Ensure that scarce resources work with var properties

Now that we have a new property type which stores JavaScript handles,
we need to ensure that scarce resources can be used with them.

Task-number: QMLNG-18
Task-number: QTBUG-21843
Change-Id: I4a920ae39e7d33cf5e33362e5e0ee21c74cb35e3
Reviewed-by: Martin Jones <martin.jones@nokia.com>

2 files changed, 49 insertions, 136 deletions

diff --git a/doc/src/declarative/basictypes.qdoc b/doc/src/declarative/basictypes.qdoc
index 98eea78a07..a908e73ca8 100644
--- a/doc/src/declarative/basictypes.qdoc
+++ b/doc/src/declarative/basictypes.qdoc
@@ -479,6 +479,19 @@
     }
     \endqml
 
+    A \c var type property can also hold an image or pixmap.
+    A \c var which contains a QPixmap or QImage is known as a
+    "scarce resource" and the declarative engine will attempt to
+    automatically release such resources after evaluation of any JavaScript
+    expression which requires one to be copied has completed.
+
+    Clients may explicitly release such a scarce resource by calling the
+    "destroy" method on the \c var property from within JavaScript.  They
+    may also explicitly preserve the scarce resource by calling the
+    "preserve" method on the \c var property from within JavaScript.
+    For more information regarding the usage of a scarce resource, please
+    see \l{Scarce Resources in JavaScript}.
+
     \sa {QML Basic Types}
 */
 
diff --git a/doc/src/declarative/javascriptblocks.qdoc b/doc/src/declarative/javascriptblocks.qdoc
index a9e383c258..34a1986db6 100644
--- a/doc/src/declarative/javascriptblocks.qdoc
+++ b/doc/src/declarative/javascriptblocks.qdoc
@@ -362,7 +362,7 @@ Item {
 
 \section1 Scarce Resources in JavaScript
 
-As described in the documentation for \l{QML Basic Types}, a \c variant type
+As described in the documentation for \l{QML Basic Types}, a \c var type
 property may hold a "scarce resource" (image or pixmap).  There are several
 important semantics of scarce resources which should be noted:
 
@@ -379,30 +379,12 @@ may be necessary for clients to manually preserve or destroy resources for
 themselves.
 
 For the following examples, imagine that we have defined the following class:
-\code
-class AvatarExample : public QObject
-{
-    Q_OBJECT
-    Q_PROPERTY(QPixmap avatar READ avatar WRITE setAvatar NOTIFY avatarChanged)
-public:
-    AvatarExample(QObject *parent = 0) : QObject(parent), m_value(100, 100) { m_value.fill(Qt::blue); }
-    ~AvatarExample() {}
-
-    QPixmap avatar() const { return m_value; }
-    void setAvatar(QPixmap v) { m_value = v; emit avatarChanged(); }
-
-signals:
-    void avatarChanged();
-
-private:
-    QPixmap m_value;
-};
-\endcode
+
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.h 0
 
 and that we have registered it with the QML type-system as follows:
-\code
-qmlRegisterType<AvatarExample>("Qt.example", 1, 0, "AvatarExample");
-\endcode
+
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 0
 
 The AvatarExample class has a property which is a pixmap.  When the property
 is accessed in JavaScript scope, a copy of the resource will be created and
@@ -414,141 +396,59 @@ unless the client explicitly preserves it.
 
 \section2 Example One: Automatic Release
 
-In this example, the resource will be automatically
+In the following example, the scarce resource will be automatically released
+after the binding evaluation is complete.
+
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleOne.qml 0
+
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 1
+
+\section2 Example Two: Automatic Release Prevented By Reference
+
+In this example, the resource will not be automatically
 released after the binding expression evaluation is
-complete.
+complete, because there is a property var referencing the
+scarce resource.
 
-\qml
-// exampleOne.qml
-import QtQuick 1.0
-import Qt.example 1.0
-
-QtObject {
-    property AvatarExample a;
-    a: AvatarExample { id: example }
-    property variant avatar: example.avatar
-}
-\endqml
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleTwo.qml 0
 
-\code
-QDeclarativeComponent component(&engine, "exampleOne.qml");
-QObject *object = component.create();
-// The scarce resource will have been released automatically
-// after the binding expression was evaluated.
-// Since the scarce resource was not released explicitly prior
-// to the binding expression being evaluated, we get the
-// expected result:
-//object->property("scarceResourceCopy").isValid() == true
-delete object;
-\endcode
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 2
 
-\section2 Example Two: Explicit Preservation
+\section2 Example Three: Explicit Preservation
 
 In this example, the resource must be explicitly preserved in order
 to prevent the declarative engine from automatically releasing the
 resource after evaluation of the imported script.
 
