Add anchors property to Popup to allow centering in parent/window

Currently, users must manually position their Popup using x and y
bindings:

    Popup {
        x: (parent.width - width) / 2
        y: (parent.height - height) / 2
    }

This patch adds an anchors property so that you can do this instead:

    Popup {
        anchors.centerIn: parent
    }

It's also possible to conveniently center within the window from
anywhere within the scene (106e7b63 also documents an alternative way
of doing this using Overlay):

    Window {
        id: window

        Pane {
            Popup {
                anchors.centerIn: window
            }
        }
    }

QQuickAnchors were never used with Popup, because we cannot use the
QQuickAnchors implementation as-is, as the visual QQuickItem parent
is not the actual parent item of QQuickPopupItem.

Currently just centerIn is supported, as that's the most common
use case.

[ChangeLog][Controls][Popup] Added anchors.centerIn to Popup to allow
a covenient way of centering a popup.

Task-number: QTBUG-60354
Change-Id: Ia030f812df9da646fea8f373ef6199a21205ffbd
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

1 files changed, 52 insertions, 1 deletions

diff --git a/src/quicktemplates2/qquickpopup.cpp b/src/quicktemplates2/qquickpopup.cpp
index 84b17d65..03dd3dd0 100644
--- a/src/quicktemplates2/qquickpopup.cpp
+++ b/src/quicktemplates2/qquickpopup.cpp
@@ -36,6 +36,7 @@
 
 #include "qquickpopup_p.h"
 #include "qquickpopup_p_p.h"
+#include "qquickpopupanchors_p.h"
 #include "qquickpopupitem_p_p.h"
 #include "qquickpopuppositioner_p_p.h"
 #include "qquickapplicationwindow_p.h"
@@ -178,6 +179,11 @@ QT_BEGIN_NAMESPACE
 
     \include qquickoverlay-popup-parent.qdocinc
 
+    Another way to center a popup in the window regardless of its parent item
+    is to use \l {anchors.centerIn}:
+
+    \snippet qtquickcontrols2-popup.qml centerIn
+
     \sa {Popup Controls}, {Customizing Popup}, ApplicationWindow
 */
 
@@ -259,7 +265,8 @@ QQuickPopupPrivate::QQuickPopupPrivate()
       exit(nullptr),
       popupItem(nullptr),
       positioner(nullptr),
-      transitionManager(this)
+      transitionManager(this),
+      anchors(nullptr)
 {
 }
 
@@ -572,6 +579,48 @@ void QQuickPopupPrivate::setBottomMargin(qreal value, bool reset)
     }
 }
 
+/*!
+    \since QtQuick.Controls 2.5 (Qt 5.12)
+    \qmlpropertygroup QtQuick.Controls::Popup::anchors
+    \qmlproperty Object QtQuick.Controls::Popup::anchors.centerIn
+
+    Anchors provide a way to position an item by specifying its
+    relationship with other items.
+
+    A common use case is to center a popup within its parent. One way to do
+    this is with the \l {Item::}{x} and \l {Item::}{y} properties. Anchors offer
+    a more convenient approach:
+
+    \qml
+    Pane {
+        // ...
+
+        Popup {
+            anchors.centerIn: parent
+        }
+    }
+    \endqml
+
+    It is also possible to center the popup in the window by using \l Overlay:
+
+    \snippet qtquickcontrols2-popup.qml centerIn
+
+    This makes it easy to center a popup in the window from any component.
+
+    \note Popups can only be centered within their immediate parent or
+    the window overlay; trying to center in other items will produce a warning.
+
+    \sa {Popup Positioning}, {Item::anchors}
+*/
+QQuickPopupAnchors *QQuickPopupPrivate::getAnchors()
+{
+    if (!anchors) {
+        Q_Q(QQuickPopup);
+        anchors = new QQuickPopupAnchors(positioner, q);
+    }
+    return anchors;
+}
+
 void QQuickPopupPrivate::setWindow(QQuickWindow *newWindow)
 {
     Q_Q(QQuickPopup);
@@ -2470,3 +2519,5 @@ bool QQuickPopup::setAccessibleProperty(const char *propertyName, const QVariant
 }
 
 QT_END_NAMESPACE
+
+#include "moc_qquickpopup_p.cpp"