Add QHeaderView::setFirstSectionMovable(), useful for flat treeviews

QTreeView makes columns movable except for the first one
(since Qt 5.0, see commit e0fc088c0c). This is indeed best
for actual trees, but not when using QTreeView as a flat
multi-column list (with no "root decoration"). Then it should
be possible to move the first column.

[ChangeLog][QtWidgets][QHeaderView] Flat treeviews can now allow
the user to move the first column (like in Qt 4.x) using the new
method QHeaderView::setFirstSectionMovable().

Change-Id: I6b5025e40850bf5c4c373124ee81f657f3f09d29
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

examples/widgets/itemviews/flattreeview/flattreeview.pro

@@ -0,0 +1,7 @@
+QT += widgets
+
+SOURCES     = main.cpp
+
+# install
+target.path = $$[QT_INSTALL_EXAMPLES]/widgets/itemviews/flattreeview
+INSTALLS += target

examples/widgets/itemviews/flattreeview/main.cpp

@@ -0,0 +1,84 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 Klaralvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author David Faure <david.faure@kdab.com>
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the examples of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*
+  main.cpp
+
+  A simple example that shows a multi-column list using QTreeView.
+  The data is not a tree, so the first column was made movable.
+*/
+
+#include <QApplication>
+#include <QHeaderView>
+#include <QStandardItemModel>
+#include <QTreeView>
+
+int main(int argc, char *argv[])
+{
+    QApplication app(argc, argv);
+
+    QStandardItemModel model(4, 2);
+    QTreeView treeView;
+    treeView.setModel(&model);
+    treeView.setRootIsDecorated(false);
+    treeView.header()->setFirstSectionMovable(true);
+    treeView.header()->setStretchLastSection(true);
+
+    for (int row = 0; row < 4; ++row) {
+        for (int column = 0; column < 2; ++column) {
+            QModelIndex index = model.index(row, column, QModelIndex());
+            model.setData(index, QVariant((row + 1) * (column + 1)));
+        }
+    }
+
+    treeView.setWindowTitle(QObject::tr("Flat Tree View"));
+    treeView.show();
+    return app.exec();
+}

examples/widgets/itemviews/itemviews.pro

@@ -8,6 +8,7 @@ SUBDIRS       = addressbook \
                 dirview \
                 editabletreemodel \
                 fetchmore \
+                flattreeview \
                 frozencolumn \
                 interview \
                 pixelator \

src/widgets/itemviews/qheaderview.cpp

@@ -1115,10 +1115,15 @@ int QHeaderView::logicalIndex(int visualIndex) const
 /*!
     \since 5.0
 
-    If \a movable is true, the header may be moved by the user; otherwise it
-    is fixed in place.
+    If \a movable is true, the header sections may be moved by the user;
+    otherwise they are fixed in place.
+
+    When used in combination with QTreeView, the first column is not
+    movable (since it contains the tree structure), by default.
+    You can make it movable with setFirstSectionMovable(true).
 
     \sa sectionsMovable(), sectionMoved()
+    \sa setFirstSectionMovable()
 */
 
 void QHeaderView::setSectionsMovable(bool movable)
@@ -1143,6 +1148,9 @@ void QHeaderView::setSectionsMovable(bool movable)
     Returns \c true if the header can be moved by the user; otherwise returns
     false.
 
+    By default, sections are movable in QTreeView (except for the first one),
+    and not movable in QTableView.
+
     \sa setSectionsMovable()
 */
 
@@ -1162,6 +1170,39 @@ bool QHeaderView::sectionsMovable() const
     \sa sectionsMovable()
 */
 
+/*!
+    \since 5.10
+
+    If \a movable is true, the first column can be moved by the user.
+    In a QTreeView, the first column holds the tree structure and is
+    therefore non-movable by default, even after setSectionsMovable(true).
+
+    It can be made movable again, for instance in the case of flat lists
+    without a tree structure, by calling this method.
+    In such a scenario, it is recommended to call QTreeView::setRootIsDecorated(false)
+    as well.
+
+    This method has no effect unless setSectionsMovable(true) is called as well.
+    \sa setSectionsMovable()
+*/
+void QHeaderView::setFirstSectionMovable(bool movable)
+{
+    Q_D(QHeaderView);
+    d->allowUserMoveOfSection0 = movable;
+}
+
+/*!
+    \since 5.10
+
+    Returns \c true if the first column can be moved by the user,
+    when this header is used in a QTreeView.
+*/
+bool QHeaderView::firstSectionMovable() const
+{
+    Q_D(const QHeaderView);
+    return d->allowUserMoveOfSection0;
+}
+
 /*!
     \since 5.0
 

src/widgets/itemviews/qheaderview.h

@@ -118,6 +118,8 @@ public:
     inline QT_DEPRECATED void setMovable(bool movable) { setSectionsMovable(movable); }
     inline QT_DEPRECATED bool isMovable() const { return sectionsMovable(); }
 #endif
+    void setFirstSectionMovable(bool movable);
+    bool firstSectionMovable() const;
 
     void setSectionsClickable(bool clickable);
     bool sectionsClickable() const;

src/widgets/itemviews/qheaderview_p.h

@@ -246,10 +246,6 @@ public:
         }
     }
 
-    inline void setAllowUserMoveOfSection0(bool b) {
-        allowUserMoveOfSection0 = b;
-    }
-
     void clear();
     void flipSortIndicator(int section);
     void cascadingResize(int visual, int newSize);

src/widgets/itemviews/qtableview.cpp

@@ -1226,7 +1226,7 @@ void QTableView::setHorizontalHeader(QHeaderView *header)
         delete d->horizontalHeader;
     d->horizontalHeader = header;
     d->horizontalHeader->setParent(this);
-    d->horizontalHeader->d_func()->setAllowUserMoveOfSection0(true);
+    d->horizontalHeader->setFirstSectionMovable(true);
     if (!d->horizontalHeader->model()) {
         d->horizontalHeader->setModel(d->model);
         if (d->selectionModel)
@@ -1264,7 +1264,7 @@ void QTableView::setVerticalHeader(QHeaderView *header)
         delete d->verticalHeader;
     d->verticalHeader = header;
     d->verticalHeader->setParent(this);
-    d->verticalHeader->d_func()->setAllowUserMoveOfSection0(true);
+    d->verticalHeader->setFirstSectionMovable(true);
     if (!d->verticalHeader->model()) {
         d->verticalHeader->setModel(d->model);
         if (d->selectionModel)

src/widgets/itemviews/qtreeview.cpp

@@ -311,7 +311,7 @@ void QTreeView::setHeader(QHeaderView *header)
         delete d->header;
     d->header = header;
     d->header->setParent(this);
-    d->header->d_func()->setAllowUserMoveOfSection0(false);
+    d->header->setFirstSectionMovable(false);
 
     if (!d->header->model()) {
         d->header->setModel(d->model);

tests/auto/widgets/itemviews/qheaderview/tst_qheaderview.cpp

@@ -514,6 +514,12 @@ void tst_QHeaderView::movable()
     QCOMPARE(view->sectionsMovable(), false);
     view->setSectionsMovable(true);
     QCOMPARE(view->sectionsMovable(), true);
+
+    QCOMPARE(view->firstSectionMovable(), true);
+    view->setFirstSectionMovable(false);
+    QCOMPARE(view->firstSectionMovable(), false);
+    view->setFirstSectionMovable(true);
+    QCOMPARE(view->firstSectionMovable(), true);
 }
 
 void tst_QHeaderView::clickable()