SwiftUI中如何利用@Published变量实时触发ForEach更新？

在SwiftUI视图中通过ForEach循环绘制图形，当前实现里ViewModel的extractFrameData函数遍历完frameDataArray后，视图的ForEach才会响应frameRectInfoList的更新。需要实现每次在extractFrameData的分支中为frameRectInfoList赋值时，立即触发视图的ForEach执行绘制操作。

视图代码 #

import SwiftUI

struct RccFrameView: View {
    @EnvironmentObject var frameViewModel: RccFrameViewModel

    var body: some View {
        VStack {
            ForEach(frameViewModel.frameRectInfoList) { frameRectInfo in
                // 基于frameRectInfo绘制图形
            }
        }
        .onReceive(self.frameViewModel.$frameData) { _ in
            frameViewModel.extractFrameData(frameDataArray: frameViewModel.frameData, deviceModel: deviceModel)
        }
    }
}

ViewModel代码 #

class RccFrameViewModel: ObservableObject {
    @Published var frameData: NSArray = []
    @Published var shapeFillColour = Color.white
    @Published var shapeStrokeColour = Color.white
    @Published var frameRectInfoList: [RccFrameRectInfo] = []
    @Published var currentFrameType: LjFocusFrameType = .unknown
    
    func extractFrameData(frameDataArray: NSArray, deviceModel: RccDeviceModelWrapper) {
        if frameDataArray.count > 0 {
            for frameDict in frameDataArray {
                if let frameType = (frameDict as AnyObject).value(forKey: "frameType") as? Int,
                   let frameRects = (frameDict as AnyObject).object(forKey: "frameRects") as? [RccFrameRectInfo] {
                    currentFrameType = LjFocusFrameType(rawValue: frameType) ?? .unknown
                    
                    switch frameType {
                    case LjFocusFrameType.contrast.rawValue:
                        if frameRects.count > 0 {
                            shapeFillColour = Color("FocusFrameGreen")
                            shapeStrokeColour = .black
                            self.frameRectInfoList = frameRects
                        }
                        
                    case LjFocusFrameType.contrastFlexibleMain.rawValue:
                        if frameRects.count > 0 {
                            shapeFillColour = .white
                            shapeStrokeColour = .black
                            self.frameRectInfoList = frameRects
                        }
                        
                    case LjFocusFrameType.unknown.rawValue:
                        if frameRects.count > 0 {
                            shapeFillColour = .white
                            shapeStrokeColour = .black
                            self.frameRectInfoList = frameRects
                        }
                    default: break
                    }
                }
            }
        }
    }
}

核心原因 #

SwiftUI的ObservableObject会合并同一运行循环（RunLoop）内的多个@Published属性更新，只触发一次视图刷新。所以你在循环中多次给frameRectInfoList赋值，最终只会在循环结束后触发一次重绘。

实现方式：拆分更新到独立运行循环 #

通过DispatchQueue.main.async将每次赋值操作放到下一个主队列运行循环中，确保每个更新都能单独触发视图刷新。

修改后的extractFrameData函数：

func extractFrameData(frameDataArray: NSArray, deviceModel: RccDeviceModelWrapper) {
    if frameDataArray.count > 0 {
        for frameDict in frameDataArray {
            if let frameType = (frameDict as AnyObject).value(forKey: "frameType") as? Int,
               let frameRects = (frameDict as AnyObject).object(forKey: "frameRects") as? [RccFrameRectInfo] {
                currentFrameType = LjFocusFrameType(rawValue: frameType) ?? .unknown
                
                switch frameType {
                case LjFocusFrameType.contrast.rawValue:
                    if frameRects.count > 0 {
                        shapeFillColour = Color("FocusFrameGreen")
                        shapeStrokeColour = .black
                        // 异步更新，触发即时重绘
                        DispatchQueue.main.async {
                            self.frameRectInfoList = frameRects
                        }
                    }
                    
                case LjFocusFrameType.contrastFlexibleMain.rawValue:
                    if frameRects.count > 0 {
                        shapeFillColour = .white
                        shapeStrokeColour = .black
                        DispatchQueue.main.async {
                            self.frameRectInfoList = frameRects
                        }
                    }
                    
                case LjFocusFrameType.unknown.rawValue:
                    if frameRects.count > 0 {
                        shapeFillColour = .white
                        shapeStrokeColour = .black
                        DispatchQueue.main.async {
                            self.frameRectInfoList = frameRects
                        }
                    }
                default: break
                }
            }
        }
    }
}

备选方案：手动发送变更通知 #

如果不想用异步队列，也可以在每次赋值前手动发送objectWillChange通知，强制触发视图更新：

// 在赋值前发送变更信号
self.objectWillChange.send()
self.frameRectInfoList = frameRects

不过这种方式在同一运行循环内多次发送通知仍可能被合并，因此DispatchQueue.main.async是更可靠的选择。

内容的提问来源于stack exchange，提问作者Tulon