Merge example_web into egui_demo_lib

egui_demo_lib/src/apps/demo/tests.rs

@@ -0,0 +1,49 @@
+#[derive(Default)]
+pub struct Tests {}
+
+impl super::Demo for Tests {
+    fn name(&self) -> &str {
+        "📋 Tests"
+    }
+
+    fn show(&mut self, ctx: &egui::CtxRef, open: &mut bool) {
+        egui::Window::new(self.name()).open(open).show(ctx, |ui| {
+            use super::View;
+            self.ui(ui);
+        });
+    }
+}
+
+impl super::View for Tests {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.heading("Name collision example");
+
+        ui.label("\
+            Widgets that store state require unique and persisting identifiers so we can track their state between frames.\n\
+            For instance, collapsable headers needs to store wether or not they are open. \
+            Their Id:s are derived from their names. \
+            If you fail to give them unique names then clicking one will open both. \
+            To help you debug this, an error message is printed on screen:");
+
+        ui.collapsing("Collapsing header", |ui| {
+            ui.label("Contents of first foldable ui");
+        });
+        ui.collapsing("Collapsing header", |ui| {
+            ui.label("Contents of second foldable ui");
+        });
+
+        ui.label("\
+            Any widget that can be interacted with also need a unique Id. \
+            For most widgets the Id is generated by a running counter. \
+            As long as elements are not added or removed, the Id stays the same. \
+            This is fine, because during interaction (i.e. while dragging a slider), \
+            the number of widgets previously in the same window is most likely not changing \
+            (and if it is, the window will have a new layout, and the slider will endup somewhere else, and so aborthing the interaction probably makes sense).");
+
+        ui.label("So these buttons have automatic Id:s, and therefore there is no name clash:");
+        let _ = ui.button("Button");
+        let _ = ui.button("Button");
+
+        ui.add(crate::__egui_github_link_file!());
+    }
+}