Para um UIScrollView *toScrollView (que é a largura da tela), quero adicionar uma borda inferior cinza (exatamente como a do campo de ação da exibição de composição do aplicativo de mensagens nativo do iPhone).

Para conseguir isso, segui o Cocoa Touch: Como alterar a cor e a espessura da borda do UIView? e apenas cobrimos a borda superior com o costume UINavigationBare fizemos a toScrollViewcoordenada x -1 e a largura 322 para que as bordas esquerda e direita fiquem fora da tela.

Parece bom, mas é uma espécie de hack, e eu queria saber se há uma maneira melhor de fazer isso.

- (void)viewDidLoad {
    [super viewDidLoad];

    // Add UINavigationBar *navigationBar at top.
    self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc]
                                             initWithBarButtonSystemItem:UIBarButtonSystemItemCancel
                                             target:self action:@selector(cancelAction)];
    UINavigationBar *navigationBar = [[UINavigationBar alloc]
                                      initWithFrame:CGRectMake(0.0f, 0.0f, 320.0f, 44.0f)];
    navigationBar.items = [NSArray arrayWithObject:self.navigationItem];

    // Add UIScrollView *toScrollView below navigationBar.
    UIScrollView *toScrollView = [[UIScrollView alloc]
                                  initWithFrame:CGRectMake(-1.0f, 43.0f, 322.0f, 45.0f)];
    toScrollView.backgroundColor = [UIColor whiteColor];
    toScrollView.layer.borderColor = [UIColor colorWithWhite:0.8f alpha:1.0f].CGColor;
    toScrollView.layer.borderWidth = 1.0f;
    [self.view addSubview:toScrollView];
    [self.view addSubview:navigationBar]; // covers top of toScrollView
}

Em vez de usar um UIView, como sugere @ ImreKelényi, você pode usar um CALayer:

// Add a bottomBorder.
CALayer *bottomBorder = [CALayer layer];

bottomBorder.frame = CGRectMake(0.0f, 43.0f, toScrollView.frame.size.width, 1.0f);

bottomBorder.backgroundColor = [UIColor colorWithWhite:0.8f 
                                                 alpha:1.0f].CGColor;

[toScrollView.layer addSublayer:bottomBorder];

Aqui está uma extensão Swift mais generalizada para criar borda para qualquer UIViewsubclasse:

import UIKit

extension UIView {      
  func addTopBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.backgroundColor = color.CGColor
    border.frame = CGRectMake(0, 0, self.frame.size.width, width)
    self.layer.addSublayer(border)
  }

  func addRightBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.backgroundColor = color.CGColor
    border.frame = CGRectMake(self.frame.size.width - width, 0, width, self.frame.size.height)
    self.layer.addSublayer(border)
  }

  func addBottomBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.backgroundColor = color.CGColor
    border.frame = CGRectMake(0, self.frame.size.height - width, self.frame.size.width, width)
    self.layer.addSublayer(border)
  }

  func addLeftBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.backgroundColor = color.CGColor
    border.frame = CGRectMake(0, 0, width, self.frame.size.height)
    self.layer.addSublayer(border)
  }
}

Swift 3

extension UIView {
    func addTopBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: 0, width: self.frame.size.width, height: width)
        self.layer.addSublayer(border)
    }

    func addRightBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: self.frame.size.width - width, y: 0, width: width, height: self.frame.size.height)
        self.layer.addSublayer(border)
    }

    func addBottomBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: self.frame.size.height - width, width: self.frame.size.width, height: width)
        self.layer.addSublayer(border)
    }

    func addLeftBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
        self.layer.addSublayer(border)
    }
}

Implementado em uma categoria como abaixo:

UIButton + Border.h:

@interface UIButton (Border)

- (void)addBottomBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addLeftBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addRightBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addTopBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

@end

UIButton + Border.m:

@implementation UIButton (Border)

- (void)addTopBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    CALayer *border = [CALayer layer];
    border.backgroundColor = color.CGColor;

    border.frame = CGRectMake(0, 0, self.frame.size.width, borderWidth);
    [self.layer addSublayer:border];
}

- (void)addBottomBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    CALayer *border = [CALayer layer];
    border.backgroundColor = color.CGColor;

    border.frame = CGRectMake(0, self.frame.size.height - borderWidth, self.frame.size.width, borderWidth);
    [self.layer addSublayer:border];
}

- (void)addLeftBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    CALayer *border = [CALayer layer];
    border.backgroundColor = color.CGColor;

    border.frame = CGRectMake(0, 0, borderWidth, self.frame.size.height);
    [self.layer addSublayer:border];
}

- (void)addRightBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    CALayer *border = [CALayer layer];
    border.backgroundColor = color.CGColor;

    border.frame = CGRectMake(self.frame.size.width - borderWidth, 0, borderWidth, self.frame.size.height);
    [self.layer addSublayer:border];
}

@end

Swift 4

Se você precisa de uma solução realmente adaptável (para todos os tamanhos de tela), é esta:

