UI: Add AbstractView base class for views, unify reconstruction in there

No user visible changes expected.

There's plenty of duplicated code in the grid and the tree view, and I expect
this to become more. This starts the process of unifying these parts, which
should also make it easier to add new views. Complexity in the view classes is
reduced, and some type shenanigans for C compatibility and general view
management can be removed, since there is now a common base type.

For the start this ports some of the view reconstruction, where the view and
its items are compared to the version of itself in the previous redraw, so that
state (highlighted, active, renaming, collapsed, ...) can be preserved.
Notifier listening is also ported.

1 files changed, 47 insertions, 0 deletions

diff --git a/source/blender/editors/include/UI_abstract_view.hh b/source/blender/editors/include/UI_abstract_view.hh
new file mode 100644
index 00000000000..477f68ca03f
--- /dev/null
+++ b/source/blender/editors/include/UI_abstract_view.hh
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup editorui
+ *
+ * Base for all views (UIs to display data sets), supporting common features.
+ * https://wiki.blender.org/wiki/Source/Interface/Views
+ *
+ * The base class manages reconstruction, most importantly keeping state over reconstructions.
+ */
+
+#pragma once
+
+struct wmNotifier;
+
+namespace blender::ui {
+
+class AbstractView {
+  bool is_reconstructed_ = false;
+
+ public:
+  virtual ~AbstractView() = default;
+
+  /** Listen to a notifier, returning true if a redraw is needed. */
+  virtual bool listen(const wmNotifier &) const;
+
+ protected:
+  AbstractView() = default;
+
+  virtual void update_children_from_old(const AbstractView &old_view) = 0;
+
+  /**
+   * Match the view and its items against an earlier version of itself (if any) and copy the old UI
+   * state (e.g. collapsed, active, selected, renaming, etc.) to the new one. See
+   * #AbstractViewItem.update_from_old().
+   * After this, reconstruction is complete (see #is_reconstructed()).
+   */
+  void update_from_old(uiBlock &new_block);
+
+  /**
+   * Check if the view is fully (re-)constructed. That means, both the build function and
+   * #update_from_old() have finished.
+   */
+  bool is_reconstructed() const;
+};
+
+}  // namespace blender::ui