Add required property documentation

Change-Id: I9e2c44ddcbb6c7e3a92b2bd7d2492ece0cd0e62d
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

2 files changed, 101 insertions, 0 deletions

diff --git a/src/quick/doc/snippets/qml/qml-data-models/listmodel-listview-required.qml b/src/quick/doc/snippets/qml/qml-data-models/listmodel-listview-required.qml
new file mode 100644
index 0000000000..e9a668ed8f
--- /dev/null
+++ b/src/quick/doc/snippets/qml/qml-data-models/listmodel-listview-required.qml
@@ -0,0 +1,81 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation 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."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+//! [document]
+import QtQuick 2.0
+
+Item {
+    width: 200
+    height: 250
+
+    ListModel {
+        id: myModel
+        ListElement { type: "Dog"; age: 8; noise: "meow" }
+        ListElement { type: "Cat"; age: 5; noise: "woof" }
+    }
+
+    component MyDelegate : Text {
+        required property string type
+        required property int age
+        text: type + ", " + age
+        // WRONG: Component.onCompleted: () => console.log(noise)
+        // The above line would cause a ReferenceError
+        // as there is no required property noise,
+        // and the presence of the required properties prevents
+        // noise from being injected into the scope
+    }
+
+    ListView {
+        anchors.fill: parent
+        model: myModel
+        delegate: MyDelegate {}
+    }
+}
+//! [document]
diff --git a/src/quick/doc/src/concepts/modelviewsdata/modelview.qdoc b/src/quick/doc/src/concepts/modelviewsdata/modelview.qdoc
index 04dbf1cf20..555f730c9e 100644
--- a/src/quick/doc/src/concepts/modelviewsdata/modelview.qdoc
+++ b/src/quick/doc/src/concepts/modelviewsdata/modelview.qdoc
@@ -164,6 +164,16 @@ To visualize data, bind the view's \c model property to a model and the
 
     \snippet qml/qml-data-models/listmodel-listview.qml document
 
+    To get finer control over which roles are accessible, and to make delegates
+    more self-contained and usable outside of views,
+    \li{Required Properties}{required properties} can be used. If a delegate
+    contains required properties, the named roles are not provided. Instead,
+    the QML engine will check if the name of a required property matches that of
+    a model role. If so, that property will be bound to the corresponding value
+    from the model.
+
+    \snippet qml/qml-data-models/listmodel-listview-required.qml document
+
     If there is a naming clash between the model's properties and the delegate's
     properties, the roles can be accessed with the qualified \e model name
     instead. For example, if a \l Text type had \e type or \e age properties,
@@ -186,6 +196,10 @@ To visualize data, bind the view's \c model property to a model and the
     modelData role is also provided for models that have only one role. In this
     case the \e modelData role contains the same data as the named role.
 
+    \note \e model, \e index, and \e modelData roles are not accessible
+    if the delegate contains required properties, unless it has also required
+    properties with matching names.
+
     QML provides several types of data models among the built-in set of QML
     types. In addition, models can be created with Qt C++ and then made
     available to \l{QQmlEngine} for use by
@@ -417,6 +431,12 @@ ListView {
 \note The \c edit role is equal to \l Qt::EditRole. See \l{QAbstractItemModel::}{roleNames}()
 for the built-in role names. However, real life models would usually register custom roles.
 
+\node If a model role is bound to a \li{Required Property}{required property}, assigning to
+that property will not modify the model. It will instead break the binding to the model (just
+like assigning to any other property breaks existing bindings). If you want to use
+required properties and change the model data, make model also a required property and assign to
+\e model.propertyName.
+
 For more information, visit the \l{qtquick-modelviewsdata-cppmodels.html#changing-model-data}{Using C++ Models with Qt Quick Views}
 article.