/**
* Extends UIView with shortcut methods
*
* @author Alexander Volkov
* @version 1.0
*/
extension UIView {

    /// Adds bottom border to the view with given side margins
    ///
    /// - Parameters:
    ///   - color: the border color
    ///   - margins: the left and right margin
    ///   - borderLineSize: the size of the border
    func addBottomBorder(color: UIColor = UIColor.red, margins: CGFloat = 0, borderLineSize: CGFloat = 1) {
        let border = UIView()
        border.backgroundColor = color
        border.translatesAutoresizingMaskIntoConstraints = false
        self.addSubview(border)
        border.addConstraint(NSLayoutConstraint(item: border,
                                                attribute: .height,
                                                relatedBy: .equal,
                                                toItem: nil,
                                                attribute: .height,
                                                multiplier: 1, constant: borderLineSize))
        self.addConstraint(NSLayoutConstraint(item: border,
                                              attribute: .bottom,
                                              relatedBy: .equal,
                                              toItem: self,
                                              attribute: .bottom,
                                              multiplier: 1, constant: 0))
        self.addConstraint(NSLayoutConstraint(item: border,
                                              attribute: .leading,
                                              relatedBy: .equal,
                                              toItem: self,
                                              attribute: .leading,
                                              multiplier: 1, constant: margins))
        self.addConstraint(NSLayoutConstraint(item: border,
                                              attribute: .trailing,
                                              relatedBy: .equal,
                                              toItem: self,
                                              attribute: .trailing,
                                              multiplier: 1, constant: margins))
    }
}

Você pode adicionar uma UIViewaltura separada de 1 ponto e cor de fundo cinza ae self.viewposicioná-la logo abaixo toScrollView.

EDIT: A menos que você tenha um bom motivo (deseja usar alguns serviços do UIView que não são oferecidos pelo CALayer), use o CALayer como sugere o @MattDiPasquale . O UIView possui uma sobrecarga maior, o que pode não ser um problema na maioria dos casos, mas ainda assim, a outra solução é mais elegante.

Solução para Swift 4

let bottomBorder = CALayer()
        bottomBorder.frame = CGRect(x: 0.0, y: calendarView.frame.size.height-1, width: calendarView.frame.width, height: 1.0)
        bottomBorder.backgroundColor = #colorLiteral(red: 0.8039215803, green: 0.8039215803, blue: 0.8039215803, alpha: 1)
        calendarView.layer.addSublayer(bottomBorder)

FundoCor claraCinza. Mude a cor se você precisar.

Também há um código aprimorado com a funcionalidade de remover bordas. Baseado em resposta confil .

import UIKit

enum viewBorder: String {
    case Left = "borderLeft"
    case Right = "borderRight"
    case Top = "borderTop"
    case Bottom = "borderBottom"
}

extension UIView {

    func addBorder(vBorder: viewBorder, color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.CGColor
        border.name = vBorder.rawValue
        switch vBorder {
            case .Left:
                border.frame = CGRectMake(0, 0, width, self.frame.size.height)
            case .Right:
                border.frame = CGRectMake(self.frame.size.width - width, 0, width, self.frame.size.height)
            case .Top:
                border.frame = CGRectMake(0, 0, self.frame.size.width, width)
            case .Bottom:
                border.frame = CGRectMake(0, self.frame.size.height - width, self.frame.size.width, width)
        }
        self.layer.addSublayer(border)
    }

    func removeBorder(border: viewBorder) {
        var layerForRemove: CALayer?
        for layer in self.layer.sublayers! {
            if layer.name == border.rawValue {
                layerForRemove = layer
            }
        }
        if let layer = layerForRemove {
            layer.removeFromSuperlayer()
        }
    }

}

Atualização: Swift 3

import UIKit

enum ViewBorder: String {
    case left, right, top, bottom
}

extension UIView {

    func add(border: ViewBorder, color: UIColor, width: CGFloat) {
        let borderLayer = CALayer()
        borderLayer.backgroundColor = color.cgColor
        borderLayer.name = border.rawValue
        switch border {
        case .left:
            borderLayer.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
        case .right:
            borderLayer.frame = CGRect(x: self.frame.size.width - width, y: 0, width: width, height: self.frame.size.height)
        case .top:
            borderLayer.frame = CGRect(x: 0, y: 0, width: self.frame.size.width, height: width)
        case .bottom:
            borderLayer.frame = CGRect(x: 0, y: self.frame.size.height - width, width: self.frame.size.width, height: width)
        }
        self.layer.addSublayer(borderLayer)
    }

    func remove(border: ViewBorder) {
        guard let sublayers = self.layer.sublayers else { return }
        var layerForRemove: CALayer?
        for layer in sublayers {
            if layer.name == border.rawValue {
                layerForRemove = layer
            }
        }
        if let layer = layerForRemove {
            layer.removeFromSuperlayer()
        }
    }

