ts 之 函数参数双向协变

enum EventType { Mouse, Keyboard }

interface Event { timestamp: number; }
interface MouseEvent extends Event { xa : number; yb: number }
interface KeyEvent extends Event { keyCode: number }



function listenEvent(eventType: EventType, handler: (n: Event) => void) {
    /* ... */
}

// Unsound, but useful and common 不健全的 但是有用的和普遍的
listenEvent(EventType.Mouse, (e: MouseEvent) => console.log(e.x + ',' + e.y));

// Undesirable alternatives in presence of soundness  不可靠的替代品
listenEvent(EventType.Mouse, (e: Event) => console.log((<MouseEvent>e).x + ',' + (<MouseEvent>e).y));
listenEvent(EventType.Mouse, <(e: Event) => void> ((e: MouseEvent) => console.log(e.x + ',' + e.y)));

// Still disallowed (clear error). Type safety enforced for wholly incompatible types
// 还不允许（清晰的错误）。完全不兼容类型强制执行的类型安全
listenEvent(EventType.Mouse, (e: number) => console.log(e));