Objective-C修改图片尺寸/生成缩略图
+(UIImage *)imageResize :(UIImage*)img andResizeTo:(CGSize)newSize
{
CGFloat scale = [[UIScreen mainScreen]scale];
//UIGraphicsBeginImageContext(newSize);
UIGraphicsBeginImageContextWithOptions(newSize, NO, scale);
[img drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
} 
