NgTypedChanges.ts

import { OnChanges, SimpleChange, SimpleChanges } from '@angular/core';

/**
 * {@link SimpleChange}, but with typed previous & current values.
 */
export type TypedChange<T> = Omit<SimpleChange, 'previousValue' | 'currentValue'> & {
	previousValue: T;
	currentValue: T;
	firstChange: boolean;
};

/**
 * {@link SimpleChanges}, but with members of the specified component type. IntelliSense will list
 * all public members of the component; not just the inputs. Use {@link ngHasChange} to detect
 * changes within this.
 */
export type TypedChanges<TComponent> = SimpleChanges & {
	[member in keyof TComponent]?: TypedChange<TComponent[member]>;
};

/**
 * {@link TypedChanges}, but with the specified members defined.
 */
export type TypedChangesWith<TComponent, TMember extends keyof TComponent> = TypedChanges<TComponent> &
	Required<Pick<TypedChanges<TComponent>, TMember>>;

/**
 * Returns whether the specified changes includes a change to the specified property of the component.
 */
export function ngHasChange<TComponent, TMember extends keyof TComponent>(
	changes: TypedChanges<TComponent>,
	member: TMember
): changes is TypedChangesWith<TComponent, TMember> {
	return member in changes;
}

/**
 * Implement this interface instead of {@link OnChanges} to receive {@link TypedChanges typed changes}
 * instead of (untyped) {@link SimpleChanges}.
 */
export interface OnTypedChanges<TComponent> extends OnChanges {
	ngOnChanges(changes: TypedChanges<TComponent>): void;
}