qml/types/qqmlbind.cpp

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#include "qqmlbind_p.h"

#include <private/qqmlnullablevalue_p.h>
#include <private/qqmlproperty_p.h>
#include <private/qqmlbinding_p.h>
#include <private/qqmlmetatype_p.h>
#include <private/qqmlvmemetaobject_p.h>
#include <private/qv4persistent_p.h>

#include <qqmlengine.h>
#include <qqmlcontext.h>
#include <qqmlproperty.h>
#include <qqmlinfo.h>

#include <QtCore/qfile.h>
#include <QtCore/qdebug.h>
#include <QtCore/qtimer.h>
#include <QtCore/qloggingcategory.h>

#include <private/qobject_p.h>

QT_BEGIN_NAMESPACE

Q_DECLARE_LOGGING_CATEGORY(lcBindingRemoval)
Q_LOGGING_CATEGORY(lcQmlBindingRestoreMode, "qt.qml.binding.restoreMode")

class QQmlBindPrivate : public QObjectPrivate
{
public:
    QQmlBindPrivate()
        : obj(nullptr)
        , prevBind(QQmlAbstractBinding::Ptr())
        , prevIsVariant(false)
        , componentComplete(true)
        , delayed(false)
        , pendingEval(false)
        , restoreBinding(true)
        , restoreValue(false)
        , restoreModeExplicit(false)
        , writingProperty(false)
    {}
    ~QQmlBindPrivate() { }

    QQmlNullableValue<bool> when;
    QPointer<QObject> obj;
    QString propName;
    QQmlNullableValue<QJSValue> value;
    QQmlProperty prop;
    QQmlAbstractBinding::Ptr prevBind;
    QV4::PersistentValue v4Value;
    QVariant prevValue;
    bool prevIsVariant:1;
    bool componentComplete:1;
    bool delayed:1;
    bool pendingEval:1;
    bool restoreBinding:1;
    bool restoreValue:1;
    bool restoreModeExplicit:1;
    bool writingProperty: 1;

    void validate(QObject *binding) const;
    void clearPrev();
};

void QQmlBindPrivate::validate(QObject *binding) const
{
    if (!obj || (when.isValid() && !when))
        return;

    if (!prop.isValid()) {
        qmlWarning(binding) << "Property '" << propName << "' does not exist on " << QQmlMetaType::prettyTypeName(obj) << ".";
        return;
    }

    if (!prop.isWritable()) {
        qmlWarning(binding) << "Property '" << propName << "' on " << QQmlMetaType::prettyTypeName(obj) << " is read-only.";
        return;
    }
}

/*!
    \qmltype Binding
    \instantiates QQmlBind
    \inqmlmodule QtQml
    \ingroup qtquick-interceptors
    \brief Enables the arbitrary creation of property bindings.

    In QML, property bindings result in a dependency between the properties of
    different objects.

    \section1 Binding to an Inaccessible Property

    Sometimes it is necessary to bind an object's property to
    that of another object that isn't directly instantiated by QML, such as a
    property of a class exported to QML by C++. You can use the Binding type
    to establish this dependency; binding any value to any object's property.

    For example, in a C++ application that maps an "app.enteredText" property
    into QML, you can use Binding to update the enteredText property.

    \code
    TextEdit { id: myTextField; text: "Please type here..." }
    Binding { target: app; property: "enteredText"; value: myTextField.text }
    \endcode

    When \c{text} changes, the C++ property \c{enteredText} will update
    automatically.

    \section1 Conditional Bindings

    In some cases you may want to modify the value of a property when a certain
    condition is met but leave it unmodified otherwise. Often, it's not possible
    to do this with direct bindings, as you have to supply values for all
    possible branches.

    For example, the code snippet below results in a warning whenever you
    release the mouse. This is because the value of the binding is undefined
    when the mouse isn't pressed.

    \qml
    // produces warning: "Unable to assign [undefined] to double value"
    value: if (mouse.pressed) mouse.mouseX
    \endqml

    The Binding type can prevent this warning.

    \qml
    Binding on value {
        when: mouse.pressed
        value: mouse.mouseX
    }
    \endqml

    The Binding type restores any previously set direct bindings on the
    property.

    \sa {Qt QML}
*/
QQmlBind::QQmlBind(QObject *parent)
    : QObject(*(new QQmlBindPrivate), parent)
{
}

QQmlBind::~QQmlBind()
{
}

/*!
    \qmlproperty bool QtQml::Binding::when

    This property holds when the binding is active.
    This should be set to an expression that evaluates to true when you want the binding to be active.

    \code
    Binding {
        target: contactName; property: 'text'
        value: name; when: list.ListView.isCurrentItem
    }
    \endcode

    When the binding becomes inactive again, any direct bindings that were previously
    set on the property will be restored.

    \note By default, a previously set literal value is not restored when the Binding becomes
    inactive. Rather, the last value set by the now inactive Binding is retained. You can customize
    the restoration behavior for literal values as well as bindings using the \l restoreMode
    property. The default will change in Qt 6.0.