}

O problema com esses métodos de extensão é que, quando o UIView / UIButton mais tarde ajusta seu tamanho, você não tem chance de alterar o tamanho do CALayer para corresponder ao novo tamanho. O que deixará você com uma borda fora do lugar. Eu achei que era melhor subclassificar meu UIButton, é claro que você poderia subclassificar outros UIViews também. Aqui está um código:

enum BorderedButtonSide {
    case Top, Right, Bottom, Left
}


class BorderedButton : UIButton {

    private var borderTop: CALayer?
    private var borderTopWidth: CGFloat?
    private var borderRight: CALayer?
    private var borderRightWidth: CGFloat?
    private var borderBottom: CALayer?
    private var borderBottomWidth: CGFloat?
    private var borderLeft: CALayer?
    private var borderLeftWidth: CGFloat?


    func setBorder(side: BorderedButtonSide, _ color: UIColor, _ width: CGFloat) {

        let border = CALayer()
        border.backgroundColor = color.CGColor

        switch side {
        case .Top:
            border.frame = CGRect(x: 0, y: 0, width: frame.size.width, height: width)
            borderTop?.removeFromSuperlayer()
            borderTop = border
            borderTopWidth = width
        case .Right:
            border.frame = CGRect(x: frame.size.width - width, y: 0, width: width, height: frame.size.height)
            borderRight?.removeFromSuperlayer()
            borderRight = border
            borderRightWidth = width
        case .Bottom:
            border.frame = CGRect(x: 0, y: frame.size.height - width, width: frame.size.width, height: width)
            borderBottom?.removeFromSuperlayer()
            borderBottom = border
            borderBottomWidth = width
        case .Left:
            border.frame = CGRect(x: 0, y: 0, width: width, height: frame.size.height)
            borderLeft?.removeFromSuperlayer()
            borderLeft = border
            borderLeftWidth = width
        }

        layer.addSublayer(border)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        borderTop?.frame = CGRect(x: 0, y: 0, width: frame.size.width, height: borderTopWidth!)
        borderRight?.frame = CGRect(x: frame.size.width - borderRightWidth!, y: 0, width: borderRightWidth!, height: frame.size.height)
        borderBottom?.frame = CGRect(x: 0, y: frame.size.height - borderBottomWidth!, width: frame.size.width, height: borderBottomWidth!)
        borderLeft?.frame = CGRect(x: 0, y: 0, width: borderLeftWidth!, height: frame.size.height)
    }

}

Ou, a maneira mais favorável ao desempenho é sobrecarregar o drawRect, simplesmente assim:

@interface TPActionSheetButton : UIButton

@property (assign) BOOL drawsTopLine;
@property (assign) BOOL drawsBottomLine;
@property (assign) BOOL drawsRightLine;
@property (assign) BOOL drawsLeftLine;
@property (strong, nonatomic) UIColor * lineColor;

@end

@implementation TPActionSheetButton

- (void) drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    CGContextSetLineWidth(ctx, 0.5f * [[UIScreen mainScreen] scale]);
    CGFloat red, green, blue, alpha;
    [self.lineColor getRed:&red green:&green blue:&blue alpha:&alpha];
    CGContextSetRGBStrokeColor(ctx, red, green, blue, alpha);

    if(self.drawsTopLine) {
        CGContextBeginPath(ctx);
        CGContextMoveToPoint(ctx, CGRectGetMinX(rect), CGRectGetMinY(rect));
        CGContextAddLineToPoint(ctx, CGRectGetMaxX(rect), CGRectGetMinY(rect));
        CGContextStrokePath(ctx);
    }
    if(self.drawsBottomLine) {
        CGContextBeginPath(ctx);
        CGContextMoveToPoint(ctx, CGRectGetMinX(rect), CGRectGetMaxY(rect));
        CGContextAddLineToPoint(ctx, CGRectGetMaxX(rect), CGRectGetMaxY(rect));
        CGContextStrokePath(ctx);
    }
    if(self.drawsLeftLine) {
        CGContextBeginPath(ctx);
        CGContextMoveToPoint(ctx, CGRectGetMinX(rect), CGRectGetMinY(rect));
        CGContextAddLineToPoint(ctx, CGRectGetMinX(rect), CGRectGetMaxY(rect));
        CGContextStrokePath(ctx);
    }
    if(self.drawsRightLine) {
        CGContextBeginPath(ctx);
        CGContextMoveToPoint(ctx, CGRectGetMaxX(rect), CGRectGetMinY(rect));
        CGContextAddLineToPoint(ctx, CGRectGetMaxX(rect), CGRectGetMaxY(rect));
        CGContextStrokePath(ctx);
    }

    [super drawRect:rect];
}

@end

Versão Swift 3 da resposta do Confile:

import UIKit

extension UIView {
    func addTopBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: 0, width: self.frame.size.width, height: width)
        self.layer.addSublayer(border)
    }

    func addRightBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: self.frame.size.width - width, y: 0, width: width, height: self.frame.size.height)
        self.layer.addSublayer(border)
    }

    func addBottomBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: self.frame.size.height - width, width: self.frame.size.width, height: width)
        self.layer.addSublayer(border)
    }

    func addLeftBorderWithColor(color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        border.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
        self.layer.addSublayer(border)
    }
}

