第一步: 新建Controller

1. 在Xcode選擇File → New → File → Cocoa Touch Class
2. 新建LoginViewController繼承自UIViewController

第二步:創建兩個UITextField

1. passwordInput: UITextField // 密碼輸入框
2. accountInput: UITextField // 帳號輸入框

第三步:添加鍵盤KVO

在viewDidLoad方法添加下面兩行代碼

//當鍵盤彈起的時候會向系統發出一個通知，
//這個時候需要注冊一個監聽器響應該通知
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(LoginViewController.keyboardWillShow(_:)), name:UIKeyboardWillShowNotification, object: nil)
//當鍵盤收起的時候會向系統發出一個通知，
//這個時候需要注冊另外一個監聽器響應該通知
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(LoginViewController.keyboardWillHide(_:)), name:UIKeyboardWillHideNotification, object: nil)

添加全局控制參數

因為連續在兩個或多個textfield之間切換時候，只會發送UIKeyboardWillShowNotification鍵盤顯示通知，而不會發送UIKeyboardWillHideNotification鍵盤隱藏通知，這就需要一個全局參數控制鍵盤只在第一次點擊輸入框時候界面上移，該參數變為false，光標移到另一個輸入框時界面不再變化。當關閉鍵盤時候，界面下移，并將這個參數恢復為默認值。
在類的第一行聲明該變量:

var keyBoardNeedLayout: Bool = true

添加兩個方法分別相應鍵盤彈起和鍵盤隱藏

鍵盤彈起響應

func keyboardWillShow(notification: NSNotification) {
        print("show")
        if let userInfo = notification.userInfo,
            value = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue,
            duration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? Double,
            curve = userInfo[UIKeyboardAnimationCurveUserInfoKey] as? UInt {
            
            let frame = value.CGRectValue()
            let intersection = CGRectIntersection(frame, self.view.frame)
            
            let deltaY = CGRectGetHeight(intersection)
            
            if keyBoardNeedLayout {
                UIView.animateWithDuration(duration, delay: 0.0,
                                           options: UIViewAnimationOptions(rawValue: curve),
                                           animations: { _ in
                                            self.view.frame = CGRectMake(0,-deltaY,self.view.bounds.width,self.view.bounds.height)
                                            self.keyBoardNeedLayout = false
                                            self.view.layoutIfNeeded()
                    }, completion: nil)
            }
            
            
        }
    }

鍵盤隱藏響應

func keyboardWillHide(notification: NSNotification) {
        print("hide")
        if let userInfo = notification.userInfo,
            value = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue,
            duration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? Double,
            curve = userInfo[UIKeyboardAnimationCurveUserInfoKey] as? UInt {
            
            let frame = value.CGRectValue()
            let intersection = CGRectIntersection(frame, self.view.frame)
            
            let deltaY = CGRectGetHeight(intersection)
            
            UIView.animateWithDuration(duration, delay: 0.0,
                                       options: UIViewAnimationOptions(rawValue: curve),
                                       animations: { _ in
                                        self.view.frame = CGRectMake(0,deltaY,self.view.bounds.width,self.view.bounds.height)
                                        self.keyBoardNeedLayout = true
                                        self.view.layoutIfNeeded()
                }, completion: nil)
            
        }
    }

更進一步

如果輸入框吸底,y的位移可以用-deltaY

self.view.frame = CGRectMake(0,-deltaY,self.view.bounds.width,self.view.bounds.height)

但是如果輸入框在偏上的位置就有可能導致某個輸入框移出界面視界，這時候可以把位移寫成deltaY/2或者deltaY/4等，自己去嘗試吧。

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