如何避免嵌套RealityViews中的不必要update调用

我有两个包含RealityView的视图First和Second，其中Second作为First的附件使用。First视图中声明了@State属性，当修改该状态时，SwiftUI会触发Second视图的update块（该状态并未在Second中使用）。如何避免Second中不必要的update块调用？

问题代码 #

import SwiftUI
import RealityKit

struct First: View {

    @State private var isButtonTapped = false

    var body: some View {
        print("Body #1")
        return VStack {
            RealityView { content, attach in
                let model = sphereModel(y: 0, color: .brown)
                content.add(model)
            } update: { content, attach in
                print(isButtonTapped)
                print("Update #1")
                guard let someView = attach.entity(for: "id")
                else { return }
                content.add(someView)
            } attachments: {
                Attachment(id: "id") {
                    Second()
                }
            }

            Button("BUTTON") { isButtonTapped.toggle() }
        }
    }
}

struct Second: View {

    var body: some View {
        print("Body #2")

        return RealityView { content in
            let model = sphereModel(y: 0, color: .red)
            content.add(model)
        } update: { content in
            print("Update #2")
        }
    }
}

func sphereModel(y: Float, color: UIColor) -> ModelEntity {
    let collisionComponent = CollisionComponent(
        shapes: [ShapeResource.generateSphere(radius: 0.01)]
    )

    let hoverEffectComponent = HoverEffectComponent(.highlight(.init(color: .blue)))

    var sphereModelMaterial = PhysicallyBasedMaterial()
    sphereModelMaterial.baseColor = .init(tint: color)

    let sphereMesh = MeshResource.generateSphere(radius: 0.01)

    let sphereModel = ModelEntity(
        mesh: sphereMesh,
        materials: [sphereModelMaterial]
    )

    sphereModel.transform.translation = .init(x: 0, y: y, z: 0)
    sphereModel.components.set(
        [
            collisionComponent,
            hoverEffectComponent
        ]
    )

    return sphereModel
}

解决方法 #

1. 让Second视图遵循Equatable并使用EquatableView包裹

SwiftUI默认会在父视图状态变化时刷新所有子视图（包括附件视图）。通过让Second遵循Equatable，并利用EquatableView，可以告诉SwiftUI只有当Second的依赖属性变化时才刷新它。因为Second没有任何外部依赖，直接返回true即可。

修改Second视图：

struct Second: View, Equatable {
    static func == (lhs: Second, rhs: Second) -> Bool {
        // 视图无外部依赖，始终视为相等
        true
    }

    // 原body代码不变
}

然后在First的附件中替换为：

Attachment(id: "id") {
    EquatableView(content: Second())
}

2. 优化First的RealityView update逻辑

原代码中First的update块每次都会执行content.add(someView)，这会重复添加实体，不仅浪费性能，也可能间接导致附件视图刷新。修改成只在实体未添加时才执行：

RealityView { content, attach in
    let model = sphereModel(y: 0, color: .brown)
    content.add(model)
    // 首次加载时添加附件实体
    if let someView = attach.entity(for: "id") {
        content.add(someView)
    }
} update: { content, attach in
    print(isButtonTapped)
    print("Update #1")
    guard let someView = attach.entity(for: "id"), 
          !content.entities.contains(someView) else { return }
    content.add(someView)
}

修改后的完整代码 #

import SwiftUI
import RealityKit

struct First: View {

    @State private var isButtonTapped = false

    var body: some View {
        print("Body #1")
        return VStack {
            RealityView { content, attach in
                let model = sphereModel(y: 0, color: .brown)
                content.add(model)
                // 首次添加附件实体
                if let someView = attach.entity(for: "id") {
                    content.add(someView)
                }
            } update: { content, attach in
                print(isButtonTapped)
                print("Update #1")
                guard let someView = attach.entity(for: "id"), 
                      !content.entities.contains(someView) else { return }
                content.add(someView)
            } attachments: {
                Attachment(id: "id") {
                    EquatableView(content: Second())
                }
            }

            Button("BUTTON") { isButtonTapped.toggle() }
        }
    }
}

struct Second: View, Equatable {
    static func == (lhs: Second, rhs: Second) -> Bool {
        true
    }

    var body: some View {
        print("Body #2")

        return RealityView { content in
            let model = sphereModel(y: 0, color: .red)
            content.add(model)
        } update: { content in
            print("Update #2")
        }
    }
}

func sphereModel(y: Float, color: UIColor) -> ModelEntity {
    let collisionComponent = CollisionComponent(
        shapes: [ShapeResource.generateSphere(radius: 0.01)]
    )

    let hoverEffectComponent = HoverEffectComponent(.highlight(.init(color: .blue)))

    var sphereModelMaterial = PhysicallyBasedMaterial()
    sphereModelMaterial.baseColor = .init(tint: color)

    let sphereMesh = MeshResource.generateSphere(radius: 0.01)

    let sphereModel = ModelEntity(
        mesh: sphereMesh,
        materials: [sphereModelMaterial]
    )

    sphereModel.transform.translation = .init(x: 0, y: y, z: 0)
    sphereModel.components.set(
        [
            collisionComponent,
            hoverEffectComponent
        ]
    )

    return sphereModel
}

效果验证 #

修改完成后，点击按钮切换isButtonTapped状态时，只会打印Body #1和Update #1，Second的Body #2和Update #2不会再被不必要地触发。

内容的提问来源于stack exchange，提问作者Pogos Anesyan