Uso ao usar o layout automático:

class CustomView: UIView {

    override func awakeFromNib() {
        super.awakeFromNib()
    }

    override func layoutSubviews() {
        addBottomBorderWithColor(color: UIColor.white, width: 1)
    }
}

Se você usar restrições (e não tiver o tamanho dos quadros), poderá adicionar uma vista de borda com as restrições necessárias

// MARK: - Add a border to one side of a view

public enum BorderSide {
    case top, bottom, left, right
}

extension UIView {
    public func addBorder(side: BorderSide, color: UIColor, width: CGFloat) {
        let border = UIView()
        border.translatesAutoresizingMaskIntoConstraints = false
        border.backgroundColor = color
        self.addSubview(border)

        let topConstraint = topAnchor.constraint(equalTo: border.topAnchor)
        let rightConstraint = trailingAnchor.constraint(equalTo: border.trailingAnchor)
        let bottomConstraint = bottomAnchor.constraint(equalTo: border.bottomAnchor)
        let leftConstraint = leadingAnchor.constraint(equalTo: border.leadingAnchor)
        let heightConstraint = border.heightAnchor.constraint(equalToConstant: width)
        let widthConstraint = border.widthAnchor.constraint(equalToConstant: width)


        switch side {
        case .top:
            NSLayoutConstraint.activate([leftConstraint, topConstraint, rightConstraint, heightConstraint])
        case .right:
            NSLayoutConstraint.activate([topConstraint, rightConstraint, bottomConstraint, widthConstraint])
        case .bottom:
            NSLayoutConstraint.activate([rightConstraint, bottomConstraint, leftConstraint, heightConstraint])
        case .left:
            NSLayoutConstraint.activate([bottomConstraint, leftConstraint, topConstraint, widthConstraint])
        }
    }
}

Em seguida, defina algo como o abaixo

myButton.addBorder(side: .left, color: UIColor.lightGray, width: 1)

(inspirado nesta resposta )

Rápido

Criar extensão UIView

private var bottomLineColorAssociatedKey : UIColor = .black
private var topLineColorAssociatedKey : UIColor = .black
private var rightLineColorAssociatedKey : UIColor = .black
private var leftLineColorAssociatedKey : UIColor = .black
extension UIView {

@IBInspectable var bottomLineColor: UIColor {
    get {
        if let color = objc_getAssociatedObject(self, &bottomLineColorAssociatedKey) as? UIColor {
            return color
        } else {
            return .black
        }
    } set {
        objc_setAssociatedObject(self, &bottomLineColorAssociatedKey, newValue, .OBJC_ASSOCIATION_RETAIN)
    }
}
@IBInspectable var bottomLineWidth: CGFloat {
    get {
        return self.bottomLineWidth
    }
    set {
        DispatchQueue.main.async {
            self.addBottomBorderWithColor(color: self.bottomLineColor, width: newValue)
        }
    }
}
@IBInspectable var topLineColor: UIColor {
    get {
        if let color = objc_getAssociatedObject(self, &topLineColorAssociatedKey) as? UIColor {
            return color
        } else {
            return .black
        }
    } set {
        objc_setAssociatedObject(self, &topLineColorAssociatedKey, newValue, .OBJC_ASSOCIATION_RETAIN)
    }
}
@IBInspectable var topLineWidth: CGFloat {
    get {
        return self.topLineWidth
    }
    set {
        DispatchQueue.main.async {
            self.addTopBorderWithColor(color: self.topLineColor, width: newValue)
        }
    }
}
@IBInspectable var rightLineColor: UIColor {
    get {
        if let color = objc_getAssociatedObject(self, &rightLineColorAssociatedKey) as? UIColor {
            return color
        } else {
            return .black
        }
    } set {
        objc_setAssociatedObject(self, &rightLineColorAssociatedKey, newValue, .OBJC_ASSOCIATION_RETAIN)
    }
}
@IBInspectable var rightLineWidth: CGFloat {
    get {
        return self.rightLineWidth
    }
    set {
        DispatchQueue.main.async {
            self.addRightBorderWithColor(color: self.rightLineColor, width: newValue)
        }
    }
}
@IBInspectable var leftLineColor: UIColor {
    get {
        if let color = objc_getAssociatedObject(self, &leftLineColorAssociatedKey) as? UIColor {
            return color
        } else {
            return .black
        }
    } set {
        objc_setAssociatedObject(self, &leftLineColorAssociatedKey, newValue, .OBJC_ASSOCIATION_RETAIN)
    }
}
@IBInspectable var leftLineWidth: CGFloat {
    get {
        return self.leftLineWidth
    }
    set {
        DispatchQueue.main.async {
            self.addLeftBorderWithColor(color: self.leftLineColor, width: newValue)
        }
    }
}
func addTopBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.name = "topBorderLayer"
    removePreviouslyAddedLayer(name: border.name ?? "")
    border.backgroundColor = color.cgColor
    border.frame = CGRect(x: 0, y : 0,width: self.frame.size.width, height: width)
    self.layer.addSublayer(border)
    self.addObserver(self, forKeyPath: #keyPath(UIView.bounds), options: .new, context: UnsafeMutableRawPointer(bitPattern: 1111) )
}

func addRightBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.name = "rightBorderLayer"
    removePreviouslyAddedLayer(name: border.name ?? "")
    border.backgroundColor = color.cgColor
    border.frame = CGRect(x: self.frame.size.width - width, y: 0, width : width, height :self.frame.size.height)
    self.layer.addSublayer(border)
     self.addObserver(self, forKeyPath: #keyPath(UIView.bounds), options: .new, context: UnsafeMutableRawPointer(bitPattern: 2222) )
}

func addBottomBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.name = "bottomBorderLayer"
    removePreviouslyAddedLayer(name: border.name ?? "")
    border.backgroundColor = color.cgColor
    border.frame = CGRect(x: 0, y: self.frame.size.height - width,width : self.frame.size.width,height: width)
    self.layer.addSublayer(border)
    self.addObserver(self, forKeyPath: #keyPath(UIView.bounds), options: .new, context: UnsafeMutableRawPointer(bitPattern: 3333) )
}
func addLeftBorderWithColor(color: UIColor, width: CGFloat) {
    let border = CALayer()
    border.name = "leftBorderLayer"
    removePreviouslyAddedLayer(name: border.name ?? "")
    border.backgroundColor = color.cgColor
    border.frame = CGRect(x:0, y:0,width : width, height : self.frame.size.height)
    self.layer.addSublayer(border)
    self.addObserver(self, forKeyPath: #keyPath(UIView.bounds), options: .new, context: UnsafeMutableRawPointer(bitPattern: 4444) )
}
override open func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {

    if let objectView = object as? UIView,
        objectView === self,
        keyPath == #keyPath(UIView.bounds) {
        switch context {
        case UnsafeMutableRawPointer(bitPattern: 1111):
            for border in self.layer.sublayers ?? [] {
                if border.name == "topBorderLayer" {
                    border.frame = CGRect(x: 0, y : 0,width: self.frame.size.width, height: border.frame.height)
                }
            }
        case UnsafeMutableRawPointer(bitPattern: 2222):
            for border in self.layer.sublayers ?? [] {
                if border.name == "rightBorderLayer" {
                     border.frame = CGRect(x: self.frame.size.width - border.frame.width, y: 0, width : border.frame.width, height :self.frame.size.height)
                }
            }
        case UnsafeMutableRawPointer(bitPattern: 3333):
            for border in self.layer.sublayers ?? [] {
                if border.name == "bottomBorderLayer" {
                    border.frame = CGRect(x: 0, y: self.frame.size.height - border.frame.height,width : self.frame.size.width,height: border.frame.height)
                }
            }
        case UnsafeMutableRawPointer(bitPattern: 4444):
            for border in self.layer.sublayers ?? [] {
                if border.name == "leftBorderLayer" {
                   border.frame = CGRect(x:0, y:0,width : border.frame.width, height : self.frame.size.height)
                }
            }
        default:
            break
        }
    }
}
func removePreviouslyAddedLayer(name : String) {
    if self.layer.sublayers?.count ?? 0 > 0 {
        self.layer.sublayers?.forEach {
            if $0.name == name {
                $0.removeFromSuperlayer()
            }
        }
    }
   }
}

Objetivo C

Criar classe de categoria de UIView

UIView + Border.h

#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
@interface UIView (Border) 

@property (nonatomic) IBInspectable UIColor *topLineColor;
@property (nonatomic) IBInspectable CGFloat topLineWidth;
@property (nonatomic) IBInspectable UIColor *bottomLineColor;
@property (nonatomic) IBInspectable CGFloat bottomLineWidth;
@property (nonatomic) IBInspectable UIColor *rightLineColor;
@property (nonatomic) IBInspectable CGFloat rightLineWidth;
@property (nonatomic) IBInspectable UIColor *leftLineColor;
@property (nonatomic) IBInspectable CGFloat leftLineWidth;

- (void)addBottomBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addLeftBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addRightBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

- (void)addTopBorderWithColor: (UIColor *) color andWidth:(CGFloat) borderWidth;

@end

UIView + Border.m

static void *topBorderContext = &topBorderContext;
static void *bottomBorderContext = &bottomBorderContext;
static void *leftBorderContext = &leftBorderContext;
static void *rightBorderContext = &rightBorderContext;
static char bottomLineColorKey,topLineColorKey,rightLineColorKey,leftLineColorKey;
@implementation UIView(Utility)
@dynamic borderColor,borderWidth,cornerRadius,bottomLineWidth,topLineWidth,rightLineWidth,leftLineWidth;

-(void)setBorderColor:(UIColor *)borderColor{
    [self.layer setBorderColor:borderColor.CGColor];
}

