Make window title LabelText instead of LocalizedString

It isn't really possible for code outside of druid to use
LocalizedString in any reasonable way at the moment, and so
this makes it much easier to set a window title, especially
a title that is generated dynamically from the data.

druid/src/app.rs

@@ -17,6 +17,7 @@
 use crate::ext_event::{ExtEventHost, ExtEventSink};
 use crate::kurbo::Size;
 use crate::shell::{Application, Error as PlatformError, WindowBuilder, WindowHandle};
+use crate::widget::LabelText;
 use crate::win_handler::{AppHandler, AppState};
 use crate::window::WindowId;
 use crate::{
@@ -40,7 +41,7 @@ pub struct AppLauncher<T> {
 /// window properties such as the title.
 pub struct WindowDesc<T> {
     pub(crate) root: Box<dyn Widget<T>>,
-    pub(crate) title: LocalizedString<T>,
+    pub(crate) title: LabelText<T>,
     pub(crate) size: Option<Size>,
     pub(crate) min_size: Option<Size>,
     pub(crate) menu: Option<MenuDesc<T>>,
@@ -148,7 +149,7 @@ impl<T: Data> WindowDesc<T> {
         // this just makes our API slightly cleaner; callers don't need to explicitly box.
         WindowDesc {
             root: root().boxed(),
-            title: LocalizedString::new("app-name"),
+            title: LocalizedString::new("app-name").into(),
             size: None,
             min_size: None,
             menu: MenuDesc::platform_default(),
@@ -158,12 +159,14 @@ impl<T: Data> WindowDesc<T> {
         }
     }
 
-    /// Set the title for this window. This is a [`LocalizedString`] that will
-    /// be kept up to date as the application's state changes.
+    /// Set the title for this window. This is a [`LabelText`]; it can be either
+    /// a `String`, a [`LocalizedString`], or a closure that computes a string;
+    /// it that will be kept up to date as the application's state changes.
     ///
+    /// [`LabelText`]: widget/enum.LocalizedString.html
     /// [`LocalizedString`]: struct.LocalizedString.html
-    pub fn title(mut self, title: LocalizedString<T>) -> Self {
-        self.title = title;
+    pub fn title(mut self, title: impl Into<LabelText<T>>) -> Self {
+        self.title = title.into();
         self
     }
 
@@ -231,7 +234,7 @@ impl<T: Data> WindowDesc<T> {
             builder.set_min_size(min_size);
         }
 
-        builder.set_title(self.title.localized_str());
+        builder.set_title(self.title.display_text());
         if let Some(menu) = platform_menu {
             builder.set_menu(menu);
         }

druid/src/widget/label.rs

@@ -30,7 +30,13 @@ const BASELINE_GUESS_FACTOR: f64 = 0.8;
 // added padding between the edges of the widget and the text.
 const LABEL_X_PADDING: f64 = 2.0;
 
-/// The text for the label
+/// The text for the label.
+///
+/// This can be one of three things; either a `String`, a [`LocalizedString`],
+/// or a closure with the signature, `Fn(&T, &Env) -> String`, where `T` is
+/// the `Data` at this point in the tree.
+///
+/// [`LocalizedString`]: ../struct.LocalizedString.html
 pub enum LabelText<T> {
     /// Localized string that will be resolved through `Env`.
     Localized(LocalizedString<T>),

druid/src/window.rs

@@ -24,11 +24,12 @@ use crate::piet::{Piet, RenderContext};
 use crate::shell::{Counter, Cursor, WindowHandle};
 
 use crate::core::{BaseState, CommandQueue, FocusChange};
+use crate::widget::LabelText;
 use crate::win_handler::RUN_COMMANDS_TOKEN;
 use crate::{
     BoxConstraints, Command, Data, Env, Event, EventCtx, InternalEvent, InternalLifeCycle,
-    LayoutCtx, LifeCycle, LifeCycleCtx, LocalizedString, MenuDesc, PaintCtx, UpdateCtx, Widget,
-    WidgetId, WidgetPod, WindowDesc,
+    LayoutCtx, LifeCycle, LifeCycleCtx, MenuDesc, PaintCtx, UpdateCtx, Widget, WidgetId, WidgetPod,
+    WindowDesc,
 };
 
 /// A unique identifier for a window.
@@ -39,7 +40,7 @@ pub struct WindowId(u64);
 pub struct Window<T> {
     pub(crate) id: WindowId,
     pub(crate) root: WidgetPod<T, Box<dyn Widget<T>>>,
-    pub(crate) title: LocalizedString<T>,
+    pub(crate) title: LabelText<T>,
     size: Size,
     pub(crate) menu: Option<MenuDesc<T>>,
     pub(crate) context_menu: Option<MenuDesc<T>>,
@@ -368,7 +369,7 @@ impl<T: Data> Window<T> {
 
     pub(crate) fn update_title(&mut self, data: &T, env: &Env) {
         if self.title.resolve(data, env) {
-            self.handle.set_title(self.title.localized_str());
+            self.handle.set_title(self.title.display_text());
         }
     }