EasyLayout makes writing auto layout code in Xamarin.iOS F# easier.

EasyLayout.fs

module EasyLayout

open System
open System.Drawing
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.Patterns
open Microsoft.FSharp.Quotations.DerivedPatterns
open MonoTouch.Foundation
open MonoTouch.UIKit

module private Utilities =

    let rec eval e =
        match e with
        | FieldGet (Some o, f) -> f.GetValue (eval o)
        | FieldGet (None, f) -> f.GetValue (null)
        | PropertyGet (None, p, i) -> p.GetValue (null, i |> Seq.map eval |> Seq.toArray)
        | PropertyGet (Some o, p, i) -> p.GetValue (eval o, i |> Seq.map eval |> Seq.toArray)
        | Value (x, _) -> x
        | _ -> raise (Exception (sprintf "Don't know how to eval %A" e))

    let toAttr m =
        match m with
        | "X" | "Left" -> NSLayoutAttribute.Left
        | "Y" | "Top" -> NSLayoutAttribute.Top
        | "Width" -> NSLayoutAttribute.Width
        | "Height" -> NSLayoutAttribute.Height
        | "Bottom" -> NSLayoutAttribute.Bottom
        | "Right" -> NSLayoutAttribute.Right
        | "CenterX" | "RectangleF.get_CenterX" -> NSLayoutAttribute.CenterX
        | "CenterY" | "RectangleF.get_CenterY" -> NSLayoutAttribute.CenterY
        | "Baseline" | "RectangleF.get_Baseline" -> NSLayoutAttribute.Baseline
        | _ -> NSLayoutAttribute.NoAttribute

    let isConstrainableProperty m = toAttr m <> NSLayoutAttribute.NoAttribute

    let (|GetFrameProp|) e =
        match e with
        | Let (_, PropertyGet (Some o, fn, _), PropertyGet (_, pn, _))
            when fn.Name = "Frame" && isConstrainableProperty pn.Name ->
            Some (eval o :?> NSObject, toAttr pn.Name)
        | Call (_, pn, [PropertyGet (Some o, fn, _)])
            when fn.Name = "Frame" && isConstrainableProperty pn.Name ->
            Some (eval o :?> NSObject, toAttr pn.Name)
        | _ -> None

    let compileLeftSide side =
        match side with
        | GetFrameProp (Some x) -> x
        | _ -> raise (Exception (sprintf "Left hand side of constraint is expected to be a UIView.Frame property. It was: %A" side))

    let (|Mul|) side =
        match side with
        | GetFrameProp (Some (x, p)) -> Some (x, p, 1.0f)
        | Call (_, m, [l; GetFrameProp (Some (x, p))]) when m.Name = "op_Multiply" -> Some (x, p, Convert.ToSingle (eval l))
        | Call (_, m, [GetFrameProp (Some (x, p)); l]) when m.Name = "op_Multiply" -> Some (x, p, Convert.ToSingle (eval l))
        | _ -> None

    let compileRightSide side =
        match side with
        | Mul (Some x) -> (Some x, 0.0f)
        | Call (_, mem, [Mul (Some x); c]) when mem.Name = "op_Addition" -> (Some x, Convert.ToSingle (eval c))
        | Call (_, mem, [Mul (Some x); c]) when mem.Name = "op_Subtraction" -> (Some x, -Convert.ToSingle (eval c))
        | Value (x, _) -> (None, Convert.ToSingle (x))
        | FieldGet _ -> (None, Convert.ToSingle (eval side))
        | _ -> raise (Exception (sprintf "Unrecognized right hand side: %A." side))

    let compileConstraint left right rel =
        let (firstObj, firstAttr) = compileLeftSide left
        let (maybeObj, add) = compileRightSide right
        match maybeObj with
        | None -> NSLayoutConstraint.Create (firstObj, firstAttr, rel, null, NSLayoutAttribute.NoAttribute, 0.0f, add)
        | Some (secObj, secAttr, mul) -> NSLayoutConstraint.Create (firstObj, firstAttr, rel, secObj, secAttr, mul, add)

