Core Media OC转Swift遇问题：CMMetadataFormatDescription创建失败

问题背景 #

我在参考AAPLTimedAnnotationWriter的Objective-C示例，在Swift 5.7（macOS）中实现同类功能时，翻译示例代码第208-218行后，调用CMMetadataFormatDescriptionCreateWithMetadataSpecifications()返回kCMFormatDescriptionError_InvalidParameter错误。

原Objective-C代码 #

CMFormatDescriptionRef metadataFormatDescription = NULL;
NSArray *specs = @[
                    @{(__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier : AAPLTimedAnnotationWriterCircleCenterCoordinateIdentifier,
                        (__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType : (__bridge NSString *)kCMMetadataBaseDataType_PointF32},
                    @{(__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier : AAPLTimedAnnotationWriterCircleRadiusIdentifier,
                        (__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType : (__bridge NSString *)kCMMetadataBaseDataType_Float64},
                    @{(__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier : AAPLTimedAnnotationWriterCommentFieldIdentifier,
                        (__bridge NSString *)kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType : (__bridge NSString *)kCMMetadataBaseDataType_UTF8}];


OSStatus err = CMMetadataFormatDescriptionCreateWithMetadataSpecifications(kCFAllocatorDefault, kCMMetadataFormatType_Boxed, (__bridge CFArrayRef)specs, &metadataFormatDescription);

我的Swift代码（存在问题） #

private func createChapterMetadataFormatDescription() -> CMMetadataFormatDescription? {
    let extra: NSArray = [
        (
            kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier: "mdta/com.example.circle.center.coordinate",
            kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType: kCMMetadataBaseDataType_PointF32
        ),
        (
            kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier: "mdta/com.example.circle.radius",
            kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType: kCMMetadataBaseDataType_Float64
        ),
        (
            kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier: "mdta/com.example.comment.field",
            kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType: kCMMetadataBaseDataType_UTF8
        )
    ]

    var formatDescription: CMMetadataFormatDescription? = nil
    if kCVReturnSuccess != CMMetadataFormatDescriptionCreateWithMetadataSpecifications(allocator: kCFAllocatorDefault,
                                                                                       metadataType: kCMMetadataFormatType_Boxed,
                                                                                       metadataSpecifications: extra,
                                                                                       formatDescriptionOut: &formatDescription) { return nil }
    return formatDescription
}

错误原因及修正 #

核心错误1：用元组替代了字典

Objective-C代码中的@{...}是NSDictionary，但你在Swift里用了元组来构造数组元素，而CMMetadataFormatDescriptionCreateWithMetadataSpecifications要求的是包含字典的数组，类型不匹配直接导致参数无效。

核心错误2：错误的成功状态判断

你用了CoreVideo的kCVReturnSuccess来判断状态，但该API返回的是OSStatus类型，成功值应为noErr（等于0）。

修正后的Swift代码

private func createChapterMetadataFormatDescription() -> CMMetadataFormatDescription? {
    // 将CFString类型的键转换为Swift String
    let identifierKey = kCMMetadataFormatDescriptionMetadataSpecificationKey_Identifier as String
    let dataTypeKey = kCMMetadataFormatDescriptionMetadataSpecificationKey_DataType as String
    
    // 构造符合要求的字典数组
    let specs: [[String: Any]] = [
        [
            identifierKey: "mdta/com.example.circle.center.coordinate",
            dataTypeKey: kCMMetadataBaseDataType_PointF32
        ],
        [
            identifierKey: "mdta/com.example.circle.radius",
            dataTypeKey: kCMMetadataBaseDataType_Float64
        ],
        [
            identifierKey: "mdta/com.example.comment.field",
            dataTypeKey: kCMMetadataBaseDataType_UTF8
        ]
    ]
    
    var formatDescription: CMMetadataFormatDescription? = nil
    let status = CMMetadataFormatDescriptionCreateWithMetadataSpecifications(
        allocator: kCFAllocatorDefault,
        metadataType: kCMMetadataFormatType_Boxed,
        metadataSpecifications: specs as CFArray,
        formatDescriptionOut: &formatDescription
    )
    
    // 使用正确的OSStatus成功值判断
    guard status == noErr else {
        return nil
    }
    
    return formatDescription
}

额外说明 #

• 所有CFString类型的常量（如数据类型标识）可直接传入字典，Swift会自动处理桥接
• 字典数组可直接转换为CFArray传入API，无需显式声明为NSArray

内容的提问来源于stack exchange，提问作者yhf8377