`UITextView` should really just have this by default.

Optional+Empty.swift

protocol TextContaining {
    var isEmpty: Bool { get }
}

extension String: TextContaining {
}

extension Optional where Wrapped: TextContaining {
    var isEmpty: Bool {
        switch self {
        case .Some(let someString):
            return someString.isEmpty
            
        default:
            return true
        }
    }
}

PlaceholderTextView.swift

final class PlaceholderTextView: UITextView {
    // MARK: - Public
    
    var placeholder: String? {
        set {
            placeholderLabel.text = newValue
        }
        get {
            return placeholderLabel.text
        }
    }
    
    var placeholderColor: UIColor {
        set {
            placeholderLabel.textColor = newValue
        }
        get {
            return placeholderLabel.textColor
        }
    }
    
    // MARK: - Private
    
    private var labelConstraints: [NSLayoutConstraint] = []
    
    private let placeholderLabel = UILabel().then {
        $0.translatesAutoresizingMaskIntoConstraints = false
        $0.lineBreakMode = .ByWordWrapping
        $0.numberOfLines = 0
        $0.textColor = .lightGrayColor()
    }
    
    // MARK: - UITextView
    
    override var font: UIFont? {
        didSet {
            placeholderLabel.font = font
        }
    }
    
    override var text: String? {
        didSet {
            updatePlaceholderVisibility()
        }
    }
    
    // MARK: - Initialization
    
    override init(frame: CGRect, textContainer: NSTextContainer?) {
        super.init(frame: frame, textContainer: textContainer)
        
        addSubview(placeholderLabel)
        
        [
            (UITextViewTextDidBeginEditingNotification, #selector(textFieldDidBeginEditing)),
            (UITextViewTextDidEndEditingNotification, #selector(textFieldDidEndEditing))
        ].forEach { name, selector in
            NSNotificationCenter.defaultCenter().addObserver(self, selector: selector, name: name, object: self)
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    // MARK: - Actions
    
    private dynamic func textFieldDidBeginEditing() {
        placeholderLabel.hidden = true
    }
    
    private dynamic func textFieldDidEndEditing() {
        updatePlaceholderVisibility()
    }
    
    // MARK: - UIView
    
    override func updateConstraints() {
        super.updateConstraints()
        
        NSLayoutConstraint.deactivateConstraints(labelConstraints)
        
        /*
         These constraints are probably not totally comprehensive but should work fine in most cases (e.g. where the
         text view is taller than the placeholder label).
         
         Don’t pin to the edges because otherwise the scroll view’s content size will be impacted by the label.
         */
        labelConstraints = [
            placeholderLabel.centerXAnchor.constraintEqualToAnchor(centerXAnchor),
            placeholderLabel.widthAnchor.constraintEqualToAnchor(widthAnchor, constant: -(textContainerInset.left + textContainerInset.right)),
            placeholderLabel.topAnchor.constraintEqualToAnchor(topAnchor, constant: textContainerInset.top),
        ]
        
        NSLayoutConstraint.activateConstraints(labelConstraints)
    }
    
    // MARK: - Private
    
    private func updatePlaceholderVisibility() {
        placeholderLabel.hidden = !text.isEmpty
    }
}

Then.swift

/// https://github.com/devxoul/Then
protocol Then {
}

extension Then {
    public func then(@noescape block: Self -> Void) -> Self {
        block(self)
        return self
    }
}