Test screen (sub-)title.

CHANGELOG.md

@@ -10,6 +10,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 - TCSS styles `layer` and `layers` can be strings https://github.com/Textualize/textual/pull/3169
+- Screen-specific (sub-)title attributes https://github.com/Textualize/textual/pull/3199:
+  - `Screen.TITLE`
+  - `Screen.SUB_TITLE`
+  - `Screen.title`
+  - `Screen.sub_title`
 
 ### Changed
 

tests/test_header.py

@@ -0,0 +1,151 @@
+from textual.app import App
+from textual.screen import Screen
+from textual.widgets import Header
+
+
+async def test_screen_title_none_is_ignored():
+    class MyScreen(Screen):
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        TITLE = "app title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test():
+        assert app.query_one("HeaderTitle").text == "app title"
+
+
+async def test_screen_title_overrides_app_title():
+    class MyScreen(Screen):
+        TITLE = "screen title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        TITLE = "app title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test():
+        assert app.query_one("HeaderTitle").text == "screen title"
+
+
+async def test_screen_title_reactive_updates_title():
+    class MyScreen(Screen):
+        TITLE = "screen title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        TITLE = "app title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test() as pilot:
+        app.screen.title = "new screen title"
+        await pilot.pause()
+        assert app.query_one("HeaderTitle").text == "new screen title"
+
+
+async def test_app_title_reactive_does_not_update_title_when_screen_title_is_set():
+    class MyScreen(Screen):
+        TITLE = "screen title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        TITLE = "app title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test() as pilot:
+        app.title = "new app title"
+        await pilot.pause()
+        assert app.query_one("HeaderTitle").text == "screen title"
+
+
+async def test_screen_sub_title_none_is_ignored():
+    class MyScreen(Screen):
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        SUB_TITLE = "app sub-title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test():
+        assert app.query_one("HeaderTitle").sub_text == "app sub-title"
+
+
+async def test_screen_sub_title_overrides_app_sub_title():
+    class MyScreen(Screen):
+        SUB_TITLE = "screen sub-title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        SUB_TITLE = "app sub-title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test():
+        assert app.query_one("HeaderTitle").sub_text == "screen sub-title"
+
+
+async def test_screen_sub_title_reactive_updates_sub_title():
+    class MyScreen(Screen):
+        SUB_TITLE = "screen sub-title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        SUB_TITLE = "app sub-title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test() as pilot:
+        app.screen.sub_title = "new screen sub-title"
+        await pilot.pause()
+        assert app.query_one("HeaderTitle").sub_text == "new screen sub-title"
+
+
+async def test_app_sub_title_reactive_does_not_update_sub_title_when_screen_sub_title_is_set():
+    class MyScreen(Screen):
+        SUB_TITLE = "screen sub-title"
+
+        def compose(self):
+            yield Header()
+
+    class MyApp(App):
+        SUB_TITLE = "app sub-title"
+
+        def on_mount(self):
+            self.push_screen(MyScreen())
+
+    app = MyApp()
+    async with app.run_test() as pilot:
+        app.sub_title = "new app sub-title"
+        await pilot.pause()
+        assert app.query_one("HeaderTitle").sub_text == "screen sub-title"