Cocoa touch（五）：UIControl

UIControl，是所有可响应事件控件的基类，通过观察者模式实现了事件的处理，

注册事件观察者

// target是注册的观察者，action为响应方法，后边是观察的事件
- (void)addTarget:(id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;

发送事件

// 触发某控件的事件
- (void)sendAction:(SEL)action to:(id)target forEvent:(UIEvent *)event;

控件具有几种状态，常用的是前面4种，看后面注释

typedef NS_OPTIONS(NSUInteger, UIControlState) {
    UIControlStateNormal       = 0,
    UIControlStateHighlighted  = 1 << 0,                  // used when UIControl isHighlighted is set
    UIControlStateDisabled     = 1 << 1,
    UIControlStateSelected     = 1 << 2,                  // flag usable by app (see below)
    UIControlStateApplication  = 0x00FF0000,              // additional flags available for application use
    UIControlStateReserved     = 0xFF000000               // flags reserved for internal framework use
};

@interface UIControl : UIView {

@property(nonatomic,getter=isEnabled) BOOL enabled;                                  // default is YES. if NO, ignores touch events and subclasses may draw differently
@property(nonatomic,getter=isSelected) BOOL selected;                                // default is NO may be used by some subclasses or by application
@property(nonatomic,getter=isHighlighted) BOOL highlighted;                          // default is NO. this gets set/cleared automatically when touch enters/exits during tracking and cleared on up
@property(nonatomic) UIControlContentVerticalAlignment contentVerticalAlignment;     // how to position content vertically inside control. default is center
@property(nonatomic) UIControlContentHorizontalAlignment contentHorizontalAlignment; // how to position content hozontally inside control. default is center

@property(nonatomic,readonly) UIControlState state;                  // could be more than one state (e.g. disabled|selected). synthesized from other flags.
@property(nonatomic,readonly,getter=isTracking) BOOL tracking;
@property(nonatomic,readonly,getter=isTouchInside) BOOL touchInside; // valid during tracking only

- (BOOL)beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (void)endTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (void)cancelTrackingWithEvent:(UIEvent *)event;   // event may be nil if cancelled for non-event reasons, e.g. removed from window

// add target/action for particular event. you can call this multiple times and you can specify multiple target/actions for a particular event.
// passing in nil as the target goes up the responder chain. The action may optionally include the sender and the event in that order
// the action cannot be NULL. Note that the target is not retained.
- (void)addTarget:(id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;

// remove the target/action for a set of events. pass in NULL for the action to remove all actions for that target
- (void)removeTarget:(id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;

// get info about target & actions. this makes it possible to enumerate all target/actions by checking for each event kind
- (NSSet *)allTargets;                                                                     // set may include NSNull to indicate at least one nil target
- (UIControlEvents)allControlEvents;                                                       // list of all events that have at least one action
- (NSArray *)actionsForTarget:(id)target forControlEvent:(UIControlEvents)controlEvent;    // single event. returns NSArray of NSString selector names. returns nil if none

// send the action. the first method is called for the event and is a point at which you can observe or override behavior. it is called repeately by the second.
- (void)sendAction:(SEL)action to:(id)target forEvent:(UIEvent *)event;
- (void)sendActionsForControlEvents:(UIControlEvents)controlEvents;                        // send all actions associated with events

@end