ios中图片拉伸用法

- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight;

Creates and returns a new image object with the specified cap values.

Deprecation Statement
Deprecated. Use the resizableImageWithCapInsets: instead, specifying cap insets such that the interior is a 1x1 area.

Declaration
SWIFT
func stretchableImageWithLeftCapWidth(_ leftCapWidth: Int,
                         topCapHeight topCapHeight: Int) -> UIImage
OBJECTIVE-C
- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth
                                 topCapHeight:(NSInteger)topCapHeight
Parameters
leftCapWidth    
The value to use for the left cap width. Specify 0 if you want the entire image to be horizontally stretchable. For a discussion of how a non-zero value affects the image, see the leftCapWidth property.
topCapHeight    
The value to use for the top cap width. Specify 0 if you want the entire image to be vertically stretchable. For a discussion of how a non-zero value affects the image, see the topCapHeight property.
Return Value
A new image object with the specified cap values.

Discussion
During scaling or resizing of the image, areas covered by a cap are not scaled or resized. Instead, the 1-pixel wide area not covered by the cap in each direction is what is scaled or resized. This technique is often used to create variable-width buttons, which retain the same rounded corners but whose center region grows or shrinks as needed.

You use this method to add cap values to an image or to change the existing cap values of an image. In both cases, you get back a new image and the original image remains untouched.

Import Statement
OBJECTIVE-C
@import UIKit;
SWIFT
import UIKit
Availability
Available in iOS 2.0 and later.
Deprecated in iOS 5.0.

stackoverflow中的注释相关信息http://stackoverflow.com/questions/12623961/stretch-background-image-for-uibutton/12625066#12625066