-\code
-// exampleTwo.js
-.import Qt.example 1.0 as QtExample
-
-var component = Qt.createComponent("exampleOne.qml");
-var exampleOneElement = component.createObject(null);
-var avatarExample = exampleOneElement.a;
-var retn = avatarExample.avatar;
-
-// without the following call, the scarce resource held
-// by retn would be automatically released by the engine
-// after the import statement in exampleTwo.qml, prior
-// to the variable assignment.
-retn.preserve();
-
-function importAvatar() {
-    return retn;
-}
-\endcode
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleThree.js 0
 
-\qml
-// exampleTwo.qml
-import QtQuick 1.0
-import Qt.example 1.0
-import "exampleTwo.js" as ExampleTwoJs
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleThree.qml 0
 
-QtObject {
-    property variant avatar: ExampleTwoJs.importAvatar()
-}
-\endqml
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 3
 
-\code
-QDeclarativeComponent component(&engine, "exampleTwo.qml");
-QObject *object = component.create();
-// The resource was preserved explicitly during evaluation of the
-// JavaScript expression.  Thus, during property assignment, the
-// scarce resource was still valid, and so we get the expected result:
-//object->property("avatar").isValid() == true
-// The scarce resource may not have been cleaned up by the JS GC yet;
-// it will continue to consume system resources until the JS GC runs.
-delete object;
-\endcode
-
-\section2 Example Three: Explicit Destruction
+\section2 Example Four: Explicit Destruction
 
 In the following example, we release (via destroy()) an explicitly preserved
 scarce resource variant.  This example shows how a client may free system
 resources by releasing the scarce resource held in a JavaScript object, if
 required, during evaluation of a JavaScript expression.
 
-\code
-// exampleThree.js
-.import Qt.example 1.0 as QtExample
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleFour.js 0
 
-var component = Qt.createComponent("exampleOne.qml");
-var exampleOneElement = component.createObject(null);
-var avatarExample = exampleOneElement.a;
-var retn = avatarExample.avatar;
-retn.preserve();
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleFour.qml 0
 
-function importAvatar() {
-    return retn;
-}
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 4
 
-function releaseAvatar() {
-    retn.destroy();
-}
-\endcode
+\section2 Example Five: Explicit Destruction And JavaScript References
 
-\qml
-// exampleThree.qml
-import QtQuick 1.0
-import Qt.example 1.0
-import "exampleThree.js" as ExampleThreeJs
-
-QtObject {
-    property variant avatarOne
-    property variant avatarTwo
-
-    Component.onCompleted: {
-        avatarOne = ExampleThreeJs.importAvatar(); // valid at this stage
-        ExampleThreeJs.releaseAvatar();            // explicit release
-        avatarTwo = ExampleThreeJs.importAvatar(); // invalid at this stage
-    }
-}
-\endqml
+One thing to be aware of when using "var" type properties is that they
+hold references to JavaScript objects.  As such, if multiple references
+to one scarce resource is held, and the client calls destroy() on one
+of those references (to explicitly release the scarce resource), all of
+the references will be affected.
 
-\code
-QDeclarativeComponent component(&engine, "exampleThree.qml");
-QObject *object = component.create();
-// The scarce resource was explicitly preserved by the client during
-// the evaluation of the imported script, and so the scarce resource
-// remains valid until the explicit call to releaseAvatar().  As such,
-// we get the expected results:
-//object->property("avatarOne").isValid() == true
-//object->property("avatarTwo").isValid() == false
-// Because the scarce resource was released explicitly, it will no longer
-// be consuming any system resources (beyond what a normal JS Object would;
-// that small overhead will exist until the JS GC runs, as per any other
-// JavaScript object).
-delete object;
-\endcode
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/exampleFive.qml 0
+
+\snippet doc/src/snippets/declarative/integrating-javascript/scarceresources/avatarExample.cpp 5
 
 */