How to remove QTreeView indentation - qt

Question

I want to have a QTreeView without an indentation on the left side increasing at each nesting level. I tried setting QTreeView::setIndentation(0). It removes the indentations just as I want, however it also hides the tree arrows.
Default behavior:
With indentations ✗
With arrows ✔
After setIndentation(0):
Without indentations ✔
Without arrows ✗
Desired behavior:
Without indentations ✔
With arrows ✔
So how can I achieve the result shown in the third example? Is there any standard way of doing it, or I will have to reimplement the QTreeView::paintEvent(), QTreeView::drawBranches(), etc.?

Answer 1

To solve the problem I used a delegate to translate the paint of the items, and paint the arrows.
#include <QtWidgets>
class BranchDelegate: public QStyledItemDelegate
{
public:
using QStyledItemDelegate::QStyledItemDelegate;
void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const override{
QStyleOptionViewItem opt(option);
if(index.column() == 0)
opt.rect.adjust(opt.rect.height(), 0, 0, 0);
QStyledItemDelegate::paint(painter, opt, index);
if(index.column() == 0){
QStyleOptionViewItem branch;
branch.rect = QRect(0, opt.rect.y(), opt.rect.height(), opt.rect.height());
branch.state = option.state;
const QWidget *widget = option.widget;
QStyle *style = widget ? widget->style() : QApplication::style();
style->drawPrimitive(QStyle::PE_IndicatorBranch, &branch, painter, widget);
}
}
};
class TreeView: public QTreeView
{
public:
TreeView(QWidget *parent=nullptr):QTreeView(parent)
{
BranchDelegate *delegate = new BranchDelegate(this);
setItemDelegate(delegate);
setIndentation(0);
}
protected:
void mousePressEvent(QMouseEvent *event) override{
QModelIndex index = indexAt(event->pos());
bool last_state = isExpanded(index);
QTreeView::mousePressEvent(event);
if(index.isValid() && last_state == isExpanded(index))
setExpanded(index, !last_state);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
TreeView w;
QFileSystemModel model;
model.setRootPath(QDir::rootPath());
w.setModel(&model);
w.setRootIndex(model.index(QDir::homePath()));
/*for (int i = 1; i< model.columnCount() ; ++i) {
w.hideColumn(i);
}*/
w.expandAll();
w.resize(640, 480);
w.show();
return a.exec();
}

Answer 2

eyllanesc's falls appart if there is horizontal scrolling. Also usually the view only expands/collapses when clicking the branch-indicator, not the index.
My Solution: Only change the Rect of indices which have a parent but no children. Also dont set the indentation to 0. No need to subclass QTreeView.
#include <QtWidgets>
class BranchDelegate: public QStyledItemDelegate
{
public:
mIndent = 50;
using QStyledItemDelegate::QStyledItemDelegate;
void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const override
{
QStyleOptionViewItem opt(option);
if(index.parent().isValid && (!index.model() || !index.model()->index(0, 0, index).isValid()))
{
opt.rect.adjust(-mIndent, 0, 0, 0);
}
QStyledItemDelegate::paint(painter, opt, index);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTreeView apView;
BranchDelegate* const apDelegate = new BranchDelegate(apView);
apDelegate->mIndent = 50;
apView->setIndentation(apDelegate->mIndent);
apView->setItemDelegateForColumn(0, apDelegate);
QFileSystemModel model;
model.setRootPath(QDir::rootPath());
apView.setModel(&model);
apView.setRootIndex(model.index(QDir::homePath()));
/*for (int i = 1; i< model.columnCount() ; ++i) {
apView.hideColumn(i);
}*/
apView.expandAll();
apView.resize(640, 480);
apView.show();
return a.exec();
}

Answer 3

ellyanesc's answer works, but is incorrect by one small detail in the line:
branch.rect = QRect(0, opt.rect.y(), opt.rect.height(), opt.rect.height());
The reason is because when the view is horizontally scrolled, option.rect.x() becomes negative. If branch.rect.x() is 0 (as in ellyanesc's answer), the branch indicator will always be shown, which also causes artifacts during scroll:
https://i.stack.imgur.com/O518A.png
https://i.stack.imgur.com/qYJMj.png
To solve this, replace the above line with:
branch.rect = QRect(option.rect.x(), opt.rect.y(), opt.rect.height(), opt.rect.height());
(I would have just pointed that out as a comment in ellyanesc's answer, but I don't have enough reputation for that.)