-(void)setBorderWidth:(CGFloat)borderWidth{
    [self.layer setBorderWidth:borderWidth];
}

-(void)setCornerRadius:(CGFloat)cornerRadius{
    [self.layer setCornerRadius:cornerRadius];
}
// for Bottom Line
- (UIColor *)bottomLineColor {
    return objc_getAssociatedObject(self, &bottomLineColorKey);
}
- (void)setBottomLineColor:(UIColor *)bottomLineColor {
    objc_setAssociatedObject(self, &bottomLineColorKey,
                             bottomLineColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
-(void)setBottomLineWidth:(CGFloat)bottomLineWidth {
    [self addBottomBorderWithColor:[self bottomLineColor] andWidth:bottomLineWidth];
}
// for top Line
- (UIColor *)topLineColor {
    return objc_getAssociatedObject(self, &topLineColorKey);
}
- (void)setTopLineColor:(UIColor *)topLineColor {
    objc_setAssociatedObject(self, &topLineColorKey,
                             topLineColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (void)setTopLineWidth:(CGFloat)topLineWidth{
    [self addTopBorderWithColor:[self topLineColor] andWidth:topLineWidth];
}
// for right Line
- (UIColor *)rightLineColor {
    return objc_getAssociatedObject(self, &rightLineColorKey);
}
-(void)setRightLineColor:(UIColor *)rightLineColor {
    objc_setAssociatedObject(self, &rightLineColorKey,
                             rightLineColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
-(void)setRightLineWidth:(CGFloat)rightLineWidth{
    [self addRightBorderWithColor:[self rightLineColor] andWidth:rightLineWidth];
}
// for left Line
-(UIColor *)leftLineColor {
    return objc_getAssociatedObject(self, &leftLineColorKey);
}
-(void)setLeftLineColor:(UIColor *)leftLineColor{
    objc_setAssociatedObject(self, &leftLineColorKey,
                             leftLineColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
-(void)setLeftLineWidth:(CGFloat)leftLineWidth{
    [self addLeftBorderWithColor:[self leftLineColor] andWidth:leftLineWidth];
}

- (void)addTopBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    dispatch_async(dispatch_get_main_queue(), ^{
        CALayer *border = [CALayer layer];
        border.name = @"topBorderLayer";
        [self removePreviouslyAddedLayer:border.name];
        border.backgroundColor = color.CGColor;
        border.frame = CGRectMake(0, 0, self.frame.size.width, borderWidth);
        [self.layer addSublayer:border];
        [self addObserver:self forKeyPath: @"bounds" options:NSKeyValueObservingOptionNew context:topBorderContext];
    });
}

- (void)addBottomBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    dispatch_async(dispatch_get_main_queue(), ^{
        CALayer *border = [CALayer layer];
        border.name = @"bottomBorderLayer";
        [self removePreviouslyAddedLayer:border.name];
        border.backgroundColor = color.CGColor;
        border.frame = CGRectMake(0, self.frame.size.height - borderWidth, self.frame.size.width, borderWidth);
        [self.layer addSublayer:border];
        [self addObserver:self forKeyPath: @"bounds" options:NSKeyValueObservingOptionNew context:bottomBorderContext];
    });
}

- (void)addLeftBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    dispatch_async(dispatch_get_main_queue(), ^{
        CALayer *border = [CALayer layer];
        border.name = @"leftBorderLayer";
        [self removePreviouslyAddedLayer:border.name];
        border.backgroundColor = color.CGColor;
        border.frame = CGRectMake(0, 0, borderWidth, self.frame.size.height);
        [self.layer addSublayer:border];
        [self addObserver:self forKeyPath: @"bounds" options:NSKeyValueObservingOptionNew context:leftBorderContext];
    });
}

- (void)addRightBorderWithColor:(UIColor *)color andWidth:(CGFloat) borderWidth {
    dispatch_async(dispatch_get_main_queue(), ^{
        CALayer *border = [CALayer layer];
        border.name = @"rightBorderLayer";
        [self removePreviouslyAddedLayer:border.name];
        border.backgroundColor = color.CGColor;
        border.frame = CGRectMake(self.frame.size.width - borderWidth, 0, borderWidth, self.frame.size.height);
        [self.layer addSublayer:border];
        [self addObserver:self forKeyPath: @"bounds" options:NSKeyValueObservingOptionNew context:rightBorderContext];
    });
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (context == topBorderContext) {
        for (CALayer *border in self.layer.sublayers) {
            if ([border.name isEqualToString:@"topBorderLayer"]) {
                [border setFrame:CGRectMake(0, 0, self.frame.size.width, border.frame.size.height)];
            }
        }
    } else if (context == bottomBorderContext) {
        for (CALayer *border in self.layer.sublayers) {
            if ([border.name isEqualToString:@"bottomBorderLayer"]) {
                [border setFrame:CGRectMake(0, self.frame.size.height - border.frame.size.height, self.frame.size.width, border.frame.size.height)];
            }
        }
    } else if (context == leftBorderContext) {
        for (CALayer *border in self.layer.sublayers) {
            if ([border.name isEqualToString:@"leftBorderLayer"]) {
                [border setFrame:CGRectMake(0, 0, border.frame.size.width, self.frame.size.height)];
            }
        }
    } else if (context == rightBorderContext) {
        for (CALayer *border in self.layer.sublayers) {
            if ([border.name isEqualToString:@"rightBorderLayer"]) {
                [border setFrame:CGRectMake(self.frame.size.width - border.frame.size.width, 0, border.frame.size.width, self.frame.size.height)];
            }
        }
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}
- (void)removePreviouslyAddedLayer:(NSString *)name {
    if (self.layer.sublayers.count > 0) {
        for (CALayer *layer in self.layer.sublayers) {
            if ([layer.name isEqualToString:name]) {
                [layer removeFromSuperlayer];
            }
        }
    }
}

