如何避免SwiftUI中ForEach不必要的Identifiable视图重复渲染？

问题场景 #

我定义了Student和Classroom两个遵循Identifiable协议的类型，当更新某班级里一名学生的姓名时，所有关联的视图（哪怕数据没变化的）都会被重渲染。原本以为Identifiable能让SwiftUI自动识别需要更新的视图，只重渲染变化的部分，但实际运行代码时修改单个学生姓名会触发所有视图重渲染，不符合预期。

原代码如下：

import SwiftUI

struct ContentView: View {
    
    @State private var school: [Classroom] = [classroom1, classroom2]
    
    var body: some View {
        
        VStack {

            Divider()

            ForEach(school) { classroom in

                ForEach(classroom.students) { student in
                    
                    StudentView(student: student, action: { value in

                        if let classroomIndex: Int = findIndex(item: classroom, array: school) {

                            if let studentIndex: Int = findIndex(item: student, array: classroom.students) {

                                school[classroomIndex].students[studentIndex].name = value

                            }
  
                        }

                    })

                }
                
                Divider()

            }

        }
        .padding()
    }
}

func findIndex<U: Identifiable>(item: U, array: [U]) -> Int? {
    return array.firstIndex(where: { element in (element.id == item.id) })
}


struct StudentView: View {
    let student: Student
    let action: (String) -> Void
    var body: some View {
        print("rendering for:", student.name)
       return Text(student.name)
            .onTapGesture {
                action(student.name + " updated! ")
            }
        
    }
}

struct Student: Identifiable {
    let id: UUID = UUID()
    var name: String
}

struct Classroom: Identifiable {
    let id: UUID = UUID()
    var name: String
    var students: [Student]
}

let classroom1: Classroom = Classroom(name: "Classroom 1", students: [Student(name: "a"), Student(name: "b"), Student(name: "c")])
let classroom2: Classroom = Classroom(name: "Classroom 2", students: [Student(name: "x"), Student(name: "y"), Student(name: "z")])

问题原因 #

Identifiable协议的作用只是让ForEach能识别数组中元素的唯一性，避免重复或错误的视图复用，但它不能直接控制SwiftUI的重渲染逻辑。

当你修改school数组中某个学生的姓名时，整个school数组会被标记为变化，触发ContentView的body重新计算。此时ForEach会重新遍历所有班级和学生，而默认情况下StudentView没有实现Equatable协议，SwiftUI无法判断新创建的StudentView实例和旧实例的输入是否一致，只能默认认为所有视图都需要重渲染，导致全量更新。

解决方法 #

通过让Student和StudentView遵循Equatable协议，让SwiftUI能够精确判断视图是否需要更新：

修改后的关键代码：

// 让Student遵循Equatable，自定义相等判断逻辑
struct Student: Identifiable, Equatable {
    let id: UUID = UUID()
    var name: String
    
    static func ==(lhs: Self, rhs: Self) -> Bool {
        return lhs.id == rhs.id && lhs.name == rhs.name
    }
}

// 让StudentView遵循Equatable，基于student属性判断视图是否需要更新
struct StudentView: View, Equatable {
    let student: Student
    let action: (String) -> Void
    var body: some View {
        print("rendering for:", student.name)
       return Text(student.name)
            .onTapGesture {
                action(student.name + " updated! ")
            }
        
    }
    
    static func ==(lhs: Self, rhs: Self) -> Bool {
         lhs.student == rhs.student
     }
}

代码说明 #

1. Student实现Equatable：通过比较id和name，确保只有当学生的唯一标识或姓名发生变化时，才判定为两个实例不相等。
2. StudentView实现Equatable：通过比较传入的student属性，让SwiftUI能够判断当前视图的输入是否真的发生变化。只有当student实例不相等时，StudentView才会被重渲染。

这样修改后，再更新单个学生的姓名时，只有对应的StudentView会触发重渲染，其他视图会被SwiftUI跳过，达到预期的性能优化效果。

内容的提问来源于stack exchange，提问作者swiftPunk