ios 等比缩小图片

//按比例缩放,size 是你要把图显示到 多大区域 CGSizeMake(300, 140)
-(UIImage *) imageCompressForSize:(UIImage *)sourceImage targetSize:(CGSize)size{
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = size.width;
    CGFloat targetHeight = size.height;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0, 0.0);
    if(CGSizeEqualToSize(imageSize, size) == NO){
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if(widthFactor > heightFactor){
            scaleFactor = widthFactor;
        }
        else{
            scaleFactor = heightFactor;
        }
        scaledWidth = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        if(widthFactor > heightFactor){
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }else if(widthFactor < heightFactor){
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    UIGraphicsBeginImageContext(size);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    if(newImage == nil){
        NSLog(@"scale image fail");
    }
    UIGraphicsEndImageContext();
    return newImage;
}

//指定宽度按比例缩放
-(UIImage *) imageCompressForWidth:(UIImage *)sourceImage targetWidth:(CGFloat)defineWidth{
    UIImage *newImage = nil;
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    CGFloat targetWidth = defineWidth;
    CGFloat targetHeight = height / (width / targetWidth);
    CGSize size = CGSizeMake(targetWidth, targetHeight);
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    CGPoint thumbnailPoint = CGPointMake(0.0, 0.0);
    if(CGSizeEqualToSize(imageSize, size) == NO){
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        if(widthFactor > heightFactor){
            scaleFactor = widthFactor;
        }
        else{
            scaleFactor = heightFactor;
        }
        scaledWidth = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        if(widthFactor > heightFactor){
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
        }else if(widthFactor < heightFactor){
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
        }
    }
    UIGraphicsBeginImageContext(size);
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    [sourceImage drawInRect:thumbnailRect];
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    if(newImage == nil){
        NSLog(@"scale image fail");
    }
    UIGraphicsEndImageContext();
    return newImage;
}