SwiftUI中创建实现函数遇到的两个编译错误求解

两个报错的原因和对应修复方式如下：

• 报错1 Cannot assign to property: 'self' is immutable：SwiftUI的View是struct值类型，普通实例方法不能直接修改未加状态包装器的存储属性；且combination本身是依赖两个选中值动态计算的结果，完全不需要定义为视图的存储属性。
• 报错2 Missing argument for parameter 'combination' in call：你定义了无默认值的存储属性combination，Swift会自动生成需要传入该参数的构造器，预览中调用时没传参数就会触发报错。

具体修改点 #

1. 删除多余的存储属性var combination: String，在returnCombo()方法内用局部变量存匹配结果直接返回，不需要给self属性赋值，同时解决两个报错的根源。
2. 修正原代码中第一个Picker的大括号闭合错误（原代码ForEach和Picker的括号闭合位置错位，会额外触发编译错误）。
3. 给drug1Value、drug2Value设置初始值，避免首次加载时强制解包firstIndex返回nil导致崩溃。
4. 修正原代码第二个Picker的标题笔误（原代码写的是"Drug One"，应为"Drug Two"）。

修复后完整可运行代码 #

import SwiftUI
struct DrugCombinationPicker: View {
    @Environment(\.colorScheme) var colorScheme
    @State var drug1Value: String = "Shrooms"
    @State var drug2Value: String = "Cannabis"
    let drugs: [String] = ["Shrooms", "Cannabis", "MDMA", "Cocaine", "Caffeine", "Alcohol", "Benzodiazephines", "Opiods"]
    
    var body: some View {
        VStack(spacing: 20) {
            Text("Pick two drug combinations")
                .font(.headline)
            
            HStack {
                Picker("Drug One", selection: $drug1Value) {
                    ForEach(drugs, id: \.self) { drug1 in
                        Text(drug1)
                    }
                }
                .pickerStyle(.menu)
                Spacer()
            }
            
            HStack {
                Picker("Drug Two", selection: $drug2Value) {
                    ForEach(drugs, id: \.self) { drug2 in
                        Text(drug2)
                    }
                }
                .pickerStyle(.menu)
            }
            
            Text("Your selected combination is \(returnCombo())")
                .padding(.top)
        }
        .padding()
    }

    func returnCombo() -> String {
        let choice1 = drugs.firstIndex(where: { $0 == drug1Value })!
        let choice2 = drugs.firstIndex(where: { $0 == drug2Value })!
        // 交互等级定义
        //0 = No interaction
        //1 = Safe & No Synergy
        //2 = Safe & Synergy
        //3 = Unsafe
        //4 = Safe/Decreases high
        //5 = Deadly
        let matrix = [
            [0,2,2,1,1,1,4,1],
            [2,0,2,1,1,1,4,2],
            [2,2,0,3,3,3,4,3],
            [1,1,3,0,3,3,4,5],
            [1,1,3,3,0,1,4,1],
            [1,1,3,3,1,0,5,5],
            [4,4,4,4,4,5,0,5],
            [1,2,3,5,1,5,5,0]
        ]
        let decision = matrix[choice1][choice2]
        
        let result: String
        switch decision {
        case 0: result = "No interaction"
        case 1: result = "Safe & No Synergy"
        case 2: result = "Safe & Synergy"
        case 3: result = "Unsafe"
        case 4: result = "Safe/Decreases high"
        case 5: result = "Deadly"
        default: result = "Unknown interaction"
        }
        return result
    }
}

struct DrugCombinationPicker_Previews: PreviewProvider {
    static var previews: some View {
        DrugCombinationPicker()
    }
}

可选优化：代码里把多分支if-else改成了switch判断，可读性更高；给视图加了基础的间距和内边距，Picker用了菜单样式，显示效果更自然。

内容的提问来源于stack exchange，提问作者Tom154ce