ios4下不使用私有API，轻松打开摄像头，获取摄像流

第一步：初始化AVCaptureSession，添加输入，输出源 

 #import <AVFoundation/AVFoundation.h>

 // Create and configure a capture session and start it running
 - (void)setupCaptureSession
 {
     NSError *error = nil;

     // Create the session
     AVCaptureSession *session = [[AVCaptureSession alloc] init];

     // Configure the session to produce lower resolution video frames, if your
     // processing algorithm can cope. We'll specify medium quality for the
     // chosen device.
     session.sessionPreset = AVCaptureSessionPresetMedium;

     // Find a suitable AVCaptureDevice
     AVCaptureDevice *device = [AVCaptureDevice
                              defaultDeviceWithMediaType:AVMediaTypeVideo];

     // Create a device input with the device and add it to the session.
     AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device
                                                                     error:&error];
     if (!input) {
         // Handling the error appropriately.
     }
     [session addInput:input];

     // Create a VideoDataOutput and add it to the session
     AVCaptureVideoDataOutput *output = [[[AVCaptureVideoDataOutput alloc] init] autorelease];
     [session addOutput:output];

     // Configure your output.
     dispatch_queue_t queue = dispatch_queue_create("myQueue", NULL);
     [output setSampleBufferDelegate:self queue:queue];
     dispatch_release(queue);

     // Specify the pixel format
     output.videoSettings =
                 [NSDictionary dictionaryWithObject:
                     [NSNumber numberWithInt:kCVPixelFormatType_32BGRA]
                     forKey:(id)kCVPixelBufferPixelFormatTypeKey];


     // If you wish to cap the frame rate to a known value, such as 15 fps, set
     // minFrameDuration.
     output.minFrameDuration = CMTimeMake(1, 15);

     // Start the session running to start the flow of data
     [session startRunning];

     // Assign session to an ivar.
     [self setSession:session];
 }

第二步：实现AVCaptureVideoDataOutputSampleBufferDelegate协议方法 

 // Delegate routine that is called when a sample buffer was written
 - (void)captureOutput:(AVCaptureOutput *)captureOutput
          didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
          fromConnection:(AVCaptureConnection *)connection
 {
     // Create a UIImage from the sample buffer data
     UIImage *image = [self imageFromSampleBuffer:sampleBuffer];

      < Add your code here that uses the image >

 }

 // Create a UIImage from sample buffer data
 - (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer
 {
     CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
     // Lock the base address of the pixel buffer
     CVPixelBufferLockBaseAddress(imageBuffer,0);

     // Get the number of bytes per row for the pixel buffer
     size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
     // Get the pixel buffer width and height
     size_t width = CVPixelBufferGetWidth(imageBuffer);
     size_t height = CVPixelBufferGetHeight(imageBuffer);

     // Create a device-dependent RGB color space
     CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
     if (!colorSpace)
     {
         NSLog(@"CGColorSpaceCreateDeviceRGB failure");
         return nil;
     }

     // Get the base address of the pixel buffer
     void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
     // Get the data size for contiguous planes of the pixel buffer.
     size_t bufferSize = CVPixelBufferGetDataSize(imageBuffer);

     // Create a Quartz direct-access data provider that uses data we supply
     CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, baseAddress, bufferSize,
                                                             NULL);
     // Create a bitmap image from data supplied by our data provider
     CGImageRef cgImage =
         CGImageCreate(width,
                         height,
                         8,
                         32,
                         bytesPerRow,
                         colorSpace,
                         kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Little,
                         provider,
                         NULL,
                         true,
                         kCGRenderingIntentDefault);
     CGDataProviderRelease(provider);
     CGColorSpaceRelease(colorSpace);

     // Create and return an image object representing the specified Quartz image
     UIImage *image = [UIImage imageWithCGImage:cgImage];
     CGImageRelease(cgImage);

     CVPixelBufferUnlockBaseAddress(imageBuffer, 0);

     return image;
 }

好了，现在就可以自由的显示和分析获取的UIImage了，再也不用使用私有API了