btn1.titleEdgeInsets= UIEdgeInsets(top: 0, left: 20, bottom: 0, right: 0)

2，扩展UIButton
如果我们想要把文字和图片位置调换下（即文字在前、图片在后），或者文字和图片改成上下排列，那么同样通过设置 titleEdgeInsets 和 imageEdgeInsets 即可实现。
为方便快速的设置图片和文字的相对位置，以及间距，这里对UIButton进行扩展。
（1）扩展代码如下：

import UIKit

extension UIButton {

    @objc func set(image anImage: UIImage?, title: String,
        titlePosition: UIViewContentMode, additionalSpacing: CGFloat, state: UIControlState){
        self.imageView?.contentMode = .Center
        self.setImage(anImage, forState: state)
        
        positionLabelRespectToImage(title, position: titlePosition, spacing: additionalSpacing)
        
        self.titleLabel?.contentMode = .Center
        self.setTitle(title, forState: state)
    }
    
    private func positionLabelRespectToImage(title: String, position: UIViewContentMode,
        spacing: CGFloat) {
        let imageSize = self.imageRectForContentRect(self.frame)
        let titleFont = self.titleLabel?.font!
        let titleSize = title.sizeWithAttributes([NSFontAttributeName: titleFont!])
        
        var titleInsets: UIEdgeInsets
        var imageInsets: UIEdgeInsets
        
        switch (position){
        case .Top:
            titleInsets = UIEdgeInsets(top: -(imageSize.height + titleSize.height + spacing),
                left: -(imageSize.width), bottom: 0, right: 0)
            imageInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: -titleSize.width)
        case .Bottom:
            titleInsets = UIEdgeInsets(top: (imageSize.height + titleSize.height + spacing),
                left: -(imageSize.width), bottom: 0, right: 0)
            imageInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: -titleSize.width)
        case .Left:
            titleInsets = UIEdgeInsets(top: 0, left: -(imageSize.width * 2), bottom: 0, right: 0)
            imageInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0,
                right: -(titleSize.width * 2 + spacing))
        case .Right:
            titleInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: -spacing)
            imageInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        default:
            titleInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
            imageInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        }
        
        self.titleEdgeInsets = titleInsets
        self.imageEdgeInsets = imageInsets
    }
}

（2）使用样例

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let btn1:UIButton = UIButton(frame: CGRect(x: 0, y: 0, width: 200, height: 32))
        btn1.center = CGPointMake(view.frame.size.width/2, 60)
        btn1.titleLabel?.font = UIFont.boldSystemFontOfSize(28) //文字大小
        btn1.setTitleColor(UIColor.orangeColor(), forState: UIControlState.Normal) //文字颜色
        btn1.set(image: UIImage(named: "alert"), title: "文字在左侧", titlePosition: .Left,
            additionalSpacing: 10.0, state: .Normal)
        view.addSubview(btn1)
        
        
        let btn2:UIButton = UIButton(frame: CGRect(x: 0, y: 0, width: 200, height: 32))
        btn2.center = CGPointMake(view.frame.size.width/2, 120)
        btn2.titleLabel?.font = UIFont.boldSystemFontOfSize(28) //文字大小
        btn2.setTitleColor(UIColor.orangeColor(), forState: UIControlState.Normal) //文字颜色
        btn2.set(image: UIImage(named: "alert"), title: "文字在右侧", titlePosition: .Right,
            additionalSpacing: 10.0, state: .Normal)
        view.addSubview(btn2)
        
        let btn3:UIButton = UIButton(frame: CGRect(x: 0, y: 0, width: 170, height: 70))
        btn3.center = CGPointMake(view.frame.size.width/2, 230)
        btn3.titleLabel?.font = UIFont.boldSystemFontOfSize(28) //文字大小
        btn3.setTitleColor(UIColor.orangeColor(), forState: UIControlState.Normal) //文字颜色
        btn3.set(image: UIImage(named: "alert"), title: "文字在上方", titlePosition: .Top,
            additionalSpacing: 10.0, state: .Normal)
        view.addSubview(btn3)
        
        let btn4:UIButton = UIButton(frame: CGRect(x: 0, y: 0, width: 170, height: 70))
        btn4.center = CGPointMake(view.frame.size.width/2, 290)
        btn4.titleLabel?.font = UIFont.boldSystemFontOfSize(28) //文字大小
        btn4.setTitleColor(UIColor.orangeColor(), forState: UIControlState.Normal) //文字颜色
        btn4.set(image: UIImage(named: "alert"), title: "文字在下方", titlePosition: .Bottom,
            additionalSpacing: 10.0, state: .Normal)
        view.addSubview(btn4)
    }

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