export function Mandatory() {
    return function(target: any, propertyKey: string) {
        const ngOnInit = target.constructor.prototype.ngOnInit;
        target.constructor.prototype.ngOnInit = function(...args) {
            if (target[propertyKey] === null || target[propertyKey] === undefined) {
                throw Error(
                    `mandatory property: ${propertyKey} not initialized. Current value is ${target[propertyKey]}`
                );
            }
            return ngOnInit && ngOnInit.apply(this, args);
        };
    };
}