我们都知道微信分享下缩略图的大小是有限制的（不超过32kb），不然地话就会出现点击分享无法调出微信的情况，为此我们必须对分享的图片进行压缩。

常规的图片压缩，有两种方法：压缩质量和压缩尺寸。

压缩质量

NSData *data = UIImageJPEGRepresentation(image, compression);
UIImage *resultImage = [UIImage imageWithData:data];

压缩尺寸

UIGraphicsBeginImageContext(size);
[image drawInRect:CGRectMake(0, 0, size.width, size.height)];
resultImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

这两种方法各有什么区别呢？

压缩图片质量的优点在于，尽可能保留图片清晰度，图片不会明显模糊，缺点在于当图片质量低于一定程度时，继续压缩没有效果，所以不能确保压缩后的大小；

压缩图片尺寸可以使图片小于指定大小，但会使图片明显模糊(比压缩图片质量模糊)。

为此，针对微信缩略图的情况，我们最终需要图片大小小于32kb，但是我们同样希望保持图片质量，那么就需要两种方法进行结合。首先循环压缩质量，如果大小仍然大于32kb，再对图片尺寸进行压缩，直到接近32kb。

这里有童鞋写出了详细的压缩方法和技术细节（戳这里），并给出了相关代码，经实地测试，代码可用

oc代码:

+ (UIImage *)compressImage:(UIImage *)image toByte:(NSUInteger)maxLength {
    // Compress by quality
    CGFloat compression = 1;
    NSData *data = UIImageJPEGRepresentation(image, compression);
    if (data.length < maxLength) return image;
    
    CGFloat max = 1;
    CGFloat min = 0;
    for (int i = 0; i < 6; ++i) {
        compression = (max + min) / 2;
        data = UIImageJPEGRepresentation(image, compression);
        if (data.length < maxLength * 0.9) {
            min = compression;
        } else if (data.length > maxLength) {
            max = compression;
        } else {
            break;
        }
    }
    UIImage *resultImage = [UIImage imageWithData:data];
    if (data.length < maxLength) return resultImage;
    
    // Compress by size
    NSUInteger lastDataLength = 0;
    while (data.length > maxLength && data.length != lastDataLength) {
        lastDataLength = data.length;
        CGFloat ratio = (CGFloat)maxLength / data.length;
        CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)),
                                 (NSUInteger)(resultImage.size.height * sqrtf(ratio))); // Use NSUInteger to prevent white blank
        UIGraphicsBeginImageContext(size);
        [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
        resultImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        data = UIImageJPEGRepresentation(resultImage, compression);
    }
    
    return resultImage;
}

swift代码：

static func compressImage(_ image: UIImage, toByte maxLength: Int) -> UIImage {
    var compression: CGFloat = 1
    guard var data = UIImageJPEGRepresentation(image, compression),
        data.count > maxLength else { return image }
    
    // Compress by size
    var max: CGFloat = 1
    var min: CGFloat = 0
    for _ in 0..<6 {
        compression = (max + min) / 2
        data = UIImageJPEGRepresentation(image, compression)!
        if CGFloat(data.count) < CGFloat(maxLength) * 0.9 {
            min = compression
        } else if data.count > maxLength {
            max = compression
        } else {
            break
        }
    }
    var resultImage: UIImage = UIImage(data: data)!
    if data.count < maxLength { return resultImage }
    
    // Compress by size
    var lastDataLength: Int = 0
    while data.count > maxLength, data.count != lastDataLength {
        lastDataLength = data.count
        let ratio: CGFloat = CGFloat(maxLength) / CGFloat(data.count)
        let size: CGSize = CGSize(width: Int(resultImage.size.width * sqrt(ratio)),
                                height: Int(resultImage.size.height * sqrt(ratio)))
        UIGraphicsBeginImageContext(size)
        resultImage.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
        resultImage = UIGraphicsGetImageFromCurrentImageContext()!
        UIGraphicsEndImageContext()
        data = UIImageJPEGRepresentation(resultImage, compression)!
    }
    return resultImage
}