@end

Uso: - Selecione qualquer controle do storyboard e, em seguida, mostre o inspetor de atributos (lado direito).

Agora você pode definir qualquer lado da cor e largura da borda.

Eu escrevi um método geral que adicionará uma borda em qualquer lado que você desejar UIView. Você pode definir espessura, cor, margens e zOrderpara cada lado.

/*
 view: the view to draw border around
 thickness: thickness of the border on the given side
 color: color of the border on the given side
 margin: space between the border's outer edge and the view's frame edge on the given side.
 zOrder: defines the order to add the borders to the view.  The borders will be added by zOrder from lowest to highest, thus making the highest priority border visible when two borders overlap at the corners.
*/

    +(void) drawBorderAroundUIView:(UIView *) view thicknessLeft:(CGFloat) thicknessLeft colorLeft:(UIColor *)colorLeft marginLeft:(CGFloat) marginLeft zOrderLeft:(int) zOrderLeft thicknessRight:(CGFloat) thicknessRight colorRight:(UIColor *)colorRight marginRight:(CGFloat) marginRight zOrderRight:(int) zOrderRight thicknessTop:(CGFloat) thicknessTop colorTop:(UIColor *)colorTop marginTop:(CGFloat) marginTop zOrderTop:(int) zOrderTop thicknessBottom:(CGFloat) thicknessBottom colorBottom:(UIColor *)colorBottom marginBottom:(CGFloat) marginBottom zOrderBottom:(int) zOrderBottom{
    //make margins be the outside edge and make positive margin represent a smaller rectangle
    marginBottom = -1 * marginBottom - thicknessBottom;
    marginTop = -1 * marginTop - thicknessTop;
    marginLeft = -1 * marginLeft - thicknessLeft;
    marginRight = -1 * marginRight - thicknessRight;

    //get reference points for corners
    CGPoint upperLeftCorner = CGPointZero;
    CGPoint lowerLeftCorner = CGPointMake(upperLeftCorner.x, upperLeftCorner.y + view.frame.size.height);
    CGPoint upperRightCorner = CGPointMake(upperLeftCorner.x + view.frame.size.width, upperLeftCorner.y);

    //left
    CALayer *leftBorder = [CALayer layer];
    leftBorder.frame = CGRectMake(upperLeftCorner.x - thicknessLeft - marginLeft, upperLeftCorner.y - thicknessTop - marginTop, thicknessLeft, view.frame.size.height + marginTop + marginBottom + thicknessBottom + thicknessTop);
    leftBorder.backgroundColor = colorLeft.CGColor;

    //right
    CALayer *rightBorder = [CALayer layer];
    rightBorder.frame = CGRectMake(upperRightCorner.x + marginRight, upperRightCorner.y - thicknessTop - marginTop, thicknessRight, view.frame.size.height + marginTop + marginBottom + thicknessBottom + thicknessTop);
    rightBorder.backgroundColor = colorRight.CGColor;

    //top
    CALayer *topBorder = [CALayer layer];
    topBorder.frame = CGRectMake(upperLeftCorner.x - thicknessLeft - marginLeft, upperLeftCorner.y - thicknessTop - marginTop, view.frame.size.width + marginLeft + marginRight + thicknessLeft + thicknessRight, thicknessTop);
    topBorder.backgroundColor = colorTop.CGColor;

    //bottom
    CALayer *bottomBorder = [CALayer layer];
    bottomBorder.frame = CGRectMake(upperLeftCorner.x - thicknessLeft - marginLeft, lowerLeftCorner.y + marginBottom, view.frame.size.width + marginLeft + marginRight + thicknessLeft + thicknessRight, thicknessBottom);
    bottomBorder.backgroundColor = colorBottom.CGColor;

    //define dictionary keys to be used for adding borders in order of zOrder
    NSString *borderDK = @"border";
    NSString *zOrderDK = @"zOrder";

