Finished when -> if workaround, but it does not work for multiple whens.

.vscode/spellright.dict

@@ -7,3 +7,4 @@ req
 fnv
 Fnv
 Deinit
+syn

src/core/var.rs

@@ -883,7 +883,7 @@ impl<T: VarValue, V: Var<T> + Clone> ObjVar<T> for ReadOnlyVar<T, V> {
     }
 }
 
-impl<T: VarValue, V: Var<T> + Clone> Clone for ReadOnlyVar<T, V> {
+impl<T: VarValue, V: Var<T>> Clone for ReadOnlyVar<T, V> {
     fn clone(&self) -> Self {
         ReadOnlyVar {
             _t: PhantomData,
@@ -892,7 +892,7 @@ impl<T: VarValue, V: Var<T> + Clone> Clone for ReadOnlyVar<T, V> {
     }
 }
 
-impl<T: VarValue, V: Var<T> + Clone> Var<T> for ReadOnlyVar<T, V> {
+impl<T: VarValue, V: Var<T>> Var<T> for ReadOnlyVar<T, V> {
     type AsReadOnly = Self;
     type AsLocal = CloningLocalVar<T, Self>;
 
@@ -931,6 +931,14 @@ impl<T: VarValue, V: Var<T> + Clone> Var<T> for ReadOnlyVar<T, V> {
     }
 }
 
+impl<T: VarValue, V: Var<T>> IntoVar<T> for ReadOnlyVar<T, V> {
+    type Var = Self;
+    #[inline]
+    fn into_var(self) -> Self::Var {
+        self
+    }
+}
+
 // #endregion ReadOnlyVar<T>
 
 // #region MapSharedVar<T> and MapBiDiSharedVar<T>
@@ -1857,6 +1865,22 @@ where
     }
 }
 
+impl<T, S, O, M, N> IntoVar<O> for MapVarBiDi<T, S, O, M, N>
+where
+    T: VarValue,
+    S: ObjVar<T>,
+    O: VarValue,
+    M: FnMut(&T) -> O + 'static,
+    N: FnMut(&O) -> T + 'static,
+{
+    type Var = Self;
+
+    #[inline]
+    fn into_var(self) -> Self::Var {
+        self
+    }
+}
+
 // #endregion MapVar<T> and MapVarBidi<T>
 
 // #region SwitchVar2<T>..SwitchVar8<T>

src/widgets/button_.rs

@@ -4,7 +4,6 @@ pub use crate::properties::{align, background_color, is_hovered, is_pressed, on_
 use crate::widget;
 use crate::widgets::{container, focusable_mixin};
 
-
 context_var! {
     /// Default background of [`button!`](crate::widgets::button) widgets.
     pub struct ButtonBackground: ColorF = rgb(0.2, 0.2, 0.2);
@@ -25,16 +24,16 @@ widget! {
         background_color: ButtonBackground;
     }
 
-    ///// When the pointer device is over this button.
-    //when self.is_hovered {
-    //    background_color: ButtonBackgroundHovered;
+    /// When the pointer device is over this button.
+    when self.is_hovered {
+        background_color: ButtonBackgroundHovered;
+    }
+
+    ///// When the mouse or touch pressed on this button and has not yet released.
+    //when  self.is_pressed  {
+    //    background_color: ButtonBackgroundPressed;
     //}
 
-    /// When the mouse or touch pressed on this button and has not yet released.
-    when self.content_align == crate::properties::Alignment::CENTER  {
-        background_color: ButtonBackgroundPressed;
-    }
-    
    ///// When the button is not enabled.
    //when {
    //    for i in 0..1000 {
@@ -44,12 +43,10 @@ widget! {
    //            return self.is_hovered.0
    //        }
    //    }
-   //    self.is_pressed && test(self.content_align)
+   //    self.is_pressed
    //} {
    //    background_color: ButtonBackgroundDisabled;
    //}
 }
 
-fn test(a: crate::properties::Alignment) -> bool {
-    a == crate::properties::Alignment::CENTER
-}
+//TODO support properties with IntoVar parameters.

zero-ui-macros/src/widget.rs

@@ -368,7 +368,8 @@ fn declare_widget(mixin: bool, mut input: WidgetInput) -> proc_macro::TokenStrea
         let pss = properties
             .clone()
             .map(|p| if defined_props.contains(p) { quote! (ps::) } else { quote!() });
-        let params = quote!(#(#param_names: &impl #pss#properties::Args),*); {}       
+        let params = quote!(#(#param_names: &impl #pss#properties::Args),*);
+        {}
 
         let local_names = property_members.keys();
         let param_names = property_members.values().map(|p| &property_params[&p.property]);
@@ -944,22 +945,35 @@ pub struct WhenBlock {
 }
 impl Parse for WhenBlock {
     fn parse(input: ParseStream) -> Result<Self> {
+        // we use `ExprIf` parse because we need the private
+        // function `syn::expr::parsing::expr_no_struct` to
+        // parse the condition expression.
+
+        // require `when`.
         input.parse::<keyword::when>()?;
-        let mut if_ = quote! ( if );
+
+        // recreate a TokenStream where `when` is `if`.
+        let mut if_ = quote!(if);
         if_.extend(input.parse::<TokenStream>()?);
 
-        panic!{"hello {}", quote!{#if_}}
-        let condition = input.parse()?;
+        let if_: ExprIf = syn::parse2(if_)?;
 
-        let inner;
-        braced!(inner in input);
+        let condition = *if_.cond;
+        let attrs = if_.attrs;
 
-        let attrs = Attribute::parse_inner(input)?;
+        let ps = if_.then_branch;
+        let ps = quote!(#ps);
 
-        let mut properties = vec![];
-        while !inner.is_empty() {
-            properties.push(inner.parse()?);
-        }
+        let parse_properties = |input: ParseStream| {
+            let inner;
+            braced!(inner in input);
+            let mut properties = vec![];
+            while !inner.is_empty() {
+                properties.push(inner.parse()?);
+            }
+            Ok(properties)
+        };
+        let properties = parse_properties.parse2(ps)?;
 
         Ok(WhenBlock {
             attrs,