe('#closeIcon', () => {
+      const closeIconRenderer = {
+        render: (host: HTMLElement, options?: any) => {
+          const renderNode = document.createElement('div');
+          renderNode.className = 'foo';
+          host.appendChild(renderNode);
+        }
+      };
+
+      it('should default to undefined', () => {
+        let title = new Title({ owner });
+        expect(title.closeIcon).to.equal(undefined);
+      });
+
+      it('should initialize from the options', () => {
+        let title = new Title({ owner, closeIcon: closeIconRenderer });
+        expect(title.closeIcon).to.equal(closeIconRenderer);
+      });
+
+      it('should be writable', () => {
+        let title = new Title({ owner });
+        expect(title.closeIcon).to.equal(undefined);
+        title.closeIcon = closeIconRenderer;
+        expect(title.closeIcon).to.equal(closeIconRenderer);
+      });
+
+      it('should emit the changed signal when the value changes', () => {
+        let called = false;
+        let title = new Title({ owner });
+        title.changed.connect((sender, arg) => {
+          expect(sender).to.equal(title);
+          expect(arg).to.equal(undefined);
+          called = true;
+        });
+        title.closeIcon = closeIconRenderer;
+        expect(called).to.equal(true);
+      });
+
+      it('should not emit the changed signal when the value does not change', () => {
+        let called = false;
+        let title = new Title({ owner, closeIcon: closeIconRenderer });
+        title.changed.connect((sender, arg) => {
+          called = true;
+        });
+        title.closeIcon = closeIconRenderer;
+        expect(called).to.equal(false);
+      });
+    });
+
     describe('#caption', () => {
       it('should default to an empty string', () => {
         let title = new Title({ owner });