app: component: Add app_id and app properties

- So that with a component at hand, it's app can be easily retrieved.

- Don't create circular dependencies.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/app.py

@@ -53,12 +53,14 @@ class App:
 
     def add(self, component):
         """Add a component to an app."""
+        component.app_id = self.app_id
         self.components[component.component_id] = component
         return self
 
     def remove(self, component_id):
         """Remove a component from the app."""
         component = self.components[component_id]
+        component.app_id = None
         del self.components[component_id]
         return component
 
@@ -167,7 +169,13 @@ class App:
 
 
 class Component:
-    """Interface for an app component."""
+    """Interface for an app component.
+
+    `app_id` is a string which is set to the value of the application's app_id
+    to which this component belongs. It is set when the component is added to
+    an app. When the component is removed from an app, it set to None.
+
+    """
 
     is_leader = False
 
@@ -177,6 +185,16 @@ class Component:
             raise ValueError('Invalid component ID')
 
         self.component_id = component_id
+        self.app_id = None
+
+    @property
+    def app(self):
+        """Return the app this component is part of.
+
+        Raises KeyError if this component is not part of any app.
+
+        """
+        return App.get(self.app_id)
 
     def enable(self):
         """Run operations to enable the component."""

plinth/tests/test_app.py

@@ -71,6 +71,7 @@ def test_app_add():
     return_value = app.add(component)
     assert len(app.components) == 1
     assert app.components['test-component'] == component
+    assert component.app_id == app.app_id
     assert return_value == app
 
 
@@ -79,6 +80,7 @@ def test_app_remove(app_with_components):
     app = app_with_components
     component = app.components['test-leader-1']
     assert app.remove('test-leader-1') == component
+    assert component.app_id is None
     assert 'test-leader-1' not in app.components
 
 
@@ -206,6 +208,18 @@ def test_component_initialization():
     assert not component.is_leader
 
 
+def test_component_app_property():
+    """Test component's app property."""
+    component = Component('test-component')
+    assert component.app_id is None
+    with pytest.raises(KeyError):
+        assert not component.app
+
+    app = AppTest()
+    app.add(component)
+    assert component.app == app
+
+
 def test_component_diagnose():
     """Test running diagnostics on component."""
     component = Component('test-component')