Use QScroller in the Dir View example

We have never had enough examples that show how easy it is to install
a QScroller.  In this case, one line makes a QTreeView flickable.

Pick-to: 5.15
Task-number: QTBUG-86090
Change-Id: Idb8b4709617befb261f3b78d63ddbdaf5ad18d6b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

examples/widgets/doc/src/dirview.qdoc

@@ -29,7 +29,7 @@
     \example itemviews/dirview
     \title Dir View Example
     \ingroup examples-itemviews
-    \brief This example demonstrates the usage of a tree view.
+    \brief This example demonstrates the usage of a tree view, and smooth flicking on a touchscreen.
 
     The Dir View example shows a tree view of the local file
     system. It uses the QFileSystemModel class to provide file
@@ -60,8 +60,14 @@
     QTreeView object \c tree visualizes the filesystem in a tree structure.
 
     \skipto tree.setAnimated(false)
-    \printuntil tree.setWindowTitle
+    \printuntil tree.setColumnWidth
 
     Sets layout options for animation, indentation, sorting, and sizing of the
     filesystem tree.
+
+    \skipto QScroller::grabGesture
+    \printuntil QScroller::grabGesture
+
+    Creates a \l QScroller instance to recognize gestures on touchscreens,
+    so that you can flick the tree view with your finger.
 */

examples/widgets/itemviews/dirview/main.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the examples of the Qt Toolkit.
@@ -52,6 +52,7 @@
 #include <QFileSystemModel>
 #include <QFileIconProvider>
 #include <QScreen>
+#include <QScroller>
 #include <QTreeView>
 #include <QCommandLineParser>
 #include <QCommandLineOption>
@@ -96,6 +97,9 @@ int main(int argc, char *argv[])
     tree.resize(availableSize / 2);
     tree.setColumnWidth(0, tree.width() / 3);
 
+    // Make it flickable on touchscreens
+    QScroller::grabGesture(&tree, QScroller::TouchGesture);
+
     tree.setWindowTitle(QObject::tr("Dir View"));
     tree.show();
 

src/widgets/util/qscroller.cpp

@@ -260,8 +260,8 @@ private:
     The scroller uses the global QAbstractAnimation timer to generate its QScrollEvents. This
     can be changed with QScrollerProperties::FrameRate on a per-QScroller basis.
 
-    Several examples in the \c scroller examples directory show how QScroller,
-    QScrollEvent and the scroller gesture can be used.
+    The \l {Dir View Example} shows one way to use a QScroller with a QTreeView.
+    An example in the \c scroller examples directory also demonstrates QScroller.
 
     Even though this kinetic scroller has a large number of settings available via
     QScrollerProperties, we recommend that you leave them all at their default, platform optimized