Store already added node viewer and load it when needed (#921)

In aiidalab-widgets-base/viewer.py, it creates a new viewer every time we select a new AiiDA node, and the viewer is not saved in the app, so we can not reuse it later. This PR stores the already added viewer and reuses it later.

Author: superstar54

src/aiidalab_qe/app/result/__init__.py

@@ -5,7 +5,6 @@
 from aiida.engine import ProcessState
 from aiida.engine.processes import control
 from aiidalab_widgets_base import (
-    AiidaNodeViewWidget,
     ProcessMonitor,
     ProcessNodesTreeWidget,
     WizardAppWidgetStep,
@@ -28,14 +27,10 @@ def __init__(self, **kwargs):
             (self, "process"),
             (self.process_tree, "value"),
         )
-
-        self.node_view = AiidaNodeViewWidget(layout={"width": "auto", "height": "auto"})
-        ipw.dlink(
-            (self.process_tree, "selected_nodes"),
-            (self.node_view, "node"),
-            transform=lambda nodes: nodes[0] if nodes else None,
-        )
-        self.process_status = ipw.VBox(children=[self.process_tree, self.node_view])
+        self.process_tree.observe(self._update_node_view, names="selected_nodes")
+        # keep track of the node views
+        self.node_views = {}
+        self.process_status = ipw.VBox(children=[self.process_tree])
 
         # Setup process monitor
         self.process_monitor = ProcessMonitor(
@@ -215,3 +210,20 @@ def _observe_process(self, _):
         self._update_state()
         self._update_kill_button_layout()
         self._update_clean_scratch_button_layout()
+
+    def _update_node_view(self, change):
+        """Callback for when the a new node is selected."""
+        from aiidalab_widgets_base.viewers import viewer
+
+        nodes = change["new"]
+        if not nodes:
+            return
+        # only show the first selected node
+        node = nodes[0]
+        # check if the viewer is already added
+        if node.uuid in self.node_views:
+            node_view = self.node_views[node.uuid]
+        else:
+            node_view = viewer(node)
+            self.node_views[node.uuid] = node_view
+        self.process_status.children = [self.process_tree, node_view]