An instance of the UIButton class implements a button on the touch screen. A button intercepts touch events and sends an action message to a target object when tapped.Methods for setting the target and actionare inherited from UIControl. This classPRovides methods for setting the title, image, and other appearance properties of a button. By using theseaccessors, you can specify a different appearance for each button state.（ios官方文档解释）
Creating Buttons
类方法：buttonWithType:
+ (id)buttonWithType:(UIButtonType)buttonType
参数：
typedefenum{
UIButtonTypeCustom=0,--No button style
UIButtonTypeSystem,--A system style button, such as those shown in navigation bars andtoolbars（Available in iOS 7.0 and later）.
UIButtonTypeDetailDisclosure,--A detail disclosure button
UIButtonTypeInfoLight,--An information button that has a light background
UIButtonTypeInfoDark,--An information button that has a dark background
UIButtonTypeContactAdd,--A contact add button
UIButtonTypeRoundedRect,--A rounded-rectangle style button
}UIButtonType;
Configuring the Button Title属性：titleLabel:
@property(nonatomic, readonly, retain) UILabel *titleLabel
A view that displays the value of the currentTitle property for a button. (read-only)
Use its own properties primarliy to configure the text of the button.
use thesetTitleColor:forState:andsetTitleShadowColor:forState:mothods of this class to make those changes.
属性：currentTitle:
@property(nonatomic, readonly, retain) NSString *currentTitle
The value for this property is set automatically whenever the button state changes.
titleForState:
Returns the title associated with the specified state.
- (NSString *)titleForState:(UIControlState)state
参数： UIControlState
方法：- setTitle:forState:
Sets the title to use for the specified state.
- (void)setTitle:(NSString *)title
forState:(UIControlState)state
方法：attributedTitleForState:
方法：setAttributedTitle:forState
方法：titleColorForState:
方法： setTitleColor:forState:
方法：titleShadowColorForState:
方法：setTitleShadowColor:forState:
属性：reversesTitleShadowWhenHighlighted：
Configuring Button Presentation属性：adjustsImageWhenHighlighted
A Boolean value that determines whether the image changes when the button is highlighted.
@property(nonatomic) BOOL adjustsImageWhenHighlighted
属性：adjustsImageWhenDisabled
A Boolean value that determines whether the image changes when the button is disabled.
@property(nonatomic) BOOL adjustsImageWhenDisabled
属性：showsTouchWhenHighlighted
A Boolean value that determines whether tapping the button causes it to glow.
@property(nonatomic) BOOL showsTouchWhenHighlighted
backgroundImageForState
imageForState:
setBackgroundImage:forState:
setImage:forState:
属性：tintColor
文档链接：https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIButton_Class/index.html
参考资料：
View Programming Guide for iOS.
Buttons.