你只要这样做就可以了

let fields = [fullNameTextField, dateOfBirthTextField, schoolNameTextField, schoolLocationTextField, gradeClassTextField]
let isVaildFields = fields.allSatisfy { field in
            !field.text.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
         } 
if isVaildFields {
    nextBtn.alpha = 1
    nextBtn.isEnabled = true
} else {
    nextBtn.alpha = 0.5
    nextBtn.isEnabled = false
}

一种方法是

class ViewController: UIViewController, UITextFieldDelegate {
    
    @IBOutlet weak var fullNameTextField: UITextField!
    @IBOutlet weak var dateOfBirthTextField: UITextField!
    @IBOutlet weak var schoolNameTextField: UITextField!
    @IBOutlet weak var schoolLocationTextField: UITextField!
    @IBOutlet weak var gradeClassTextField: UITextField!
    @IBOutlet weak var nextBtn: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        nextBtn.isEnabled = false
        nextBtn.alpha = 0.5
    }

    func textFieldDidBeginEditing(_ textField: UITextField) {
        let fields = [fullNameTextField, dateOfBirthTextField, schoolNameTextField,
                      schoolLocationTextField, gradeClassTextField]
        let isValidFields = fields.allSatisfy {
            !($0?.text?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty ?? true)
        }
        
        if isValidFields {
            nextBtn.alpha = 1
            nextBtn.isEnabled = true
        } else {
            nextBtn.alpha = 0.5
            nextBtn.isEnabled = false
        }
    }
}

您也可以通过创建文本字段@IBOutlet的列表来实现，如下所示：

class ViewController: UIViewController, UITextFieldDelegate {
    
    @IBOutlet var textFields: [UITextField]!
    @IBOutlet weak var nextBtn: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        nextBtn.isEnabled = false
        nextBtn.alpha = 0.5
    }

    func textFieldDidBeginEditing(_ textField: UITextField) {
        let isValidFields = textFields.allSatisfy {
            !($0.text?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty ?? true)
        }
        
        if isValidFields {
            nextBtn.alpha = 1
            nextBtn.isEnabled = true
        } else {
            nextBtn.alpha = 0.5
            nextBtn.isEnabled = false
        }
    }
}

如果你只想为单个文本域做这件事，你可以简单地像这样做。

class ViewController: UIViewController, UITextFieldDelegate {
    
    @IBOutlet weak var fullNameTextField: UITextField!
    @IBOutlet weak var dateOfBirthTextField: UITextField!
    @IBOutlet weak var schoolNameTextField: UITextField!
    @IBOutlet weak var schoolLocationTextField: UITextField!
    @IBOutlet weak var gradeClassTextField: UITextField!
    @IBOutlet weak var nextBtn: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        nextBtn.isEnabled = false
        nextBtn.alpha = 0.5
    }
    
    func isValid(_ textField: UITextField) -> Bool {
        return !(textField.text?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty ?? true)
    }

    func textFieldDidBeginEditing(_ textField: UITextField) {
        if isValid(fullNameTextField), isValid(dateOfBirthTextField),
           isValid(schoolNameTextField), isValid(schoolLocationTextField),
           isValid(gradeClassTextField) {
            nextBtn.alpha = 1
            nextBtn.isEnabled = true
        } else {
            nextBtn.alpha = 0.5
            nextBtn.isEnabled = false
        }
    }
}