Angular2从调用服务的构造函数中的类访问@input

Question

我对Angular2和打字是新手。我试图从我的类中的构造函数获得对@input的访问权。一个服务是从构造函数内部调用的，我需要给它传递一个参数。

@Component({
    selector:'side-menu',
    templateUrl:'menu.html',
    providers:    [MenuService]
 })

export class SideMenuComponent {
    @Input() blockType;

    menuItems

    constructor(menuService:MenuService){
        this.menuItems = menuService.getMenuItems(this.blockType); // Sends Undefined
        this.menuItems = menuService.getMenuItems('HardCodedString'); //This works
    }


}

    @Injectable()

export class MenuService {
    getMenuItems(type) {
        console.log(type)

        switch(type){
            case 'Thing':

                return [];
                break;
            case 'Otherthing':
                return [];
                break;
            default:
                return [];


        }

    }
}

Answer 1

调用构造函数时尚未设置输入。将构造函数代码移到ngoninit()。

@Component({
    selector:'side-menu',
    templateUrl:'menu.html',
    providers:    [MenuService]
 })

export class SideMenuComponent {
    @Input() blockType;

    menuItems

    constructor(private menuService:MenuService){}

    ngOnInit() {
        this.menuItems = this.menuService.getMenuItems(this.blockType); // Sends Undefined
        this.menuItems = this.menuService.getMenuItems('HardCodedString'); //This works
    }
}

每次更新@input()时，都会调用ngonchanges()。
ngonchanges()在第一次调用ngonchanges()之后调用ngoninit()。