我們都知道微信分享下縮略圖的大小是有限制的（不超過(guò)32kb），不然地話就會(huì)出現(xiàn)點(diǎn)擊分享無(wú)法調(diào)出微信的情況，為此我們必須對(duì)分享的圖片進(jìn)行壓縮。

常規(guī)的圖片壓縮，有兩種方法：壓縮質(zhì)量和壓縮尺寸。

壓縮質(zhì)量

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

壓縮尺寸

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

這兩種方法各有什么區(qū)別呢？

壓縮圖片質(zhì)量的優(yōu)點(diǎn)在于，盡可能保留圖片清晰度，圖片不會(huì)明顯模糊，缺點(diǎn)在于當(dāng)圖片質(zhì)量低于一定程度時(shí)，繼續(xù)壓縮沒(méi)有效果，所以不能確保壓縮后的大??；

壓縮圖片尺寸可以使圖片小于指定大小，但會(huì)使圖片明顯模糊(比壓縮圖片質(zhì)量模糊)。

為此，針對(duì)微信縮略圖的情況，我們最終需要圖片大小小于32kb，但是我們同樣希望保持圖片質(zhì)量，那么就需要兩種方法進(jìn)行結(jié)合。首先循環(huán)壓縮質(zhì)量，如果大小仍然大于32kb，再對(duì)圖片尺寸進(jìn)行壓縮，直到接近32kb。

這里有童鞋寫出了詳細(xì)的壓縮方法和技術(shù)細(xì)節(jié)（戳這里），并給出了相關(guān)代碼，經(jīng)實(shí)地測(cè)試，代碼可用

oc代碼:

+ (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代碼：

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
}