首先需要引入用的的Cocoa类CommonCrypto，由于CommonCrypto没有兼容 Swift，需要将以下导入到项目的Bridging-Header.h文件中：

#import <CommonCrypto/CommonCrypto.h>

由于移动设备的内存有限，以下代码实现是将文件分块读出并且计算md5值的方法，兼容Swift 3.0：

func fileMD5(_ path: String) -> String? {
    
    let handle = FileHandle(forReadingAtPath: path)
    
    if handle == nil {
        return nil
    }

    let ctx = UnsafeMutablePointer<CC_MD5_CTX>.allocate(capacity: MemoryLayout<CC_MD5_CTX>.size)
    
    CC_MD5_Init(ctx)
    
    var done = false
    
    while !done {
        let fileData = handle?.readData(ofLength: 256)
        
        fileData?.withUnsafeBytes {(bytes: UnsafePointer<CChar>)->Void in
            //Use `bytes` inside this closure
            //...
            CC_MD5_Update(ctx, bytes, CC_LONG(fileData!.count))
        }
        
        if fileData?.count == 0 {
            done = true
        }
    }
    
    //unsigned char digest[CC_MD5_DIGEST_LENGTH];
    let digestLen = Int(CC_MD5_DIGEST_LENGTH)
    let digest = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: digestLen)
    CC_MD5_Final(digest, ctx);
    
    var hash = ""
    for i in 0..<digestLen {
        hash +=  String(format: "%02x", (digest[i]))
    }
    
    digest.deinitialize()
    ctx.deinitialize()
    
    return hash;

}