doc: Set Drag.imageSource before Drag.active to avoid race condition

grabToImage() is asynchronous. 4b982c744f538a24e21a2af146c45f93d27dd1cb
started it on mouse press, which generally gave it enough time before
the QDrag became active; but 5971a6faaa1124f5ef3f0b42d4ed0298cf8096a3
changed it to call grabToImage() in DragHandler.onActiveChanged, which
means waiting until the drag threshold is exceeded. (DragHandler doesn't
offer a way to detect when the point is pressed. One could probably use
a separate TapHandler or PointHandler just to detect the press; but
that's also inefficient and inconvenient.) If we also bind Drag.active
to DragHandler.active, it's very likely that the binding is updated
before grabToImage() is done; and as documented, setting imageSource
after the drag has started has no effect (that in turn is a limitation
in QDrag and the platform support, probably a limitation on actual
platforms). So just wait until the screen-grab is done before setting
Drag.active, as a workaround for now.

This is a step back in declarativeness, using JS rather than a binding.
But setting imageSource to a screen-grab unfortunately requires JS
anyway; so we might as well delay making the Drag active until the
screen-grab is done.

Fixes: QTBUG-112673
Fixes: QTBUG-115491
Pick-to: 6.2 6.5 6.6
Change-Id: I0317613aa0288ad99b55ebd8a470500432b2ea02
Reviewed-by: Richard Moe Gustavsen <[email protected]>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/quick/doc/snippets/qml/externaldrag.qml b/src/quick/doc/snippets/qml/externaldrag.qml
index 072612f0c8..723701a7a5 100644
--- a/src/quick/doc/snippets/qml/externaldrag.qml
+++ b/src/quick/doc/snippets/qml/externaldrag.qml
@@ -12,7 +12,6 @@ Item {
         color: "green"
         radius: 5
 
-        Drag.active: dragHandler.active
         Drag.dragType: Drag.Automatic
         Drag.supportedActions: Qt.CopyAction
         Drag.mimeData: {
@@ -30,8 +29,11 @@ Item {
             onActiveChanged:
                 if (active) {
                     parent.grabToImage(function(result) {
-                        parent.Drag.imageSource = result.url;
+                        parent.Drag.imageSource = result.url
+                        parent.Drag.active = true
                     })
+                } else {
+                    parent.Drag.active = false
                 }
         }
     }