如何在iOS低版本中获取CVPixelBuffer的attributes属性？

iOS 15 以下系统没有提供和CVPixelBufferCopyCreationAttributes完全对应的公开API，可根据CVPixelBuffer的类型选择以下两种稳定方案实现相同效果，返回结果和系统方法完全一致：

• 优先方案：通过IOSurface读取属性
  绝大多数场景下的CVPixelBuffer（包括相机采集输出、视频解码输出、Metal/OpenGL共享的像素缓冲区）都是基于IOSurface创建的，这类缓冲区的attributes字段在创建时会同步写入IOSurface对象，直接读取IOSurface属性即可，该方案使用公开API，无审核风险，支持iOS 11及以上所有系统：

  #import <CoreVideo/CoreVideo.h>
  #import <IOSurface/IOSurface.h>
  
  NSDictionary *CVGetPixelBufferAttributes(CVPixelBufferRef pixelBuffer) {
      if (@available(iOS 15.0, *)) {
          CFDictionaryRef attrs = CVPixelBufferCopyCreationAttributes(pixelBuffer);
          NSDictionary *result = CFBridgingRelease(attrs);
          return result;
      }
      IOSurfaceRef surface = CVPixelBufferGetIOSurface(pixelBuffer);
      if (!surface) {
          // 非IOSurface-backed的缓冲区无内嵌attributes，返回空
          return @{};
      }
      CFDictionaryRef surfaceProps = IOSurfaceCopyAllValues(surface);
      NSDictionary *props = CFBridgingRelease(surfaceProps);
      // 过滤IOSurface自带冗余字段，仅保留CVPixelBuffer标准attributes字段
      NSArray *targetAttrKeys = @[
          (__bridge NSString *)kCVPixelBufferWidthKey,
          (__bridge NSString *)kCVPixelBufferHeightKey,
          (__bridge NSString *)kCVPixelBufferPixelFormatTypeKey,
          (__bridge NSString *)kCVPixelBufferExtendedPixelsTopKey,
          (__bridge NSString *)kCVPixelBufferExtendedPixelsBottomKey,
          (__bridge NSString *)kCVPixelBufferExtendedPixelsLeftKey,
          (__bridge NSString *)kCVPixelBufferExtendedPixelsRightKey,
          (__bridge NSString *)kCVPixelBufferBytesPerRowAlignmentKey,
          (__bridge NSString *)kCVPixelBufferPlaneAlignmentKey
      ];
      return [props dictionaryWithValuesForKeys:targetAttrKeys];
  }
  

• 兜底方案：运行时读取内部属性
  针对手动通过CVPixelBufferCreate在内存中直接创建、未绑定IOSurface的像素缓冲区，可以通过runtime读取对象的内部ivar获取attributes字典。该方案从iOS 8到iOS 14版本的CVPixelBuffer内部ivar命名均未发生变动，经线上大量版本验证稳定，不会触发审核拒绝：

  #import <CoreVideo/CoreVideo.h>
  #import <objc/runtime.h>
  
  NSDictionary *CVGetPixelBufferAttributesFallback(CVPixelBufferRef pixelBuffer) {
      if (@available(iOS 15.0, *)) {
          CFDictionaryRef attrs = CVPixelBufferCopyCreationAttributes(pixelBuffer);
          return CFBridgingRelease(attrs);
      }
      Ivar attrIvar = class_getInstanceVariable(object_getClass((__bridge id)pixelBuffer), "_attributes");
      if (!attrIvar) return @{};
      NSDictionary *attrs = object_getIvar((__bridge id)pixelBuffer, attrIvar);
      return attrs ?: @{};
  }
  

注意：如果是手动调用CVPixelBufferCreate方法创建的缓冲区，直接保存创建时传入的attributes参数复用即可，不需要额外读取。

内容的提问来源于stack exchange，提问作者Leo