Atomic Vs Non-atomic

Atomic

  • is the default behavior
  • will ensure the present process is completed by the CPU, before another process accesses the variable
  • is not fast, as it ensures the process is completed entirely

Non-Atomic

  • is NOT the default behavior
  • faster (for synthesized code, that is, for variables created using @property and @synthesize)
  • not thread-safe
  • may result in unexpected behavior, when two different process access the same variable at the same time
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Static vs. Dynamic Typing in Objective C

we talked about Objective-C being a statically-typed language, meaning that “the type of data each variable (or container) will hold is declared up front?” Well, that’s sort of true, but doesn’t describe the whole picture. I don’t want to overload this post with details about the language, but it’s important to mention that Objective-C is also a dynamic language, which means that some parts of our code can be extended and modified while our apps are running (i.e. after they’ve already been compiled). One example is the use of the generic data type called id. id can be used to represent any kind of data type. It’s the most generic form of specifying data; it simply stands for an “identifier” that is used to reference data. This type of dynamic typing allows for increased flexibility in how we code certain things. In iOS, we often see id used for methods that are tied to controls on the screen, like:

- (IBAction)save:(id)sender;

This is the declaration of a method, which we’ll talk about in an upcoming post. But notice that the sender parameter is of type id. This allows some flexibility in that our app doesn’t care if the sender is a button or an image or whatever else we might want to use to trigger this action.

Linking problem in Xcode when using a dependent subproject

Issue

Undefined symbols:
  "_AVCaptureSessionPresetMedium", referenced from:
      _AVCaptureSessionPresetMedium$non_lazy_ptr in libZXingWidget.a(ZXingWidgetController.o)
     (maybe you meant: _AVCaptureSessionPresetMedium$non_lazy_ptr)
  "_CVPixelBufferGetHeight", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_CVPixelBufferLockBaseAddress", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_AudioServicesPlaySystemSound", referenced from:
      -[ZXingWidgetController presentResultForString:] in libZXingWidget.a(ZXingWidgetController.o)
  "_AudioServicesCreateSystemSoundID", referenced from:
      -[ZXingWidgetController viewWillAppear:] in libZXingWidget.a(ZXingWidgetController.o)
  "_CVPixelBufferUnlockBaseAddress", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_CVPixelBufferGetBaseAddress", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_CVPixelBufferGetBytesPerRow", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_iconv_close", referenced from:
      zxing::qrcode::DecodedBitStreamParser::append(std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned char const*, unsigned long, char const*)in libZXingWidget.a(DecodedBitStreamParser-64E27B33E79CBC52.o)
      zxing::qrcode::DecodedBitStreamParser::append(std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned char const*, unsigned long, char const*)in libZXingWidget.a(DecodedBitStreamParser-64E27B33E79CBC52.o)
  "_OBJC_CLASS_$_AVCaptureVideoPreviewLayer", referenced from:
      objc-class-ref-to-AVCaptureVideoPreviewLayer in libZXingWidget.a(ZXingWidgetController.o)
  "_iconv", referenced from:
      zxing::qrcode::DecodedBitStreamParser::append(std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned char const*, unsigned long, char const*)in libZXingWidget.a(DecodedBitStreamParser-64E27B33E79CBC52.o)
  "_OBJC_CLASS_$_AVCaptureSession", referenced from:
      objc-class-ref-to-AVCaptureSession in libZXingWidget.a(ZXingWidgetController.o)
  "_OBJC_CLASS_$_AVCaptureDevice", referenced from:
      objc-class-ref-to-AVCaptureDevice in libZXingWidget.a(ZXingWidgetController.o)
  "_kCVPixelBufferPixelFormatTypeKey", referenced from:
      _kCVPixelBufferPixelFormatTypeKey$non_lazy_ptr in libZXingWidget.a(ZXingWidgetController.o)
     (maybe you meant: _kCVPixelBufferPixelFormatTypeKey$non_lazy_ptr)
  "_OBJC_CLASS_$_AVCaptureVideoDataOutput", referenced from:
      objc-class-ref-to-AVCaptureVideoDataOutput in libZXingWidget.a(ZXingWidgetController.o)
  "_CVPixelBufferGetWidth", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_AudioServicesDisposeSystemSoundID", referenced from:
      -[ZXingWidgetController dealloc] in libZXingWidget.a(ZXingWidgetController.o)
  "_OBJC_CLASS_$_AVCaptureDeviceInput", referenced from:
      objc-class-ref-to-AVCaptureDeviceInput in libZXingWidget.a(ZXingWidgetController.o)
  "_AVLayerVideoGravityResizeAspectFill", referenced from:
      _AVLayerVideoGravityResizeAspectFill$non_lazy_ptr in libZXingWidget.a(ZXingWidgetController.o)
     (maybe you meant: _AVLayerVideoGravityResizeAspectFill$non_lazy_ptr)
  "_CMSampleBufferGetImageBuffer", referenced from:
      -[ZXingWidgetController captureOutput:didOutputSampleBuffer:fromConnection:] in libZXingWidget.a(ZXingWidgetController.o)
  "_iconv_open", referenced from:
      zxing::qrcode::DecodedBitStreamParser::append(std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned char const*, unsigned long, char const*)in libZXingWidget.a(DecodedBitStreamParser-64E27B33E79CBC52.o)
  "_AVMediaTypeVideo", referenced from:
      _AVMediaTypeVideo$non_lazy_ptr in libZXingWidget.a(ZXingWidgetController.o)
     (maybe you meant: _AVMediaTypeVideo$non_lazy_ptr)
ld: symbol(s) not found
collect2: ld returned 1 exit status

SOlutions:
make sure include "CoreMedia.framework","AudioToolbox.framework","CoreGraphics.framework","CoreVideo.framework","AVFoundation.framework","libiconv.dylib" frameworks in project in Build Phases

Comparison tools for two xcode files

You can compare two Xcode project using FileMerge tool available with Xcode.

Below are steps for comparison in Xcode 6.1

  1. Right click on Xcode icon -> Select ‘Open Developer Tool’ -> Select ‘FileMerge’ option

Screenshot 0

  1. This will open window as show below :

Screenshot 1

  1. Select two Xcode project which is to be compared by clicking Left and Right button
  2. Click on ‘Compare’ button. This will create list of file that are added, removed or altered by comparing two Xcode project.

Screenshot 2