do not destroy and rerender sub widgets if not necessary

add failing test on rerendering sub widgets...

web/static/src/ts/core/Widget.ts

@@ -25,6 +25,8 @@ interface Meta<T extends WEnv> {
   isDestroyed: boolean;
   parent: Widget<T> | null;
   children: { [key: number]: Widget<T> };
+  // children mapping: from templateID to widgetID
+  cmap: { [key: number]: number };
 }
 
 const patch = init([sdListeners, sdAttrs]);
@@ -43,6 +45,7 @@ export class Widget<T extends WEnv> {
 
   env: T;
   state: Object = {};
+  props: any;
   refs: { [key: string]: Widget<T> | HTMLElement | undefined } = {}; // either HTMLElement or Widget
 
   //--------------------------------------------------------------------------
@@ -51,6 +54,7 @@ export class Widget<T extends WEnv> {
 
   constructor(parent: Widget<T> | T, props?: any) {
     wl.push(this);
+    this.props = props;
     let id: number;
     let p: Widget<T> | null = null;
     if (parent instanceof Widget) {
@@ -69,7 +73,8 @@ export class Widget<T extends WEnv> {
       isMounted: false,
       isDestroyed: false,
       parent: p,
-      children: {}
+      children: {},
+      cmap: {}
     };
   }
 
@@ -139,6 +144,11 @@ export class Widget<T extends WEnv> {
     }
   }
 
+  updateProps(props?: any): Promise<void> {
+    this.props = props;
+    return this.render();
+  }
+
   //--------------------------------------------------------------------------
   // Private
   //--------------------------------------------------------------------------

web/static/src/ts/core/qweb_vdom.ts

@@ -691,11 +691,6 @@ const widgetDirective: Directive = {
   priority: 100,
   atNodeEncounter({ ctx, value, node, qweb }): boolean {
     ctx.rootContext.shouldDefineOwner = true;
-    let dummyID = ctx.generateID();
-    let defID = ctx.generateID();
-    ctx.addLine(`let _${dummyID} = {}; // DUMMY`);
-    ctx.addLine(`let _${dummyID}_index = c${ctx.parentNode}.length;`);
-    ctx.addLine(`c${ctx.parentNode}.push(_${dummyID});`);
     let props = node.getAttribute("t-props");
     if (props) {
       props = props.trim();
@@ -713,21 +708,48 @@ const widgetDirective: Directive = {
         props = qweb._formatExpression(props);
       }
     }
+    let dummyID = ctx.generateID();
+    let defID = ctx.generateID();
     let widgetID = ctx.generateID();
+    ctx.addLine(`let _${dummyID} = {}; // DUMMY`);
+    ctx.addLine(`let _${dummyID}_index = c${ctx.parentNode}.length;`);
+    ctx.addLine(`c${ctx.parentNode}.push(_${dummyID});`);
+    ctx.addLine(`let def${defID};`);
+
+    ctx.addLine(`if (${widgetID} in context.__widget__.cmap) {`);
+    ctx.indent();
+    ctx.addLine(
+      `let curWidget = context.__widget__.children[context.__widget__.cmap[${widgetID}]]`
+    );
+    ctx.addLine(
+      `def${defID} = curWidget.updateProps(${props}).then(()=>{vnode=curWidget.__widget__.vnode;c${
+        ctx.parentNode
+      }[_${dummyID}_index]=vnode;vnode.data.hook = {remove(){curWidget.destroy()}}});`
+    );
+    ctx.dedent();
+    ctx.addLine("} else {");
+    ctx.indent();
+
     ctx.addLine(
       `let _${widgetID} = new context.widgets['${value}'](owner, ${props});`
     );
     ctx.addLine(
-      `let def${defID} = _${widgetID}._start().then(() => _${widgetID}._render()).then(vnode=>{c${
+      `context.__widget__.cmap[${widgetID}] = _${widgetID}.__widget__.id`
+    );
+    ctx.addLine(
+      `def${defID} = _${widgetID}._start().then(() => _${widgetID}._render()).then(vnode=>{c${
         ctx.parentNode
       }[_${dummyID}_index]=vnode;vnode.data.hook = {create(_,vn){_${widgetID}._mount(vn)},remove(){_${widgetID}.destroy()}}});`
     );
-    ctx.addLine(`extra.promises.push(def${defID});`);
 
     let ref = node.getAttribute("t-ref");
     if (ref) {
       ctx.addLine(`context.refs['${ref}'] = _${widgetID};`);
     }
+    ctx.dedent();
+    ctx.addLine("}");
+    ctx.addLine(`extra.promises.push(def${defID});`);
+
     return true;
   }
 };

web/static/src/ts/widgets/Discuss.ts

@@ -56,10 +56,5 @@ export class Discuss extends Widget<Env> {
 
 class ColorWidget extends Widget<Env> {
   name = "colorwidget";
-  template = `<div>Current Color: <t t-esc="state.color"/></div>`;
-  state: { color: "red" | "blue" };
-  constructor(parent: Widget<Env>, props: { color: "red" | "blue" }) {
-    super(parent);
-    this.state = { color: props.color };
-  }
+  template = `<div>Current Color: <t t-esc="props.color"/></div>`;
 }

web/static/tests/core/widget.test.ts

@@ -516,6 +516,25 @@ describe("composition", () => {
 
     expect(children(widget)[0].env).toBe(env);
   });
+
+  test("rerendering a widget with a sub widget", async () => {
+    class ParentWidget extends Widget<WEnv> {
+      template = `<div><t t-widget="Counter"/></div>`;
+      widgets = { Counter };
+    }
+    const widget = new ParentWidget(env);
+    await widget.mount(fixture);
+    const button = fixture.getElementsByTagName("button")[0];
+    await button.click();
+    await nextTick();
+    expect(fixture.innerHTML).toBe(
+      "<div><div>1<button>Inc</button></div></div>"
+    );
+    await widget.render();
+    expect(fixture.innerHTML).toBe(
+      "<div><div>1<button>Inc</button></div></div>"
+    );
+  });
 });
 
 describe("props evaluation (with t-props directive)", () => {