NSInputStream 和 NSOutputStream

对于 NSInputStream 和 NSOutputStream 一直没怎么搞清楚，今天抽一些时间在此记录一下！
  
NSInputStream 与 NSOutputStream 都继承于 NSStream, NSStream 是一个抽象的基类, 规定了Stream共有的一些行为…
  
什么是Stream
  
Stream翻译成为流，它是对我们读写文件的一个抽象。 你可以这样想象，当你读文件和写文件的时候，文件的内容就像水流一样哗哗的 像你流过来或者流给别人，这样岂不是很爽。 而Stream就帮我们做了这样的事情， 实际上，它是把文件的内容，一小段一小段的读出或 写入，来到达这样的效果
  
NSStream
  
NSStream 是Cocoa平台下对流这个概念的实现类， NSInputStream 和 NSOutputStream 则是它的两个子类，分别对应了读文件和 写文件。
  
NSInputStream
  
NSInputStream 对应的是读文件，所以要记住它是要将文件的内容读到内存(你声明的一段buffer)里, 下面一段是测试代码
         
- (void)doTestInputStream {  NSString *path = @"/Users/usr/Desktop/stream.txt";    NSInputStream *readStream = [[NSInputStream alloc]initWithFileAtPath:path];  [readStream setDelegate:self];    [readStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];  [readStream open]; //调用open开始读文件}- (void)stream:(NSStream *)aStream handleEvent:(NSStreamEvent)eventCode {  switch (eventCode) {      case NSStreamEventHasBytesAvailable:{                    uint8_t buf[1024];                    NSInputStream *reads = (NSInputStream *)aStream;          NSInteger blength = [reads read:buf maxLength:sizeof(buf)]; //把流的数据放入buffer          NSData *data = [NSData dataWithBytes:(void *)buf length:blength];                    NSString *string = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];          NSLog(@"%@",string);      }          break;                        //错误和无事件处理      case NSStreamEventErrorOccurred:{                }          break;      case NSStreamEventNone:          break;            //打开完成      case NSStreamEventOpenCompleted: {          NSLog(@"NSStreamEventOpenCompleted");      }          break;                default:          break;  }}@end
        
NSOutputStream
  
NSOutputStream 对应的是写文件，它是要将已存在的内存(buffer)里的数据写入文件, 下面同样一段是测试代码
         
- (NSData *)dataWillWrite {  static  NSData *data = nil;  static dispatch_once_t onceToken;  dispatch_once(&onceToken, ^{      data = [NSData dataWithContentsOfFile:@"/Users/usr/Desktop/stream.txt"];  });    return data;}- (void)doTestOutputStream {  NSString *path = @"/Users/usr/Desktop/stream-write.txt";    NSOutputStream *writeStream = [[NSOutputStream alloc] initToFileAtPath:path append:YES];  [writeStream setDelegate:self];    [writeStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];  [writeStream open];}- (void)stream:(NSStream *)aStream handleEvent:(NSStreamEvent)eventCode {  switch (eventCode) {      case NSStreamEventHasSpaceAvailable: {                    NSInteger bufSize = 5;          uint8_t buf[bufSize];                    if (self.location + bufSize > [self dataWillWrite].length) {              [[self dataWillWrite] getBytes:buf                                       range:NSMakeRange(self.location, self.location + bufSize - [self dataWillWrite].length)];          }          else {              [[self dataWillWrite] getBytes:buf range:NSMakeRange(self.location, bufSize)];          }                    NSOutputStream *writeStream = (NSOutputStream *)aStream;          [writeStream write:buf maxLength:sizeof(buf)]; //把buffer里的数据，写入文件                    self.location += bufSize;          if (self.location >= [[self dataWillWrite] length] ) { //写完后关闭流              [aStream close];          }                }          break;                case NSStreamEventEndEncountered: {          [aStream close];      }          break;            //错误和无事件处理      case NSStreamEventErrorOccurred:{                }          break;      case NSStreamEventNone:          break;            //打开完成      case NSStreamEventOpenCompleted: {          NSLog(@"NSStreamEventOpenCompleted");                          }          break;                default:          break;  }}
        
用途
  
NSInputStream 和 NSOutputStream 常用与网络传输中，比如要将一个很大的文件传送给服务器，那么NSInputStream这时候是 很好的选择, 我们可以查看到 NSURLRequest 有一个属性叫HTTPBodyStream， 这时只要设置好一个NSInputStream的实例就可以 了，最大的好处就是可以节省我们很多的内存。
  
另外要说明的是，NSInputStream 和 NSOutputStream其实是对 CoreFoundation 层对应的CFReadStreamRef 和 CFWriteStreamRef 的高层抽象。在使用CFNetwork时，常常会使用到CFReadStreamRef 与 CFWriteStreamRef。 下面是一段相关代码
                           
 // Keep a reference to self to use for controller callbacks    //  CFStreamClientContext ctx = {0, (__bridge void *)(self), NULL, NULL, NULL};    // Get callbacks for stream data, stream end, and any errors    //  CFOptionFlags registeredEvents = (kCFStreamEventHasBytesAvailable | kCFStreamEventEndEncountered | kCFStreamEventErrorOccurred);    // Create a read-only socket    //  CFReadStreamRef readStream;  CFStreamCreatePairWithSocketToHost(kCFAllocatorDefault, (__bridge CFStringRef)host, (UInt32)port, &readStream, NULL);    // Schedule the stream on the run loop to enable callbacks    //  if (CFReadStreamSetClient(readStream, registeredEvents, socketCallback, &ctx)) {      CFReadStreamScheduleWithRunLoop(readStream, CFRunLoopGetCurrent(), kCFRunLoopCommonModes);        }    else {        [self networkFailedWithErrorMessage:@"Failed to assign callback method"];      return;  }    // Open the stream for reading    //  if (CFReadStreamOpen(readStream) == NO) {        [self networkFailedWithErrorMessage:@"Failed to open read stream"];            return;  }    CFErrorRef error = CFReadStreamCopyError(readStream);  if (error != NULL) {      if (CFErrorGetCode(error) != 0) {          NSString * errorInfo = [NSString stringWithFormat:@"Failed to connect stream; error '%@' (code %ld)", (__bridge NSString*)CFErrorGetDomain(error), CFErrorGetCode(error)];            [self networkFailedWithErrorMessage:errorInfo];      }            CFRelease(error);            return;  }