如何检查NSData对象是否包含子NSData？

Question

我有一个NSData对象,其中包含我需要的一些数据.我想要做的是找出数据"FF D8"的位置(JPEG数据的开始)

我怎样才能实现这样的工作？

Answer 1

首先获取范围,然后获取数据:

// The magic start data object is only created once safely and 
// then reused each time
static NSData* magicStartData = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
  static const uint8_t magic[] = { 0xff, 0xd8 };
  magicStartData = [NSData dataWithBytesNoCopy:(void*)magic length:2 freeWhenDone:NO];
});

// assume data is the NSData with embedded data
NSRange range = [data rangeOfData:magicStartData options:0 range:NSMakeRange(0, [data length])];
if (range.location != NSNotFound) {
  // This assumes the subdata doesn't have a specific range and is just everything
  // after the magic, otherwise adjust
  NSData* subdata = [data subdataWithRange:NSMakeRange(range.location, [data length] - range.location)];
}