哪种CMTime无效？

Question

kCMTimeInvalid is invalid CMTime, but based on Apple document, there are more invalid CMTime, what are they? What does CMTime "invalid" means? It's overflow, uninitiated or anything else?

https://developer.apple.com/documentation/coremedia/kcmtimeinvalid

All fields are 0, so you can calloc or fill with 0's to make lots of them. Do not test against this using (time == kCMTimeInvalid), there are many CMTimes other than this that are also invalid. Use CMTIME_IS_INVALID(time) instead.

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I found some cases when CMTime is invalid:

• When flags(CMTimeFlags) kCMTimeFlags_Valid not set.

  kCMTimeFlags_Valid Must be set, or the CMTime is considered invalid.

• Some operations of CMTimeAdd

+infinity + +infinity == +infinity

-infinity + -infinity == -infinity

+infinity + -infinity == invalid

-infinity + +infinity == invalid

Answer 1

There are five possible states:

1. +Infinity: This is similar to Float.Infinity. This is a valid value, just greater than any finite number. How might you use it? For example, imagine an API that gives you information about a time range within a video, identified by two CMTimes. You might invoke it with (-Infinity, +Infinity) to ask for information about the entire video.
2. -Infinity: This is again similar to -Float.Infinity.
3. Indefinite: This is similar to Float.NaN, as I understand. Use this when you don't know what value to use, like the duration of a live stream, as Apple suggests . It wouldn't be right to use infinity, for example, since a live stream doesn't go on for forever. It has a finite duration; we just don't know it yet.
4. Invalid: This is a CMTime structure that doesn't obey the rules of CMTime. I assume that means things like a zero or negative denominator. Since CMTime is a C struct, it's not encapsulated, so someone can create one with invalid values like this. C structs can't have initialisers that throw an exception or return nil.
5. Numeric: This is the normal case of a finite value. Use CMTIME_IS_NUMERIC to check for this. It returns false for all the weird cases above.

Answer 2

From the documentation you posted, it says:

Use CMTIME_IS_INVALID(time) instead.

From CMTime.h , it looks like CMTIME_IS_INVALID is defined as:

#define CMTIME_IS_INVALID(time) (! CMTIME_IS_VALID(time))

And CMTIME_IS_VALID is defined as:

#define CMTIME_IS_VALID(time) ((Boolean)(((time).flags & kCMTimeFlags_Valid) != 0))

So it looks like the only thing that determines if a CMTime is valid or not is whether the kCMTimeFlags_Valid flag is set.