refactor: let target be a trait

This way target can be updated, and works better in combination with
Reacton and Solara.

ipypopout/popout_button.py

@@ -35,6 +35,7 @@ class PopoutButton(v.VuetifyTemplate):
     template_file = (__file__, "popout_button.vue")
     kernel_id = traitlets.Unicode('').tag(sync=True)
     target_model_id = traitlets.Unicode().tag(sync=True)
+    target = traitlets.Instance(ipywidgets.Widget, allow_none=True)
     echo_available = traitlets.Bool(False).tag(sync=True)
 
     is_displayed = traitlets.Bool(False).tag(sync=True)
@@ -48,17 +49,38 @@ class PopoutButton(v.VuetifyTemplate):
     # See: https://developer.mozilla.org/en-US/docs/Web/API/Window/open#window_features
     window_features = traitlets.Unicode('popup').tag(sync=True)
 
-    def __init__(self, target, **kwargs):
-        self.kernel_id = get_kernel_id()
-        self.target_model_id = target._model_id
-        self.window_name = target._model_id
+    def __init__(self, target=None, **kwargs):
+        kwargs = kwargs.copy()
 
         if os.environ.get("JUPYTER_WIDGETS_ECHO") is None:
             ipywidgets.widgets.widget.JUPYTER_WIDGETS_ECHO = True
 
         self.echo_available = ipywidgets.widgets.widget.JUPYTER_WIDGETS_ECHO
+        if target is not None:
+            kwargs = {**kwargs, **{'target': target}}
         super(PopoutButton, self).__init__(**kwargs)
 
+    @traitlets.observe('target')
+    def _on_target_change(self, change):
+        if change['new'] is not None:
+            self.target_model_id = change['new']._model_id
+            self.window_name = change['new']._model_id
+
+    @traitlets.default("target_model_id")
+    def _default_target_model_id(self):
+        if self.target is not None:
+            return self.target._model_id
+        return ""
+
+    @traitlets.default("window_name")
+    def _default_window_name(self):
+        return self.target_model_id or ""
+
+    @traitlets.default("kernel_id")
+    def _default_kernel_id(self):
+        return get_kernel_id()
+
+
     def open_window(self):
         if self.is_displayed:
             self.send({

tests/unit/popout_test.py

@@ -0,0 +1,15 @@
+import ipypopout
+import ipywidgets as widgets
+
+
+def test_create_target():
+    box = widgets.VBox()
+    button = ipypopout.PopoutButton(target=box)
+    assert button.target_model_id == box._model_id
+    assert button.window_name == box._model_id
+
+    box2 = widgets.VBox()
+    button.target = box2
+    assert button.target_model_id == box2._model_id
+    assert button.window_name == box2._model_id
+