Add optional callback to forceUpdate method

src/component.js

@@ -83,9 +83,11 @@ extend(Component.prototype, {
 
 
 	/** Immediately perform a synchronous re-render of the component.
+	 *	@param {function} callback		A function to be called after component is re-rendered.
 	 *	@private
 	 */
-	forceUpdate() {
+	forceUpdate(callback) {
+		if (callback) (this._renderCallbacks = (this._renderCallbacks || [])).push(callback);
 		renderComponent(this, FORCE_RENDER);
 	},
 

src/preact.d.ts

@@ -62,7 +62,7 @@ declare namespace preact {
 		setState<K extends keyof StateType>(state:Pick<StateType, K>, callback?:() => void):void;
 		setState<K extends keyof StateType>(fn:(prevState:StateType, props:PropsType) => Pick<StateType, K>, callback?:() => void):void;
 
-		forceUpdate(): void;
+		forceUpdate(callback?:() => void): void;
 
 		abstract render(props?:PropsType & ComponentProps<this>, state?:StateType, context?:any):JSX.Element;
 	}

test/browser/spec.js

@@ -124,4 +124,49 @@ describe('Component spec', () => {
 			expect(WithDefaultProps.prototype.getDefaultProps).to.be.calledOnce;
 		});
 	});
+
+	describe('forceUpdate', () => {
+		it('should force a rerender', () => {
+			let forceUpdate;
+			class ForceUpdateComponent extends Component {
+				componentWillUpdate() {}
+				componentDidMount() {
+					forceUpdate = () => this.forceUpdate();
+				}
+				render() {
+					return <div />;
+				}
+			}
+			sinon.spy(ForceUpdateComponent.prototype, 'componentWillUpdate');
+			sinon.spy(ForceUpdateComponent.prototype, 'forceUpdate');
+			render(<ForceUpdateComponent />, scratch);
+			expect(ForceUpdateComponent.prototype.componentWillUpdate).not.to.have.been.called;
+
+			forceUpdate();
+
+			expect(ForceUpdateComponent.prototype.componentWillUpdate).to.have.been.called;
+			expect(ForceUpdateComponent.prototype.forceUpdate).to.have.been.called;
+		});
+
+		it('should add callback to renderCallbacks', () => {
+			let forceUpdate;
+			let callback = sinon.spy();
+			class ForceUpdateComponent extends Component {
+				componentDidMount() {
+					forceUpdate = () => this.forceUpdate(callback);
+				}
+				render() {
+					return <div />;
+				}
+			}
+			sinon.spy(ForceUpdateComponent.prototype, 'forceUpdate');
+			render(<ForceUpdateComponent />, scratch);
+
+			forceUpdate();
+
+			expect(ForceUpdateComponent.prototype.forceUpdate).to.have.been.called;
+			expect(ForceUpdateComponent.prototype.forceUpdate).to.have.been.calledWith(callback);
+			expect(callback).to.have.been.called;
+		});
+	});
 });