一、场景描述：

情景1：image尺寸比button大，直接设置AspectFill
情景2：image尺寸比button小，默认会居中显示且不会充满整个button，如果希望充满整个button需要自定义button并重写imageRect方法，参见以下代码。

图片在左文字在右结构的button示例

import UIKit

class RTSettingBtn: UIButton {

    override init(frame: CGRect) {
        
        super.init(frame: frame)
    
        setImage(UIImage(named:"L_set"), for: UIControl.State.normal)
        titleLabel?.textAlignment = NSTextAlignment.center
        titleLabel?.font = UIFont.systemFont(ofSize: 18, weight: UIFont.Weight.medium)
        setTitle("设置", for: UIControl.State.normal)
        setTitleColor(UIColor.black, for: UIControl.State.normal)
        
        let space = 10
        titleEdgeInsets = UIEdgeInsets(top: 0, left: space/2.0, bottom: 0, right: -space/2.0)
        imageEdgeInsets = UIEdgeInsets(top: 0, left: -space/2.0, bottom: 0, right: space/2.0)
        imageView?.contentMode = .scaleAspectFill

        sizeToFit()
    }
    
    override func imageRect(forContentRect contentRect: CGRect) -> CGRect {
        return CGRect(x: 0, y: 0, width: self.height, height: self.height)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

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