创建

来自 NSString：

NSString *str = @"Hello world";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];

来自 Int：

int i = 1;
NSData *data = [NSData dataWithBytes: &i length: sizeof(i)];

你还可以使用以下方法：

+ dataWithContentsOfURL:
+ dataWithContentsOfURL:options:error:
+ dataWithData:
- initWithBase64EncodedData:options:
- initWithBase64EncodedString:options:
- initWithBase64Encoding:
- initWithBytesNoCopy:length:
- initWithBytesNoCopy:length:deallocator:
- initWithBytesNoCopy:length:freeWhenDone:
- initWithContentsOfFile:
- initWithContentsOfFile:options:error:
- initWithContentsOfMappedFile:
- initWithContentsOfURL:
- initWithContentsOfURL:options:error:
- initWithData: