要測試 TextField 的 binding，先建立一個 MemoInputData 並讓這個物件 conform ObservableObject

step1: 建立 MemoInputData

import Foundation

class MemoInputData: ObservableObject {
}

step2: 在 MemoInputView 加上 MemoInputData

/// MemoInputView.swift
struct MemoInputView: View {
    
    @State var inputedText: String = ""
    @StateObject var data: MemoInputData

preview 也要改

struct MemoInputView_Previews: PreviewProvider {
    
    static let data: MemoInputData = .init()
    
    static var previews: some View {
        MemoInputView(data: data)
    }
}

step3: TextField 的 binding 需變更

struct MemoInputView: View {
    
    @StateObject var data: MemoInputData
    
    var displayText: String {
        if data.inputedString.isEmpty {
            return "請輸入文字"
        } else {
            return "你輸入的是: \(data.inputedString)"
        }
    }
    
    var body: some View {
        
        VStack {
            Text(displayText)
            TextField("請輸入交易心得", text: $data.inputedString)
        }
        .padding()
    }
}

step4: Unit testing 並測試是否通過測試

func testMemoInputViewStart() throws {
        
        let expect = "請輸入文字"
        /// MemoInputView 裡面裝載了 vStack, 而 Text 在 0 個位置
        let string = try sut.inspect().vStack().text(0).string()
        XCTAssertEqual(expect, string)
    }
    
    func testMemoInputTextWithDummy() throws {
        
        let dummyText = "foo"
        let expect = "你輸入的是: foo"
        try sut.inspect().vStack().textField(1).setInput(dummyText)
        let string = try sut.inspect().vStack().text(0).string()
        XCTAssertEqual(string, expect)
    }

這樣就可以去測試 TextField 和 UI 元件的交互行為