Swift and C disagree about whether a CMTimeRange is valid

Originator:fcanas
Number:rdar://30576556 Date Originated:17-Feb-2017
Status:Open Resolved:
Product:iOS + SDK Product Version:
Classification: Reproducible:Always
 
Area:
Core Media

Summary:
The documentation states that the value of a CMTimeRange's duration must not be negative. 

"The epoch in a CMTime that represents a duration should always be 0, and the value must be non-negative."

The C macro CMTIMERANGE_IS_VALID includes a check (range.duration.value >= 0).

#define CMTIMERANGE_IS_VALID(range) ((Boolean)(CMTIME_IS_VALID(range.start) && CMTIME_IS_VALID(range.duration) && (range.duration.epoch == 0) && (range.duration.value >= 0)))

And a time range with a negative-value duration is invalid when checked with the C macro. But when checked in Swift via CMTimeRange's public var isValid: Bool { get }, a range with a negative duration is considered valid.

Using the public function CMTIMERANGE_IS_VALID in Swift is consistent with .isValid, and inconsistent with the C macro.

Steps to Reproduce:
Example project includes the following components.

1) Wrap the CMTIMERANGE_IS_VALID macro in a C function

Boolean timeRangeIsValid(CMTimeRange range)
{
    return CMTIMERANGE_IS_VALID(range);
}

2) In a swift program, we can now directly compare the outputs of the 3 methods of validating a time range.

let positiveOne = CMTime(value: 1, timescale: 1)
let negativeOne = CMTime(value: -1, timescale: 1)
let rangeWithNegativeDuration = CMTimeRange(start: positiveOne, duration: negativeOne)

print("swift : range is valid: \(rangeWithNegativeDuration.isValid)")
print("c     : range is valid: \(timeRangeIsValid(rangeWithNegativeDuration))")
print("macro : range is valid: \(CMTIMERANGE_IS_VALID(rangeWithNegativeDuration))")


Expected Results:
I expect CMTimeRange validity to be independent of language and mechanism used. And for it to be consistent with the documentation.

Program output above should be

swift : range is valid: false
c     : range is valid: false
macro : range is valid: false

Actual Results:
CMTimeRange validity only agrees with documentation when using the C macro for checking validity. Validating with the two mechanisms available in Swift is incorrect.

Program output above is

swift : range is valid: true
c     : range is valid: false
macro : range is valid: true

Comments


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