这是我的视图控制器

class SecondViewController: UIViewController {
// MARK: - IBOutlet
@IBOutlet weak var stringTextField: UITextField!
@IBOutlet weak var characterTextField: UITextField!
@IBOutlet weak var countLabel: UILabel!

// MARK: - Variable

// MARK: - Life Cycle Method
override func viewDidLoad() {
    super.viewDidLoad()
    intlizer()
}

// MARK: - Intliztion Method
func intlizer() {
    
}

func checkCharacter(char: Character, testString: String) -> Int {
    let count = testString.filter { $0 == char }.count
    return count
}

@IBAction func checkCount(_ sender: Any) {
    let count = checkCharacter(char: Character(characterTextField.text!.lowercased()), testString: stringTextField.text!.lowercased())
    countLabel.text = "\(characterTextField.text!) is contain \(count) time"
}
}

无论字符是小写还是大写（此代码为全部计数），主代码如下

let count = "yourString".filter { $0 == "Character you want to find" }.count

这是最终结果