为什么计时器更改会触发LazyvGrid视图更新？

Question

import SwiftUI

struct ContentView: View {
    @State private var set = Set<Int>()
    @State private var count = "10"
    private let columns:[GridItem] = Array(repeating: .init(.flexible()), count: 3)
    
    @State private var timer:Timer? = nil
    @State private var time = 0
    
    
    var body: some View {
        VStack {
            ScrollView {
                LazyVGrid(columns: columns) {
                    ForEach(Array(set)) { num in
                        Text(String(num))
                    }
                }
            }
            .frame(width: 400, height: 400, alignment: .center)
            
            HStack{
                TextField("Create \(count) items", text: $count)
                
                Button {
                    createSet(count: Int(count)!)
                } label: {
                    Text("Create")
                }
            }
            
            if let _ = timer {
                Text(String(time))
                    .font(.title2)
                    .foregroundColor(.green)
            }
            
            HStack {
                Button {
                    time = 100
                    
                    let timer = Timer.scheduledTimer(withTimeInterval: 10, repeats: true) { _ in
                        time -= 10
                        
                        if time == 0 {
                            self.timer?.invalidate()
                            self.timer = nil
                        }
                    }
                    
                    self.timer = timer
                } label: {
                    Text("Start Timer")
                }
                
                Button {
                    self.timer?.invalidate()
                    self.timer = nil
                } label: {
                    Text("Stop Timer")
                }
            }
        }
        .padding()
    }
    
    private func createSet(count:Int) {
        set.removeAll(keepingCapacity: true)
        
        repeat {
            let num = Int.random(in: 1...10000)
            set.insert(num)
        } while set.count < count
    }
}

extension Int:Identifiable {
    public var id:Self { self }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

我在text（String（num））上提出了一个断点。每次计时器触发时，GridView都会更新。为什么会发生这种情况？由于网格模型没有改变。

如果我将计时器放置在另一个视图中，则已更新

，网格视图不会触发。

import SwiftUI

struct ContentView: View {
    @State private var set = Set<Int>()
    @State private var count = "10"
    private let columns:[GridItem] = Array(repeating: .init(.flexible()), count: 3)
    
    var body: some View {
        VStack {
            ScrollView {
                LazyVGrid(columns: columns) {
                    ForEach(Array(set)) { num in
                        Text(String(num))
                    }
                }
            }
            .frame(width: 400, height: 400, alignment: .center)
            
            HStack{
                TextField("Create \(count) items", text: $count)
                
                Button {
                    createSet(count: Int(count)!)
                } label: {
                    Text("Create")
                }
            }
            
            TimerView()
        }
        .padding()
    }
    
    private func createSet(count:Int) {
        set.removeAll(keepingCapacity: true)
        
        repeat {
            let num = Int.random(in: 1...10000)
            set.insert(num)
        } while set.count < count
    }
}

extension Int:Identifiable {
    public var id:Self { self }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

import SwiftUI

struct TimerView: View {
    @State private var timer:Timer? = nil
    @State private var time = 0
    
    var body: some View {
        if let _ = timer {
            Text(String(time))
                .font(.title2)
                .foregroundColor(.green)
        }
        
        HStack {
            Button {
                time = 100
                
                let timer = Timer.scheduledTimer(withTimeInterval: 10, repeats: true) { _ in
                    time -= 10
                    
                    if time == 0 {
                        self.timer?.invalidate()
                        self.timer = nil
                    }
                }
                
                self.timer = timer
            } label: {
                Text("Start Timer")
            }
            
            Button {
                self.timer?.invalidate()
                self.timer = nil
            } label: {
                Text("Stop Timer")
            }
        }
    }
}

struct TimerView_Previews: PreviewProvider {
    static var previews: some View {
        TimerView()
    }
}

Answer 1

这几乎是Swiftui的工作方式。每个更改@State var触发重新评估的视图。如果将foreach放在另一个视图中，则只有更改更改视图的VAR时，才会重新评估。例如set或列。

struct ExtractedView: View {
    var columns: [GridItem]
    var set: Set<Int>
    var body: some View {
        ScrollView {
            LazyVGrid(columns: columns) {
                ForEach(Array(set)) { num in
                    Text(String(num))
                }
            }
        }
        .frame(width: 400, height: 400, alignment: .center)
    }
}

在swiftui中鼓励它制作许多小型视图。驾驶此驱动的系统在识别需要更改的内容和不进行的系统方面非常好。有一个非常好的WWDC视频描述了这一点。

wwdc