    let toRel m =
        match m with
        | "op_Equality" -> Some NSLayoutRelation.Equal
        | "op_LessThanOrEqual" -> Some NSLayoutRelation.LessThanOrEqual
        | "op_GreaterThanOrEqual" -> Some NSLayoutRelation.GreaterThanOrEqual
        | _ -> None

    let rec compileConstraints expr  =
        match expr with
        | NewArray (_, es) -> es |> Seq.collect compileConstraints |> Seq.toList
        | IfThenElse (i, t, e) -> compileConstraints i @ compileConstraints t @ compileConstraints e
        | Call (_, m, [l; r]) when (toRel m.Name).IsSome -> [compileConstraint l r (toRel m.Name).Value]
        | Value _ -> []
        | _ -> raise (Exception (sprintf "Unable to recognize constraints in expression: %A" expr))

type RectangleF with
    member this.CenterX = this.X + this.Width / 2.0f
    member this.CenterY = this.Y + this.Height / 2.0f
    member this.Baseline = 0.0f

type UIView with
    /// <summary>
    /// <para>Constrains the layout of subviews according to equations and
    /// inequalities specified in <paramref name="constraints"/>.  Issue
    /// multiple constraints per call using the &amp;&amp; operator.</para>
    /// <code><@ button.Frame.Left &gt;= text.Frame.Right + 22 &amp;&amp;
    /// button.Frame.Width = View.Frame.Width * 0.42f @></code>
    /// </summary>
    /// <param name="constraints">Constraint equations and inequalities.</param>
    member this.ConstrainLayout (constraints) =
        let cs = Utilities.compileConstraints constraints |> Seq.toArray
        this.AddConstraints (cs)
        for x in cs do
            (x.FirstItem :?> UIView).TranslatesAutoresizingMaskIntoConstraints <- false
        cs

I'm curious: since the C# version was created a while ago (and presumably like the rest of us your code skill has improved in the meantime), would you think that it's possible to re-work the C# version so it's not so verbose compared to F#? Or are we condemned to always writing about 3x more code if we write C#?

In C# you are always condemned to writing more code.

Hmm... I wonder whether it is necessary to use quotations. It might be possible to achieve similar structuring of the desired constraints using a combinator library rather than quotation interpretation. The main difficulty with a typical interpreter approach is that the interpreter is one closed piece of functionality and to extend it, you need to modify the interpreter. The interpreter then accumulates complexity and becomes a code structuring bottleneck. In the example, you are using individual properties (Width, Top, and so on). Suppose you wish to express more complicated constraints that deal with multiple properties at once. With a combinator library, the user of the constraint mechanism could likely easily write such more complicated constraints as ordinary F# expressions and use those.

@VesaKarvonen Thats why I mentioned the custom symbol earlier on, that way you could use infix notation for the constraints still. You could also use a computation expression to hide the complexity of the combinators too.

I think it's better to rewrite

let (|GetFrameProp|) e = ...

match side with
| GetFrameProp (Some x) -> ...

with

let (|GetFrameProp|_|) e = ...

match side with
| GetFrameProp x -> ...

Thanks for building this. I'm keen to keep my auto-layout code tight!

I'm trying EasyLayout and it works fine on the simulator. However, I am getting an exception when debugging on a real iPhone. The code that is crashing is as follows:

override __.ViewDidLoad() =
    base.ViewDidLoad()

    let rootView = __.View
    rootView.BackgroundColor <- UIColor.LightGray

    // Create a sub content view.
    let contentView = new UIView(BackgroundColor = UIColor.LightGray)
    rootView.AddSubview(contentView)
    rootView.ConstrainLayout(
        <@[| contentView.Frame.Top = rootView.Frame.Top + 45.0f
             contentView.Frame.Left = rootView.Frame.Left
             contentView.Frame.Right = rootView.Frame.Right
             contentView.Frame.Bottom = rootView.Frame.Bottom |]@>) |> ignore

The exception in questions is as follows:

Failed to bind property 'Frame'. Parameter name: propName.

Anybody experienced this?

awesome!

Awesome Stuff! I updated it to work with the Unified API: https://gist.github.com/hussamal/0127e676a789fc704d8e