动态缩小UIImage的函数

有时候需要动态缩小图片，这样你可以用UIImage来缩小图片，代码如下：

@implementation UIImage (Extras) 
#pragma mark - 
#pragma mark Scale and crop image 
- (UIImage*)imageByScalingAndCroppingForSize:(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; // scale to fit height 
else 
scaleFactor = heightFactor; // scale to fit width 
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; 
} 
} 
UIGraphicsBeginImageContext(targetSize); // this will crop 
CGRect thumbnailRect = CGRectZero; 
thumbnailRect.origin = thumbnailPoint; 
thumbnailRect.size.width = scaledWidth; 
thumbnailRect.size.height = scaledHeight; 
[sourceImage drawInRect:thumbnailRect]; 
newImage = UIGraphicsGetImageFromCurrentImageContext(); 
if(newImage == nil) 
NSLog(@"could not scale image"); 
//pop the context to get back to the default 
UIGraphicsEndImageContext(); 
return newImage; 
}