add on change decorator

src/lib/utils/decorators/on-change.decorator.ts

@@ -0,0 +1,60 @@
+import { FunctionKeys } from '../types/utility-types';
+
+export interface SimpleChange<T> {
+    readonly previousValue: T;
+    readonly currentValue: T;
+    readonly isFirstChange: boolean;
+}
+
+export type CallBackFunction<T> = (value: T, change: SimpleChange<T>) => void;
+
+type TypedPropertyDecorator<C> = (target: C, key: PropertyKey) => void;
+
+const CACHED_VALUE_KEY = Symbol();
+const IS_FIRST_CHANGE_KEY = Symbol();
+
+interface Instance<T> {
+    [CACHED_VALUE_KEY]: T;
+    [IS_FIRST_CHANGE_KEY]: boolean;
+
+    [key: string]: unknown;
+}
+
+export function OnChange<T>(callback: CallBackFunction<T> | string): PropertyDecorator;
+// eslint-disable-next-line @typescript-eslint/ban-types
+export function OnChange<T, C extends Object = Object>(callback: CallBackFunction<T> | FunctionKeys<C>): TypedPropertyDecorator<C>;
+// eslint-disable-next-line @typescript-eslint/ban-types
+export function OnChange<T, C extends Object = Object>(callback: CallBackFunction<T> | FunctionKeys<C>): TypedPropertyDecorator<C> {
+    return function _onChange(target: C, key: PropertyKey) {
+        Object.defineProperty(target, key, {
+            set(value: T) {
+                const instance = this as Instance<T>;
+                const callBackFn = <CallBackFunction<T>>(typeof callback === 'string' ? instance[callback] : callback);
+                if (!callBackFn) {
+                    throw new Error(`Cannot find method ${String(callback)} in class ${target.constructor.name}`);
+                }
+
+                instance[IS_FIRST_CHANGE_KEY] = instance[IS_FIRST_CHANGE_KEY] === undefined;
+
+                // No operation if new value is same as old value
+                if (!instance[IS_FIRST_CHANGE_KEY] && instance[CACHED_VALUE_KEY] === value) {
+                    return;
+                }
+
+                const oldValue = instance[CACHED_VALUE_KEY];
+                instance[CACHED_VALUE_KEY] = value;
+
+                const simpleChange: SimpleChange<T> = {
+                    previousValue: oldValue,
+                    currentValue: instance[CACHED_VALUE_KEY],
+                    isFirstChange: instance[IS_FIRST_CHANGE_KEY]
+                };
+
+                callBackFn.call(instance, instance[CACHED_VALUE_KEY], simpleChange);
+            },
+            get(): T {
+                return (this as Instance<T>)[CACHED_VALUE_KEY];
+            }
+        });
+    };
+}

src/lib/utils/index.ts

@@ -8,3 +8,4 @@ export * from './types/tooltip-positions.type';
 export * from './decorators/bind.decorator';
 export * from './decorators/required.decorator';
 export * from './decorators/debounce.decorator';
+export * from './decorators/on-change.decorator';