使用NSMutableData实例的字节，同时允许它自动释放

I've been using

我一直在用

NSMutableData* mutableData = [NSMutableData dataWithLength: someLength];
void* bitmapData = [mutableData mutableBytes];
CGContextRef context = CGBitmapContextCreate(bitmapData,...);
// ...use context
CGContextRelease(context);
NSMutableData* mutab