    //storing borders in dictionaries in preparation to add them in order of zOrder
    NSDictionary *leftBorderDictionary = [NSDictionary dictionaryWithObjectsAndKeys:leftBorder, borderDK, [NSNumber numberWithInt:zOrderLeft], zOrderDK, nil];
    NSDictionary *rightBorderDictionary = [NSDictionary dictionaryWithObjectsAndKeys:rightBorder, borderDK, [NSNumber numberWithInt:zOrderRight], zOrderDK, nil];
    NSDictionary *topBorderDictionary = [NSDictionary dictionaryWithObjectsAndKeys:topBorder, borderDK, [NSNumber numberWithInt:zOrderTop], zOrderDK, nil];
    NSDictionary *bottomBorderDictionary = [NSDictionary dictionaryWithObjectsAndKeys:bottomBorder, borderDK, [NSNumber numberWithInt:zOrderBottom], zOrderDK, nil];

    NSMutableArray *borders = [NSMutableArray arrayWithObjects:leftBorderDictionary, rightBorderDictionary, topBorderDictionary, bottomBorderDictionary, nil];

    //add borders in order of zOrder (lowest -> highest).  Thus the highest zOrder will be added last so it will be on top.
    while (borders.count)
    {
        //look for the next lowest zOrder border to add
        NSDictionary *nextBorderToLayDown = [borders objectAtIndex:0];
        for (int indexOfBorder = 0; indexOfBorder < borders.count; indexOfBorder++)
        {
            NSDictionary *borderAtIndex = [borders objectAtIndex:indexOfBorder];
            if ([[borderAtIndex objectForKey:zOrderDK] intValue] < [[nextBorderToLayDown objectForKey:zOrderDK] intValue])
            {
                nextBorderToLayDown = borderAtIndex;
            }
        }
        //add the border to the view
        [view.layer addSublayer:[nextBorderToLayDown objectForKey:borderDK]];
        [borders removeObject:nextBorderToLayDown];
    }
}

Você não precisa adicionar uma camada para cada borda, basta usar um caminho de bezier para desenhá-las uma vez.

CGRect rect = self.bounds;

CGPoint destPoint[4] = {CGPointZero,
    (CGPoint){0, rect.size.height},
    (CGPoint){rect.size.width, rect.size.height},
    (CGPoint){rect.size.width, 0}};

BOOL position[4] = {_top, _left, _bottom, _right};

UIBezierPath *path = [UIBezierPath new];
[path moveToPoint:destPoint[3]];

for (int i = 0; i < 4; ++i) {
    if (position[i]) {
        [path addLineToPoint:destPoint[i]];
    } else {
        [path moveToPoint:destPoint[i]];
    }
}

CAShapeLayer *borderLayer = [CAShapeLayer new];
borderLayer.frame = self.bounds;
borderLayer.path  = path.CGPath;
borderLayer.lineWidth   = _borderWidth ?: 1 / [UIScreen mainScreen].scale;
borderLayer.strokeColor = _borderColor.CGColor;
borderLayer.fillColor   = [UIColor clearColor].CGColor;

[self.layer addSublayer:borderLayer];

Swift 4/3

Você pode usar esta solução abaixo. Ele funciona em UIBezierPaths, que são mais leves que as camadas, causando tempos de inicialização rápidos. É fácil de usar, veja as instruções abaixo.

class ResizeBorderView: UIView {
    var color = UIColor.white
    var lineWidth: CGFloat = 1
    var edges = [UIRectEdge](){
        didSet {
            setNeedsDisplay()
        }
    }
    override func draw(_ rect: CGRect) {
        if edges.contains(.top) || edges.contains(.all){
            let path = UIBezierPath()
            path.lineWidth = lineWidth
            color.setStroke()
            UIColor.blue.setFill()
            path.move(to: CGPoint(x: 0, y: 0 + lineWidth / 2))
            path.addLine(to: CGPoint(x: self.bounds.width, y: 0 + lineWidth / 2))
            path.stroke()
        }
        if edges.contains(.bottom) || edges.contains(.all){
            let path = UIBezierPath()
            path.lineWidth = lineWidth
            color.setStroke()
            UIColor.blue.setFill()
            path.move(to: CGPoint(x: 0, y: self.bounds.height - lineWidth / 2))
            path.addLine(to: CGPoint(x: self.bounds.width, y: self.bounds.height - lineWidth / 2))
            path.stroke()
        }
        if edges.contains(.left) || edges.contains(.all){
            let path = UIBezierPath()
            path.lineWidth = lineWidth
            color.setStroke()
            UIColor.blue.setFill()
            path.move(to: CGPoint(x: 0 + lineWidth / 2, y: 0))
            path.addLine(to: CGPoint(x: 0 + lineWidth / 2, y: self.bounds.height))
            path.stroke()
        }
        if edges.contains(.right) || edges.contains(.all){
            let path = UIBezierPath()
            path.lineWidth = lineWidth
            color.setStroke()
            UIColor.blue.setFill()
            path.move(to: CGPoint(x: self.bounds.width - lineWidth / 2, y: 0))
            path.addLine(to: CGPoint(x: self.bounds.width - lineWidth / 2, y: self.bounds.height))
            path.stroke()
        }
    }
}