qmlpreview: do not reload all files constantly

The logic to reload a file would actually trigger a reload of all files.
In a real application of a slightly bigger size that slows down things a
lot. It triggered rebuilding the entire type registry.

Pick-to: 6.12
Change-Id: Ie484c3514d066b7950f79f4b5bba77a33d2b5213
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

2 files changed, 10 insertions, 1 deletions

diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.cpp b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.cpp
index 2e5a470248..7ce8835f1a 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.cpp
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.cpp
@@ -78,7 +78,14 @@ void QQmlPreviewServiceImpl::messageReceived(const QByteArray &data)
                 ? QUrl(QLatin1String("qrc") + path)
                 : QUrl::fromLocalFile(path);
 
-        emit drop(url);
+        // Only a re-serve of a URL we already saw is an actual edit that needs relinking. The
+        // first time any file is served (e.g. while the whole app is still starting up) isn't
+        // dropping anything; treating it as such piles up the entire type registry as "dropped"
+        // and causes a mass relink storm on the first Load.
+        if (m_servedUrls.contains(url))
+            emit drop(url);
+        else
+            m_servedUrls.insert(url);
         emit file(path, contents);
 
         // Remember the first .qml file as the current URL, so that a Load command
diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.h b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.h
index 1506b816e0..cccdda619f 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.h
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewservice.h
@@ -20,6 +20,7 @@
 #include "qqmlpreviewfileengine.h"
 #include <private/qqmldebugserviceinterfaces_p.h>
 
+#include <QtCore/qset.h>
 #include <memory>
 
 QT_BEGIN_NAMESPACE
@@ -82,6 +83,7 @@ private:
     QScopedPointer<QQmlPreviewFileLoader> m_loader;
     std::unique_ptr<QQmlPreviewHandler> m_handler;
     QUrl m_currentUrl;
+    QSet<QUrl> m_servedUrls;
 };
 
 QT_END_NAMESPACE