[FIX] store: properly call onUpdate functions in some cases

Before this commit, the following scenario could happen:

Suppose that we have a parent component A, connected to a store,
 and a child component B, also connected to the store and using
the onUpdate feature.

Then, we remount the A component in some other places and a
rendering is initiated in A.  We immediately update the store state.
What happens next is:

- rendering A is done, A internal revid is updated
- store update A (but nothing is done because the state change here
  does not modify A)
- rendering B is done (from parent), B internal revid is updated
- store update B, notice internal revid is updated, does not call the
  onUpdate function

We then have the B component which has not its internal state updated,
because we did not call its onUpdate function.

The solution is to move the onUpdate call in a "preupdate" event, to be
sure that it is called everytime the store is updated.

closes #816

src/store.ts

@@ -75,6 +75,11 @@ export class Store extends Context {
     );
     return result;
   }
+
+  __notifyComponents(): Promise<void> {
+    this.trigger("before-update");
+    return super.__notifyComponents();
+  }
 }
 
 interface SelectorOptions {
@@ -105,13 +110,16 @@ export function useStore(selector, options: SelectorOptions = {}): any {
     const newRevNumber = hashFn(result);
     if ((newRevNumber > 0 && revNumber !== newRevNumber) || !isEqual(oldResult, result)) {
       revNumber = newRevNumber;
-      if (options.onUpdate) {
-        options.onUpdate(result);
-      }
       return true;
     }
     return false;
   }
+  if (options.onUpdate) {
+    store.on("before-update", component, () => {
+      const newValue = selector(store!.state, component.props!);
+      options.onUpdate(newValue);
+    });
+  }
   store.updateFunctions[componentId].push(function (): boolean {
     return selectCompareUpdate(store!.state, component.props);
   });
@@ -132,6 +140,9 @@ export function useStore(selector, options: SelectorOptions = {}): any {
   const __destroy = component.__destroy;
   component.__destroy = (parent) => {
     delete store.updateFunctions[componentId];
+    if (options.onUpdate) {
+      store.off("before-update", component);
+    }
     __destroy.call(component, parent);
   };