imp: add willPatch hook

src/component.ts

@@ -149,6 +149,14 @@ export class Component<
    */
   mounted() {}
 
+  /**
+   * The willPatch hook is called just before the DOM patching process starts.
+   * It is not called on the initial render.  This is useful to get some
+   * information which are in the DOM.  For example, the current position of the
+   * scrollbar
+   */
+  willPatch() {}
+
   /**
    * This hook is called whenever a component did actually update its props,
    * state or env.
@@ -340,10 +348,12 @@ export class Component<
 
   _patch(vnode) {
     this.__widget__.renderPromise = null;
-    this.__widget__.vnode = patch(
-      this.__widget__.vnode || document.createElement(vnode.sel!),
-      vnode
-    );
+    if (this.__widget__.vnode) {
+      this.willPatch();
+      this.__widget__.vnode = patch(this.__widget__.vnode, vnode);
+    } else {
+      this.__widget__.vnode = patch(document.createElement(vnode.sel!), vnode);
+    }
   }
   async _start(): Promise<VNode> {
     this.__widget__.renderProps = this.props;

tests/component.test.ts

@@ -498,6 +498,45 @@ describe("lifecycle hooks", () => {
     await widget.updateState({ flag: false });
     expect(destroyed).toBe(true);
   });
+
+  test("willPatch/updated hook", async () => {
+    const steps: string[] = [];
+    class ParentWidget extends Widget {
+      inlineTemplate = `
+        <div>
+            <t t-widget="child" t-props="{v: state.n}"/>
+        </div>`;
+      widgets = { child: ChildWidget };
+      state = { n: 1 };
+      willPatch() {
+        steps.push("parent:willPatch");
+      }
+      componentDidUpdate() {
+        steps.push("parent:updated");
+      }
+    }
+
+    class ChildWidget extends Widget {
+      willPatch() {
+        steps.push("child:willPatch");
+      }
+      componentDidUpdate() {
+        steps.push("child:updated");
+      }
+    }
+    const widget = new ParentWidget(env);
+    await widget.mount(fixture);
+    expect(steps).toEqual([]);
+    await widget.updateState({ n: 2 });
+
+    // Not sure about this order.  If you disagree, feel free to open an issue...
+    expect(steps).toEqual([
+      "child:willPatch",
+      "child:updated",
+      "parent:willPatch",
+      "parent:updated"
+    ]);
+  });
 });
 
 describe("destroy method", () => {