UIImage *image = [UIImage imageNamed:@"123"];
    NSData *data = UIImagePNGRepresentation(image);
    // 每次读取1024的长度
    NSInteger readLength = 1024;
    NSInteger count = data.length / readLength;
    NSMutableData *mutData = [NSMutableData data];
    char bytes[data.length];
    for (int i = 0; i < count; i++) {
        [data getBytes:bytes range:NSMakeRange(i * readLength, readLength)];
        NSData *newData = [NSData dataWithBytes:bytes length:readLength];
        [mutData appendData:newData];
    // 最后剩余的部分
    NSInteger res = data.length % readLength;
    if (res > 0) {
        [data getBytes:bytes range:NSMakeRange(data.length - res, res)];
        NSData *newData = [NSData dataWithBytes:bytes length:res];
        [mutData appendData:newData];
    }
　　
　　NSLog(@"resulteData:%ld", mutData.length);



==================swift版本=======================

if let image = UIImage( named: "123"), let data = image.pngData(), data. count > 0 {

let nsdata = data as NSData

var bytes = [ UInt8](repeating: 0, count: nsdata. count)

nsdata. getBytes(&bytes, length: nsdata. count)

let readLength = 1024

let mutData = NSMutableData()

let count = bytes. count / readLength

for i in 0..<count {

let start = i * readLength

let values = bytes[start..<(start + readLength)]

let value = NSData( bytes: Array(values), length: values.count)

mutData. append(value as Data)

let reset = bytes.count % (readLength)

if reset > 0 {

let start = bytes. count - reset

let values = bytes[start..<bytes. count]

let value = NSData( bytes: Array(values), length: values.count)

mutData. append(value as Data)

print( "mutData.count:\(mutData. count) ")