UIResponder Class Reference
Managing the Responder Chain
- 1.- (UIResponder *)nextResponder
UIResponder类不自动存储和设置下一个响应者，而是默认返回nil。子类必须override这个方法来设置下一个响应者。
UIView实现了这个方法，因为可以返回管理这个UIView的UIViewController或者它的父类；
UIViewController实现了这个方法，返回UIViewController的View的父View；
UIWindow发挥UIApplication对象；
UIApplication返回nil
- 2.- (BOOL)isFirstResponder
- 3.- (BOOL)canBecomeFirstResponder
如果一个响应对象通过这个方法返回YES，那么它成为了第一响应对象，并且可以接收触摸事件和动作消息。
子类必须overrider这个方法才可以成为第一响应者。
You must not send this message to a view that is not currently attached to the view hierarchy. The result is undefined.
- 3.- (BOOL)becomeFirstResponder
子类可以override这个方法来更新状态或者执行一些行为，比如高亮选中项。
一个响应对象只有当前响应者可以放弃第一响应者状态，并且新的响应者可以成为第一响应者，才能成为第一响应对象。
- 4.- (BOOL)canResignFirstResponder
- 5.- (BOOL)resignFirstResponder
如果返回NO，拒绝放弃第一响应状态。
如果你override这个方法，必须调用父类的实现[super resignFirstResponder].
Managing Input Views
- 1.@property (readonly, retain) UIView *inputView
This property is typically used to replace the system-supplied keyboard that is presented for UITextField and UITextView objects.
UITextField和UITextView如果设置了inputView那么在 becomeFirstResponder时不会显示键盘，而现实自定义的inputView；如果设置了inputAccessoryView那么在 becomeFirstResponder时会在键盘的顶端显示自定义的inputAccessoryView。
- 2.@property (readonly, retain) UIView *inputAccessoryView
This property is typically used to attach an accessory view to the system-supplied keyboard that is presented for UITextField and UITextView objects.
- 3.- (void)reloadInputViews
Responding to Touch Events
- 1.- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
将消息转发给下一个响应者，将消息发送给父类，不要将消息直接传递给下一个响应者。
如果你override这个方法而没有调用super..，你必须同样override其它响应触摸事件的方法，你要是空实现就好。
默认是不支持多点触摸的，如果想要响应多点触摸，你只要吧UIView的 multipleTouchEnabled 属性设置为YES即可。
- 2.- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
- 3.- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
- 4.- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
Responding to Motion Events
- 1.- (void)motionBegan:(UIEventSubtype)motion withEvent:(UIEvent *)event
- 2.- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event
- 3.- (void)motionCancelled:(UIEventSubtype)motion withEvent:(UIEvent *)event
Responding to Remote-Control Events
- 1.- (void)remoteControlReceivedWithEvent:(UIEvent *)event
Getting the Undo Manager
- 1.@property(readonly) NSUndoManager *undoManager
Validating Commands
- 1.- (BOOL)canPerformAction:(SEL)action withSender:(id)sender
YES if the the command identified by action should be enabled or NO if it should be disabled. Returning YES means that your class can handle the command in the current context.