Stevia：一款开源、简单、直观的纯代码自动布局类库

就自动布局而言，AutoLayout已经成为iOS开发者应用开发中必不可少的一部分，苹果一直主张开发者使用AutoLayout来布局，而关于iOS自动布局教程与经验分享亦数不胜数。但不可否认的是，Autolayout太过繁琐复杂，由此出现了Autosizing。除此之外，还有一种解决方案，就是Stevia（GitHub托管地址）。

Stevia是一款非常直观的纯代码自动布局类库，其主要贡献者S4cha在GitHub上写明了为什么会开发出这样一个自动布局类库：因为没有什么并纯代码更赞，Xibs和storyboards太过繁重、很难维护以及合并，将视图拆分成两个独立文件来调试简直就是一场噩梦，我们必须拥有一个更为便捷、效率更高的解决方案。

以登陆界面为例：

使用原生AutoLayout：

class LoginViewNative:UIView {

    let email = UITextField()
    let password = UITextField()
    let login = UIButton()

    convenience init() {
        self.init(frame:CGRectZero)
        backgroundColor = .whiteColor()

        addSubview(email)
        addSubview(password)
        addSubview(login)

        email.translatesAutoresizingMaskIntoConstraints = false
        password.translatesAutoresizingMaskIntoConstraints = false
        login.translatesAutoresizingMaskIntoConstraints = false

        addConstraint(NSLayoutConstraint(
                item: email,
                attribute: .Left,
                relatedBy: .Equal,
                toItem: self,
                attribute: .Left,
                multiplier: 1,
                constant: 8)
        )
        addConstraint(NSLayoutConstraint(
                item: email,
                attribute: .Right,
                relatedBy: .Equal,
                toItem: self,
                attribute: .Right,
                multiplier: 1,
                constant: -8)
        )
        addConstraint(NSLayoutConstraint(
            item: password,
            attribute: .Left,
                relatedBy: .Equal,
                toItem: self,
                attribute: .Left,
                multiplier: 1,
                constant: 8)
        )
        addConstraint(NSLayoutConstraint(
            item: password,
            attribute: .Right,
            relatedBy: .Equal,
            toItem: self,
            attribute: .Right,
            multiplier: 1,
            constant: -8)
        )
        addConstraint(NSLayoutConstraint(
            item: login,
            attribute: .Left,
            relatedBy: .Equal,
            toItem: self,
            attribute: .Left,
            multiplier: 1,
            constant: 0)
        )
        addConstraint(NSLayoutConstraint(
            item: login,
            attribute: .Right,
            relatedBy: .Equal,
            toItem: self,
            attribute: .Right,
            multiplier: 1,
            constant: 0)
        )
        for b in [email, password, login] {
            addConstraint(NSLayoutConstraint(
                item: b,
                attribute: .Height,
                relatedBy: .Equal,
                toItem: nil,
                attribute: .NotAnAttribute,
                multiplier: 1,
                constant: 80)
            )
        }
        addConstraint(NSLayoutConstraint(
            item: email,
            attribute: .Top,
            relatedBy: .Equal,
            toItem: self,
            attribute: .Top,
            multiplier: 1,
            constant: 100)
        )
        addConstraint(NSLayoutConstraint(
            item:email,
            attribute: .Bottom,
            relatedBy: .Equal,
            toItem: password,
            attribute: .Top,
            multiplier: 1,
            constant: -8)
        )
        addConstraint(NSLayoutConstraint(
            item: login,
            attribute: .Bottom,
            relatedBy: .Equal,
            toItem: self,
            attribute: .Bottom,
            multiplier: 1,
            constant: 0)
        )

        email.placeholder = "Email"
        email.borderStyle = .RoundedRect
        email.autocorrectionType = .No
        email.keyboardType = .EmailAddress
        email.font = UIFont(name: "HelveticaNeue-Light", size: 26)
        email.returnKeyType = .Next

        password.placeholder = "Password"
        password.borderStyle = .RoundedRect
        password.font = UIFont(name: "HelveticaNeue-Light", size: 26)
        password.secureTextEntry = true
        password.returnKeyType = .Done

        login.setTitle("Login", forState: .Normal)
        login.backgroundColor = .lightGrayColor()
        login.addTarget(self, action: "loginTapped", forControlEvents: .TouchUpInside)
        login.setTitle(NSLocalizedString("Login", comment: ""), forState: .Normal)
    }

    func loginTapped() {
        //Do something
    }
}

使用Stevia：

class LoginViewStevia:UIView {

    let email = UITextField()
    let password = UITextField()
    let login = UIButton()

    convenience init() {
        self.init(frame:CGRectZero)
        backgroundColor = .whiteColor()

        sv(
            email.placeholder("Email").style(fieldStyle), //.style(emailFieldStyle),
            password.placeholder("Password").style(fieldStyle).style(passwordFieldStyle),
            login.text("Login").style(buttonStyle).tap(loginTapped)
        )

        layout(
            100,
            |-email-| ~ 80,
            8,
            |-password-| ~ 80,
            "",
            |login| ~ 80,
            0
        )
    }

    func fieldStyle(f:UITextField) {
        f.borderStyle = .RoundedRect
        f.font = UIFont(name: "HelveticaNeue-Light", size: 26)
        f.returnKeyType = .Next
    }

    func passwordFieldStyle(f:UITextField) {
        f.secureTextEntry = true
        f.returnKeyType = .Done
    }

    func buttonStyle(b:UIButton) {
        b.backgroundColor = .lightGrayColor()
    }

    func loginTapped() {
        //Do something
    }
}