settings_ui: Add placeholder view (#15019)

This PR adds a placeholder view for the settings UI. It does not contain
any functionality, as of yet.

This view is staff-shipped behind a feature flag.

Release Notes:

- N/A

crates/settings_ui/src/settings_ui.rs

@@ -0,0 +1,107 @@
+use std::any::TypeId;
+
+use command_palette_hooks::CommandPaletteFilter;
+use feature_flags::{FeatureFlag, FeatureFlagViewExt};
+use gpui::{actions, AppContext, EventEmitter, FocusHandle, FocusableView, View};
+use ui::prelude::*;
+use workspace::item::{Item, ItemEvent};
+use workspace::Workspace;
+
+pub struct SettingsUiFeatureFlag;
+
+impl FeatureFlag for SettingsUiFeatureFlag {
+    const NAME: &'static str = "settings-ui";
+}
+
+actions!(zed, [OpenSettingsEditor]);
+
+pub fn init(cx: &mut AppContext) {
+    cx.observe_new_views(|workspace: &mut Workspace, cx| {
+        workspace.register_action(|workspace, _: &OpenSettingsEditor, cx| {
+            let existing = workspace
+                .active_pane()
+                .read(cx)
+                .items()
+                .find_map(|item| item.downcast::<SettingsPage>());
+
+            if let Some(existing) = existing {
+                workspace.activate_item(&existing, true, true, cx);
+            } else {
+                let settings_page = SettingsPage::new(workspace, cx);
+                workspace.add_item_to_active_pane(Box::new(settings_page), None, true, cx)
+            }
+        });
+
+        let settings_ui_actions = [TypeId::of::<OpenSettingsEditor>()];
+
+        CommandPaletteFilter::update_global(cx, |filter, _cx| {
+            filter.hide_action_types(&settings_ui_actions);
+        });
+
+        cx.observe_flag::<SettingsUiFeatureFlag, _>(move |is_enabled, _view, cx| {
+            if is_enabled {
+                CommandPaletteFilter::update_global(cx, |filter, _cx| {
+                    filter.show_action_types(settings_ui_actions.iter());
+                });
+            } else {
+                CommandPaletteFilter::update_global(cx, |filter, _cx| {
+                    filter.hide_action_types(&settings_ui_actions);
+                });
+            }
+        })
+        .detach();
+    })
+    .detach();
+}
+
+pub struct SettingsPage {
+    focus_handle: FocusHandle,
+}
+
+impl SettingsPage {
+    pub fn new(_workspace: &Workspace, cx: &mut ViewContext<Workspace>) -> View<Self> {
+        cx.new_view(|cx| Self {
+            focus_handle: cx.focus_handle(),
+        })
+    }
+}
+
+impl EventEmitter<ItemEvent> for SettingsPage {}
+
+impl FocusableView for SettingsPage {
+    fn focus_handle(&self, _cx: &AppContext) -> FocusHandle {
+        self.focus_handle.clone()
+    }
+}
+
+impl Item for SettingsPage {
+    type Event = ItemEvent;
+
+    fn tab_icon(&self, _cx: &WindowContext) -> Option<Icon> {
+        Some(Icon::new(IconName::Settings))
+    }
+
+    fn tab_content_text(&self, _cx: &WindowContext) -> Option<SharedString> {
+        Some("Settings".into())
+    }
+
+    fn show_toolbar(&self) -> bool {
+        false
+    }
+
+    fn to_item_events(event: &Self::Event, mut f: impl FnMut(ItemEvent)) {
+        f(*event)
+    }
+}
+
+impl Render for SettingsPage {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        v_flex()
+            .p_4()
+            .size_full()
+            .child(Label::new("Settings").size(LabelSize::Large))
+            .child(Label::new(
+                "Nothing to see here yet. Feature-flagged for staff.",
+            ))
+    }
+}