如何判断Entity已成功附加到AnchorEntity后执行指定函数

你当前doSomething()触发过早的核心原因是Entity.loadAsync为异步操作，sink的两个闭包都会在加载完成后才异步执行，而你把doSomething()放在了异步流程外部，调用addModel时会同步立即执行，与加载流程完全无关。
要实现「Entity加载完成且成功附加到AnchorEntity后再执行」的需求，可按以下逻辑调整：

基础实现（满足绝大多数场景） #

直接将doSomething()移入receiveValue闭包末尾，同时增加附加成功的校验：

func addModel(to planeAnchor: AnchorEntity) {
    Entity.loadAsync(named: "SomeUSDZModel")
        .sink { completion in
            switch completion {
            case .finished:
                print("Ok")
            case .failure(let error):
                print(error.localizedDescription)
            }
        } receiveValue: { model in
            // Entity should be added before the animation is started.
            planeAnchor.addChild(model)
            if let walkingAnimation = model.availableAnimations.first {
                model.playAnimation(walkingAnimation.repeat(duration: .infinity),
                                    transitionDuration: 1.25,
                                    blendLayerOffset: 0,
                                    separateAnimatedValue: false,
                                    startsPaused: false)
            }
            self.model = model
            
            // 新增校验逻辑：确认model已经成功附加到目标锚点
            guard model.parent == planeAnchor else { return }
            doSomething()
        }
        .store(in: &subscriptions)
}

更稳妥的实现（避免渲染帧延迟问题） #

如果你的场景中存在RealityKit内部渲染帧延迟导致addChild后parent未立即更新的特殊情况，可以等待下一个渲染循环后再校验执行：

func addModel(to planeAnchor: AnchorEntity) {
    Entity.loadAsync(named: "SomeUSDZModel")
        .sink { completion in
            switch completion {
            case .finished:
                print("Ok")
            case .failure(let error):
                print(error.localizedDescription)
            }
        } receiveValue: { model in
            // Entity should be added before the animation is started.
            planeAnchor.addChild(model)
            if let walkingAnimation = model.availableAnimations.first {
                model.playAnimation(walkingAnimation.repeat(duration: .infinity),
                                    transitionDuration: 1.25,
                                    blendLayerOffset: 0,
                                    separateAnimatedValue: false,
                                    startsPaused: false)
            }
            self.model = model
            
            // 等待下一个渲染帧后再校验执行
            Task { @MainActor in
                try? await Task.sleep(nanoseconds: 16_000_000) // 约等于1帧的时长
                guard model.parent == planeAnchor else { return }
                doSomething()
            }
        }
        .store(in: &subscriptions)
}

内容的提问来源于stack exchange，提问作者Kuba Gawecki