Swift自定義View和OC自定義View的原理都是一樣的，重寫init()方法或initWithFrame()方法，下面簡單說說如何自定義swift UIView

主要是重寫init（frame：CGRect)方法，其中required init?(coder aDecoder: NSCoder) 是必不可少的，swift語言強制要求的

//
//  LoginImgFieldView.swift
//  SmartMilk
//
//  Created by mac on 2017/6/9.
//  Copyright © 2017年 mac. All rights reserved.
//

import UIKit

let G_HEADIMAGE_HEIGHT:CGFloat = 30


class LoginImgFieldView: UIView {

    var headImgView:UIImageView?
    var field:UITextField?
    var line:UILabel?
    
    
    
    /*
    // Only override draw() if you perform custom drawing.
    // An empty implementation adversely affects performance during animation.
    override func draw(_ rect: CGRect) {
        // Drawing code
    }
    */
//    override init(){
//        super.init()
//        setupSubViews()
//    }
    
    override init(frame:CGRect){
        super.init(frame: frame)
        setupSubViews()
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func setupSubViews() {
        
        let frame = self.bounds
        //uiimageVIew
        headImgView = UIImageView(frame:CGRect(x:0,y:0,width:G_HEADIMAGE_HEIGHT,height:G_HEADIMAGE_HEIGHT))
        headImgView?.backgroundColor = UIColor.blue
        
        //uiTextField
        field = UITextField(frame:CGRect(x:0,y:0,width:100,height:G_HEADIMAGE_HEIGHT))
//        field?.font = UIFont.systemFont(ofSize: 15)
        
        //uiLabel
        
        line = UILabel(frame: CGRect(x:0,y:frame.size.height-1,width:frame.size.width,height:1))
        line?.backgroundColor = UIColor.orange
        
        self.addSubview(headImgView!)
        self.addSubview(field!)
        self.addSubview(line!)
    }
    
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        let frame = self.bounds
        let imgY = (frame.size.height - G_HEADIMAGE_HEIGHT)/2
        headImgView?.frame = CGRect(x:0,y:imgY,width:G_HEADIMAGE_HEIGHT,height:G_HEADIMAGE_HEIGHT)
        
        //field
        let fieldx = G_HEADIMAGE_HEIGHT+5
        let fieldWidth = frame.size.width - fieldx
        
        field?.frame = CGRect(x: fieldx, y: imgY, width: fieldWidth, height: G_HEADIMAGE_HEIGHT)
        
        //label
        var lineFrame = line?.frame
        lineFrame?.origin.y = frame.size.height - 2
        lineFrame?.size.width = frame.size.width
        
        line?.frame = lineFrame!
        
    }

}