Can I create a StackLayout with F# in Xamarin Forms? - f#

Question

In his Xamarin 3 F# Awesomeness blog posting, Dave Thomas shows a StackLayout being created:
StackLayout.Create(
[
Entry (Placeholder = "Username")
Entry (Placeholder = "Password", IsPassword = true)
Button (Text = "Login", TextColor = Color.White, BackgroundColor = Color.FromHex "77D065")
],...
However, I get an error The field, constructor or member 'Create' is not defined
If I take out the .Create before the parens, the error message changes to The member of object constructor 'StackLayout' takes 0 arguments but is here given 1.
My interpretation is that Create is a static method taking a tuple but I can't see it defined anywhere in the assembly browser.
I'm also a bit bothered in his sample by a lowercase create being used on TabbedPage so it looks like the code is inconsistent and possibly typed in without compiling, although he shows screenshots below.
I will take any suggestion on how to do this - if there's no missing magical extension to add Create, I'm happy to take other approaches.
The parent class declaration, in the docs, is:
[Xamarin.Forms.ContentProperty("Children")]
public abstract class Layout<T> : Layout, IViewContainer<T>
where T : Xamarin.Forms.View
I figure there's an idiom for setting that content Children property available as initialisation to C# but maybe not to F#?
For example, I've got compiling code from the Forms Gallery sample such as:
StackLayout stackLayout = new StackLayout
{
Spacing = 0,
VerticalOptions = LayoutOptions.FillAndExpand,
Children =
{
new Label
{
Text = "StackLayout",
HorizontalOptions = LayoutOptions.Start
},
I tried transcribing this into F# syntax - the following is legal syntax but with the Children property being set gets the error The member of object constructor 'StackLayout' has no argument or settable return property 'Children'. The required signature is StackLayout(): unit. (FS0495)
type App() =
static member GetMainPage =
let sl = new StackLayout (
Spacing = 20,
VerticalOptions = LayoutOptions.FillAndExpand,
Children = [
new Label (Text = "StackLayout");
new Label (Text = "SuckLayout")
]
)
new ContentPage( Content = sl )
So, consider me baffled - I have no idea why Children is accessible from C# and not F#. The only suspicion that makes sense is that somehow the XAML content property annotation makes a difference to one compiler and not the other.
I am a very experienced C++ and Python programmer learning F# (and Swift) so I'm likely to stumble over syntax. I get tuples, I'm getting used to the weird role of commas and thankfully my Python background made me relaxed about whitespace.
Someone else had the same question in the comments on the blog post but no answer was posted.
I am using Xamarin.iOS 8.10.0.258
Xamarin.Forms 1.4.0.6341
My fallback is to give up on F# for GUI and use C# there with F# for main logic but I really liked the much more compact syntax.
Note that I also asked this over on their forums

Answer 1

The C# code you show is using the collection initialisers feature of C#, it will compile to calls to Children.Add(), it is not setting the Children property.
F# does not have the same collection initialisers feature so you will have to manually call Children.Add() which is most likely what the StackLayout.Create function is doing.
The StackLayout.Create function could be a local function defined in a module called StackLayout which would explain why you don't see it in the docs.

Answer 2

Thanks to Leaf Garland above, here's the fixed function - I just needed that hint that C# was using Add.
type App() =
static member GetMainPage =
let sl = new StackLayout (
Spacing = 20.0,
VerticalOptions = LayoutOptions.FillAndExpand
)
let slc : View[] = [|
new Entry (Placeholder = "Username")
new Entry (Placeholder = "Password", IsPassword = true)
new Button (Text = "Login", TextColor = Color.White, BackgroundColor = Color.FromHex "77D065")
|]
slc |> Seq.iter (fun vc -> sl.Children.Add(vc))
new ContentPage( Content = sl )
Note that I had to create the temp variable slc to get an array typed as the superclass View
If you had all the same types you could use a literal list and slightly simpler code like this:
type App() =
static member GetMainPage =
let sl = new StackLayout (
Spacing = 20.0,
VerticalOptions = LayoutOptions.FillAndExpand
)
[
new Label (Text = "StackLayout")
new Label (Text = "SuckLayout")
] |> Seq.iter (fun vc -> sl.Children.Add(vc))
new ContentPage( Content = sl )
Here's a nice article about the C# collection initialisers if you're interested.
I"m too busy tonight but will try to write the StackLayout.Create function as an exercise.