在设置AVVideoComposition时，需要为AVVideoCompositionCoreAnimationTool设置delegate，并实现其中的“animationTool:didAnimateProperty:ofLayer:atTime:withValue:”方法。在该方法内，判断自定义属性的键是否符合动画处理，如果是则对其进行相应的动画处理。

示例代码：

// 自定义属性的键
let customAnimationKey = "customAnimationKey"

// 设置AVAssetExportSession
let exporter = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality)
exporter.outputFileType = .mp4
exporter.videoComposition = videoComposition

// 设置AVVideoCompositionCoreAnimationTool
let animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: videoComposition)
animationTool.delegate = self // 设置delegate

videoComposition.animationTool = animationTool

// 实现AVVideoCompositionCoreAnimationToolDelegate中的方法，进行动画处理
extension ViewController: AVVideoCompositionCoreAnimationToolDelegate {
    func animationTool(_ animationTool: AVVideoCompositionCoreAnimationTool, didAnimateProperty prop: String, ofLayer layer: CALayer, atTime time: CMTime, withValue value: Any) {

        if prop == customAnimationKey {
            // 对自定义属性进行动画处理
            let anim = CABasicAnimation(keyPath: "opacity")
            anim.fromValue = 0
            anim.toValue = 1
            anim.duration = 1
            layer.opacity = 1
            layer.add(anim, forKey: "opacity")
        }
    }
}