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 NS_CLASS_AVAILABLE_IOS(2_0) @interface UIControl : UIView

 //1.1设置控件的状态
     启用 禁用：一个按钮可不可以点击

 @property(nonatomic,getter=isEnabled) BOOL enabled;                                  // default is YES. if NO, ignores touch events and subclasses may draw differently
     选中 不选中：

 @property(nonatomic,getter=isSelected) BOOL selected;                                // default is NO may be used by some subclasses or by application
     高亮或者不高亮

 @property(nonatomic,getter=isHighlighted) BOOL highlighted;                          // default is NO. this gets set/cleared automatically when touch enters/exits during tracking and cleared on up

 
1.2设置控件内容的布局
   垂直居中方向
9 @property(nonatomic) UIControlContentVerticalAlignment contentVerticalAlignment;     // how to position content vertically inside control. default is center
   水平居中方向

 @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:(nullable UIEvent *)event;

 - (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(nullable UIEvent *)event;

 - (void)endTrackingWithTouch:(nullable UITouch *)touch withEvent:(nullable UIEvent *)event; // touch is sometimes nil if cancelTracking calls through to this.

 - (void)cancelTrackingWithEvent:(nullable 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.

 添加或者删除监听方法

- (void)addTarget:(nullable 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:(nullable id)target action:(nullable 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

 - (nullable NSArray<NSString *> *)actionsForTarget:(nullable 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:(nullable id)target forEvent:(nullable UIEvent *)event;

 - (void)sendActionsForControlEvents:(UIControlEvents)controlEvents;                        // send all actions associated with events

 @end

 NS_ASSUME_NONNULL_END