Utils: Emerge widgets from LayoutItem

This allows us to call e.g. PushButton{}.emerge(); without having to
add another unnecessary layout.

Change-Id: Ica26f4776dee1843e1f40c238e1f3c1a54d14ea5
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/layoutbuilder.cpp

@@ -573,11 +573,28 @@ void LayoutItem::attachTo(QWidget *w) const
 
 QWidget *LayoutItem::emerge()
 {
+    LayoutBuilder builder;
+
+    builder.stack.append(Slice());
+    addItemHelper(builder, *this);
+
+    if (builder.stack.empty())
+        return nullptr;
+
+    QTC_ASSERT(builder.stack.last().pendingItems.size() == 1, return nullptr);
+    ResultItem ri = builder.stack.last().pendingItems.takeFirst();
+
+    QTC_ASSERT(ri.layout || ri.widget, return nullptr);
+
+    if (ri.layout) {
         auto w = new QWidget;
-    attachTo(w);
+        w->setLayout(ri.layout);
         return w;
     }
 
+    return ri.widget;
+}
+
 static void layoutExit(LayoutBuilder &builder)
 {
     builder.stack.last().flush();
@@ -861,6 +878,17 @@ LayoutItem title(const QString &title)
     };
 }
 
+LayoutItem windowTitle(const QString &windowTitle)
+{
+    return [windowTitle](QObject *target) {
+        if (auto widget = qobject_cast<QWidget *>(target)) {
+            widget->setWindowTitle(windowTitle);
+        } else {
+            QTC_CHECK(false);
+        }
+    };
+}
+
 LayoutItem text(const QString &text)
 {
     return [text](QObject *target) {

tests/manual/widgets/layoutbuilder/tst_manual_widgets_layoutbuilder.cpp

@@ -38,5 +38,16 @@ int main(int argc, char *argv[])
         },
     }.emerge()->show();
 
+    Group {
+        windowTitle("Group without parent layout"),
+        title("This group was emerged without parent layout"),
+        Column {
+            Splitter {
+                new QTextEdit("First Widget"),
+                new QTextEdit("Second Widget"),
+            },
+        }
+    }.emerge()->show();
+
     return app.exec();
 }