refactor: improve the typing of fromProp

Author: rocwang

src/Grid.vue

@@ -70,9 +70,9 @@ export default defineComponent({
   },
   setup(props) {
     // region: props
-    const length$ = fromProp<number>("length", props);
-    const pageSize$ = fromProp<number>("pageSize", props);
-    const pageProvider$ = fromProp<PageProvider>("pageProvider", props);
+    const length$ = fromProp(props, "length");
+    const pageSize$ = fromProp(props, "pageSize");
+    const pageProvider$ = fromProp(props, "pageProvider");
     // endregion
 
     // region: refs

src/utilites.ts

@@ -4,11 +4,11 @@ import { partial, pipe, unary } from "ramda";
 import { useResizeObserver } from "@vueuse/core";
 import { mergeAll } from "rxjs/operators";
 
-export function fromProp<T>(
-  propName: string,
-  props: Record<string, any>
-): Observable<T> {
-  return new Observable<T>((subscriber) =>
+export function fromProp<T, U extends keyof T>(
+  props: T,
+  propName: U
+): Observable<T[U]> {
+  return new Observable((subscriber) =>
     watchEffect(() => subscriber.next(props[propName]))
   );
 }