Buttons: defer the execution of the delegates

In practice, deferring the execution of the delegates (until
accessed) means that the default delegate item does not get created
at all, when a custom control replaces it at construction time.

    Button {
        background: Rectangle { ... }
    }

Before, such custom Button would never be faster than the original
one, because the default delegate was first created and then thrown
away at construction time. Originally, this was not considered a huge
problem, because the plan was to keep the default delegates so light-
weight that it wouldn't matter. However, then came the fancy styles
with shadows and effects and thus, heavier default delegates. There's
also a growing demand for more features, and the default delegates
are slowly getting heavier...

Now, after this patch, if you replace a heavy default delegate with
a light and simple custom delegate, the result is a much faster
control. For example, replacing Material style Button's background,
which has a shadow effect, with a plain Rectangle gives a ~10x boost,
because the default background with its heavy shadow effect is not
executed at all.

At the same time, deferring the execution of the default delegates
avoids troubles with asynchronous incubation, because we don't need
to destroy an object in the middle of the incubation process.

Task-number: QTBUG-50992
Change-Id: I2274bff99b9ff126d3748278d58d859222910c98
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 110 insertions, 0 deletions

diff --git a/src/quicktemplates2/qquickdeferredexecute.cpp b/src/quicktemplates2/qquickdeferredexecute.cpp
new file mode 100644
index 00000000..7ec4e8e8
--- /dev/null
+++ b/src/quicktemplates2/qquickdeferredexecute.cpp
@@ -0,0 +1,110 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the Qt Quick Templates 2 module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL3$
+** 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 http://www.qt.io/terms-conditions. For further
+** information use the contact form at http://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPLv3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or later as published by the Free
+** Software Foundation and appearing in the file LICENSE.GPL included in
+** the packaging of this file. Please review the following information to
+** ensure the GNU General Public License version 2.0 requirements will be
+** met: http://www.gnu.org/licenses/gpl-2.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qquickdeferredexecute_p_p.h"
+
+#include <QtQml/qqmlengine.h>
+#include <QtQml/private/qqmldata_p.h>
+#include <QtQml/private/qqmlcomponent_p.h>
+#include <QtQml/private/qqmlobjectcreator_p.h>
+
+QT_BEGIN_NAMESPACE
+
+namespace QtQuickPrivate {
+
+static void beginDeferred(QQmlEnginePrivate *enginePriv, const QQmlProperty &property, QQmlComponentPrivate::DeferredState *deferredState)
+{
+    QObject *object = property.object();
+    QQmlData *ddata = QQmlData::get(object);
+    Q_ASSERT(!ddata->deferredData.isEmpty());
+
+    int propertyIndex = property.index();
+
+    for (auto dit = ddata->deferredData.rbegin(); dit != ddata->deferredData.rend(); ++dit) {
+        QQmlData::DeferredData *deferData = *dit;
+
+        auto range = deferData->bindings.equal_range(propertyIndex);
+        if (range.first == deferData->bindings.end())
+            continue;
+
+        QQmlComponentPrivate::ConstructionState *state = new QQmlComponentPrivate::ConstructionState;
+        state->completePending = true;
+
+        QQmlContextData *creationContext = nullptr;
+        state->creator.reset(new QQmlObjectCreator(deferData->context->parent, deferData->compilationUnit, creationContext));
+
+        enginePriv->inProgressCreations++;
+
+        typedef QMultiHash<int, const QV4::CompiledData::Binding *> QV4PropertyBindingHash;
+        auto it = std::reverse_iterator<QV4PropertyBindingHash::iterator>(range.second);
+        auto last = std::reverse_iterator<QV4PropertyBindingHash::iterator>(range.first);
+        while (it != last) {
+            if (!state->creator->populateDeferredBinding(property, deferData, *it))
+                state->errors << state->creator->errors;
+            ++it;
+        }
+
+        deferredState->constructionStates += state;
+
+        // Cleanup any remaining deferred bindings for this property, also in inner contexts,
+        // to avoid executing them later and overriding the property that was just populated.
+        while (dit != ddata->deferredData.rend()) {
+            (*dit)->bindings.remove(propertyIndex);
+            ++dit;
+        }
+        break;
+    }
+}
+
+void executeDeferred(QObject *object, const QString &property)
+{
+    QQmlData *data = QQmlData::get(object);
+    if (data && !data->deferredData.isEmpty() && !data->wasDeleted(object)) {
+        QQmlEnginePrivate *ep = QQmlEnginePrivate::get(data->context->engine);
+
+        QQmlComponentPrivate::DeferredState state;
+        beginDeferred(ep, QQmlProperty(object, property), &state);
+
+        // Release deferred data for those compilation units that no longer have deferred bindings
+        data->releaseDeferredData();
+
+        QQmlComponentPrivate::completeDeferred(ep, &state);
+    }
+}
+
+} // QtQuickPrivate
+
+QT_END_NAMESPACE