以下内容属于转载
在iOS中，uiimage没有用于修改大小的属性，要在代码中改变uiimage图片的大小，需要扩展UIImage类，如下：
头文件：
#import<UIKit/UIKit.h>
@interface UIImage (UIImageExtras)
- (UIImage *)imageByScalingToSize:(CGSize)targetSize;
@end
#import"UIImage+UIImageExtras.h"
@implementation UIImage (UIImageExtras)
- (UIImage *)imageByScalingToSize:(CGSize)targetSize
{
UIImage *sourceImage = self;
UIImage *newImage = nil;
CGSize imageSize = sourceImage.size;
CGFloat width = imageSize.width;
CGFloat height = imageSize.height;
CGFloat targetWidth = targetSize.width;
CGFloat targetHeight = targetSize.height;
CGFloat scaleFactor = 0.0;
CGFloat scaledWidth = targetWidth;
CGFloat scaledHeight = targetHeight;
CGPoint thumbnailPoint = CGPointMake(0.0,0.0);
if (CGSizeEqualToSize(imageSize, targetSize) ==NO) {
CGFloat widthFactor = targetWidth / width;
CGFloat heightFactor = targetHeight / height;
if (widthFactor < heightFactor)
scaleFactor = widthFactor;
else
scaleFactor = heightFactor;
scaledWidth = width * scaleFactor;
scaledHeight = height * scaleFactor;
// center the image
if (widthFactor < heightFactor) {
thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
} else if (widthFactor > heightFactor) {
thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
}
}
// this is actually the interesting part:
UIGraphicsBeginImageContext(targetSize);
CGRect thumbnailRect = CGRectZero;
thumbnailRect.origin = thumbnailPoint;
thumbnailRect.size.width = scaledWidth;
thumbnailRect.size.height = scaledHeight;
[sourceImage drawInRect:thumbnailRect];
newImage =UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
if(newImage == nil)
NSLog(@"could not scale image");
return newImage ;
}
@end
写好扩展类之后，就在要修改UIImage大小的viewcontroller.h头文件空先导入你的写好的扩展类，我的是#import “UIImage+UIImageExtras.h”
这样，你就可以调用你对UIImage写的那个扩展方法了。
我调用的地方
UIImage *tabbarimage=[UIImageimageNamed:@"xxx.png"];
CGSize imagesize=CGSizeMake(50.0, 50.0);
self.tabBarItem.image=[tabbarimageimageByScalingToSize:imagesize];
下面是网上另外一个扩展UIImage类的版本，头文件我就不发了，直接上.m文件
@implementationUIImage
(Category)-
(UIImage*)transformWidth:(CGFloat)width height:(CGFloat)height
{ CGFloat
destW = width; CGFloat
destH = height; CGFloat
sourceW = width; CGFloat
sourceH = height; CGImageRef
imageRef = self.CGImage; CGContextRef
bitmap = CGBitmapContextCreate(NULL, destW, destH, CGImageGetBitsPerComponent(imageRef), 4*destW, CGImageGetColorSpace(imageRef), (kCGBitmapByteOrder32Little
| kCGImageAlphaPremultipliedFirst)); CGContextDrawImage(bitmap,
CGRectMake(0, 0, sourceW, sourceH), imageRef); CGImageRef
ref = CGBitmapContextCreateImage(bitmap); UIImage
*result = [UIImage imageWithCGImage:ref]; CGContextRelease(bitmap); CGImageRelease(ref); returnresult;}@end原文：http://www.cnblogs.com/yfming/p/5634613.html