Swift常量&变量

前言

优先使用常量[let]，只有发现标识符需要修改时，再使用变量，防止在其他不希望修改的地方，不小心将值改掉

常量的本质：指向内存地址不可以修改，可以通过内存地址找到对应的对象，之后修改对象内部的属性

OC创建对象&Swift创建对象[类型()]：

OC:UIView *view = [[UIView alloc] init];
Swift:var view : UIView = UIView()

通过内存地址找到对应的对象，之后修改对象内部的属性

let view2 : UIView = UIView()
view2.alpha = 0.5
view2.backgroundColor = UIColor.red
view2.frame = CGRect(x: 0, y: 0, width: 100, height: 100)

例子[创建一个UIView对象，并在其中添加一个UIButton，点击按钮，切换UIView对象的背景颜色]

import UIKit

fileprivate let SCREEN_WIDTH : CGFloat = UIScreen.main.bounds.width
fileprivate let SCREEN_HEIGHT : CGFloat = UIScreen.main.bounds.height

class ViewController: UIViewController {

    var customView : UIView = UIView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        setupUI()
    }

    func setupUI() {
        //创建UIView对象
        let viewRect = CGRect(x: 0, y: 0, width: SCREEN_WIDTH, height: SCREEN_HEIGHT)
        let view : UIView = UIView(frame: viewRect)
        
        //设置view属性
        view.backgroundColor = UIColor.purple
        
        //将view加入到当前控制器中
        self.view .addSubview(view)
        customView = view
        
        //创建UIButton对象
        let btn : UIButton = UIButton()
        
        //给btn设置属性
        btn.frame = CGRect(x: 10, y: 300, width: SCREEN_WIDTH-20, height: 30)
        btn.backgroundColor = UIColor.white
        btn.titleLabel?.textAlignment = .center
        btn .setTitleColor(UIColor.gray, for: .normal)
        btn.setTitle("点我换色", for: .normal)
        btn.layer.cornerRadius = 5;
        btn.layer.masksToBounds = true
        btn.addTarget(self, action: #selector(clickBtn), for: .touchUpInside)
        
        
        //将btn加入到view中
        view.addSubview(btn)
    }
    @objc
    func clickBtn(btn : UIButton) {
        btn.setTitle("再换一种", for: .normal)
        customView.backgroundColor = UIColor.randomColor
    }


}

extension UIColor {
    //返回随机颜色
    class var randomColor: UIColor {
        get {
            let red = CGFloat(arc4random()%256)/255.0
            let green = CGFloat(arc4random()%256)/255.0
            let blue = CGFloat(arc4random()%256)/255.0
            return UIColor(red: red, green: green, blue: blue, alpha: 1.0)
        }
    }
}

效果图

clickBtn.gif