1 files changed, 44 insertions, 0 deletions
diff --git a/FrontEnd/src/app/common.ts b/FrontEnd/src/app/common.ts
new file mode 100644
index 00000000..0a2d345f
--- /dev/null
+++ b/FrontEnd/src/app/common.ts
@@ -0,0 +1,44 @@
+import React from "react";
+import { Observable, Subject } from "rxjs";
+
+// This error is thrown when ui goes wrong with bad logic.
+// Such as a variable should not be null, but it does.
+// This error should never occur. If it does, it indicates there is some logic bug in codes.
+export class UiLogicError extends Error {}
+
+export function useEventEmiiter(): [() => Observable<null>, () => void] {
+  const ref = React.useRef<Subject<null> | null>(null);
+
+  return React.useMemo(() => {
+    const getter = (): Subject<null> => {
+      if (ref.current == null) {
+        ref.current = new Subject<null>();
+      }
+      return ref.current;
+    };
+    const trigger = (): void => {
+      getter().next(null);
+    };
+    return [getter, trigger];
+  }, []);
+}
+
+export function useValueEventEmiiter<T>(): [
+  () => Observable<T>,
+  (value: T) => void
+] {
+  const ref = React.useRef<Subject<T> | null>(null);
+
+  return React.useMemo(() => {
+    const getter = (): Subject<T> => {
+      if (ref.current == null) {
+        ref.current = new Subject<T>();
+      }
+      return ref.current;
+    };
+    const trigger = (value: T): void => {
+      getter().next(value);
+    };
+    return [getter, trigger];
+  }, []);
+}