Question

我正在阅读关于AVCaptureSession等的示例代码（AVCam）。我注意到以下行（link to the code）：

// In iOS 9 and later, the userInfo dictionary contains information
// on why the session was interrupted.
if ( &AVCaptureSessionInterruptionReasonKey ) {
   ...
}

代码中的注释是有道理的。但是对我来说没有意义的是我们为什么要获得AVCaptureSessionInterruptionReasonKey的地址。它定义如下（在AVCaptureSection.h中）：

AVF_EXPORT NSString *const AVCaptureSessionInterruptionReasonKey NS_AVAILABLE_IOS(9_0);

如果定义了密钥，其地址将如何nil？如果没有定义这个键，代码将永远不会被编译，对吧？有人可以向我解释这个if语句是如何工作的吗？

Answer 1

AVCaptureSessionInterruptionReasonKey was added in iOS 9.0. Such an if statement is needed only if your app also supports iOS 8 or earlier.

When the code is run on a device with iOS 9 or later, the value will be non-nil and the if statement will be true. On devices with iOS 8 or earlier, the value will be nil and the if statement will be false.

If your app only supports iOS 9 or later, there is no need for the if statement.

Read the SDK Compatibility Guide in the iOS docs for more details on this type of check.

为什么我们在AVCam示例代码中获取AVCaptureSessionInterruptionReasonKey的地址？

1 个答案: