实例代码：将iOS中 UIImage根据屏宽调整size

#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 

CGSize size = image.size; 
image = [image imageByScalingToSize:CGSizeMake([UIScreen mainScreen].bounds.size.width,[UIScreen mainScreen].bounds.size.width * (size.height / size.width))]; 
self.imageview.image = image;