Improve handling of QTBUG-57971 (ListModel initializing roles on firs…

…t insertion)

So far the provided workaround was missing some corner cases when there
is more than one SFPM (or other proxies) in a chain, providing some
custom roles like proxy roles in SFPM. Detailed explanation provided as
a comment directly in the source.

qqmlsortfilterproxymodel.cpp

@@ -380,10 +380,39 @@ void QQmlSortFilterProxyModel::resetInternalData()
 
 void QQmlSortFilterProxyModel::setSourceModel(QAbstractItemModel *sourceModel)
 {
-    if (sourceModel && sourceModel->roleNames().isEmpty()) { // workaround for when a model has no roles and roles are added when the model is populated (ListModel)
-        // QTBUG-57971
-        connect(sourceModel, &QAbstractItemModel::rowsInserted, this, &QQmlSortFilterProxyModel::initRoles);
+    // QML built-in type ListModel behaves in a specific way regarding roles
+    // initialization (QTBUG-57971). Empty model has no roles, they become
+    // available after first insertion. However modelAboutToBeReset/modelReset
+    // is not emited. It means that roles may change not only in between model
+    // resets but also on the first insertion.
+    // In a simple case, where ListModel (or other model behaving in that way)
+    // is direct source of SFPM, situation is relatively simple - if source
+    // has no roles, SFPM should try to initialize them on first insertion
+    // However this behavior has far-reaching consequences, because the
+    // ListModel-like model may not be a direct source for SFPM. E.g. there may
+    // by another SFPM in between, with proxy roles defined:
+    //
+    // ListModel -> SFPM 1 (with proxy roles) -> SFPM 2
+    //
+    // In such scenario SFPM 2 will always have model with roles as it's source
+    // (at least proxy roles). It means that on first insertion right after
+    // SFPM creation or after source model reset it's necessary to re-initialize
+    // role names if the source was empty before insertion.
+
+    if (sourceModel && sourceModel->rowCount() == 0)
+        connect(sourceModel, &QAbstractItemModel::rowsInserted, this,
+                &QQmlSortFilterProxyModel::initRoles, Qt::UniqueConnection);
+
+    if (sourceModel) {
+        connect(sourceModel, &QAbstractItemModel::modelReset, this, [sourceModel, this]() {
+            if (sourceModel->rowCount() != 0)
+                return;
+
+            connect(sourceModel, &QAbstractItemModel::rowsInserted, this,
+                    &QQmlSortFilterProxyModel::initRoles, Qt::UniqueConnection);
+        });
     }
+
     QSortFilterProxyModel::setSourceModel(sourceModel);
 }