    \sa restoreMode
*/
bool QQmlBind::when() const
{
    Q_D(const QQmlBind);
    return d->when;
}

void QQmlBind::setWhen(bool v)
{
    Q_D(QQmlBind);
    if (!d->when.isNull && d->when == v)
        return;

    d->when = v;
    if (v && d->componentComplete)
        d->validate(this);
    eval();
}

/*!
    \qmlproperty Object QtQml::Binding::target

    The object to be updated.
*/
QObject *QQmlBind::object()
{
    Q_D(const QQmlBind);
    return d->obj;
}

void QQmlBind::setObject(QObject *obj)
{
    Q_D(QQmlBind);
    if (d->obj && d->when.isValid() && d->when) {
        /* if we switch the object at runtime, we need to restore the
           previous binding on the old object before continuing */
        d->when = false;
        eval();
        d->when = true;
    }
    d->obj = obj;
    if (d->componentComplete) {
        setTarget(QQmlProperty(d->obj, d->propName, qmlContext(this)));
        d->validate(this);
    }
    eval();
}

/*!
    \qmlproperty string QtQml::Binding::property

    The property to be updated.

    This can be a group property if the expression results in accessing a
    property of a \l {QML Basic Types}{value type}. For example:

    \qml
    Item {
        id: item

        property rect rectangle: Qt.rect(0, 0, 200, 200)
    }

    Binding {
        target: item
        property: "rectangle.x"
        value: 100
    }
    \endqml
*/
QString QQmlBind::property() const
{
    Q_D(const QQmlBind);
    return d->propName;
}

void QQmlBind::setProperty(const QString &p)
{
    Q_D(QQmlBind);
    if (!d->propName.isEmpty() && d->when.isValid() && d->when) {
        /* if we switch the property name at runtime, we need to restore the
           previous binding on the old object before continuing */
        d->when = false;
        eval();
        d->when = true;
    }
    d->propName = p;
    if (d->componentComplete) {
        setTarget(QQmlProperty(d->obj, d->propName, qmlContext(this)));
        d->validate(this);
    }
    eval();
}

/*!
    \qmlproperty any QtQml::Binding::value

    The value to be set on the target object and property.  This can be a
    constant (which isn't very useful), or a bound expression.
*/
QJSValue QQmlBind::value() const
{
    Q_D(const QQmlBind);
    return d->value.value;
}

void QQmlBind::setValue(const QJSValue &v)
{
    Q_D(QQmlBind);
    d->value = v;
    prepareEval();
}

/*!
    \qmlproperty bool QtQml::Binding::delayed
    \since 5.8

    This property holds whether the binding should be delayed.

    A delayed binding will not immediately update the target, but rather wait
    until the event queue has been cleared. This can be used as an optimization,
    or to prevent intermediary values from being assigned.

    \code
    Binding {
        target: contactName; property: 'text'
        value: givenName + " " + familyName; when: list.ListView.isCurrentItem
        delayed: true
    }
    \endcode
*/
bool QQmlBind::delayed() const
{
    Q_D(const QQmlBind);
    return d->delayed;
}

void QQmlBind::setDelayed(bool delayed)
{
    Q_D(QQmlBind);
    if (d->delayed == delayed)
        return;

    d->delayed = delayed;

    if (!d->delayed)
        eval();
}

/*!
    \qmlproperty enumeration QtQml::Binding::restoreMode
    \since 5.14

    This property can be used to describe if and how the original value should
    be restored when the binding is disabled.

    The possible values are:
    \list
    \li Binding.RestoreNone The original value is not restored at all
    \li Binding.RestoreBinding The original value is restored if it was another
        binding. In that case the old binding is in effect again.
    \li Binding.RestoreValue The original value is restored if it was a plain
        value rather than a binding.
    \li Binding.RestoreBindingOrValue The original value is always restored.
    \endlist

    \warning The default value is Binding.RestoreBinding. This will change in
    Qt 6.0 to Binding.RestoreBindingOrValue.

    If you rely on any specific behavior regarding the restoration of plain
    values when bindings get disabled you should migrate to explicitly set the
    restoreMode.

    Reliance on a restoreMode that doesn't restore the previous binding or value
    for a specific property results in a run-time warning.
*/
QQmlBind::RestorationMode QQmlBind::restoreMode() const
{
    Q_D(const QQmlBind);
    unsigned result = RestoreNone;
    if (d->restoreValue)
        result |= RestoreValue;
    if (d->restoreBinding)
        result |= RestoreBinding;
    return RestorationMode(result);
}

void QQmlBind::setRestoreMode(RestorationMode newMode)
{
    Q_D(QQmlBind);
    d->restoreModeExplicit = true;
    if (newMode != restoreMode()) {
        d->restoreValue = (newMode & RestoreValue);
        d->restoreBinding = (newMode & RestoreBinding);
        emit restoreModeChanged();
    }
}

void QQmlBind::setTarget(const QQmlProperty &p)
{
    Q_D(QQmlBind);

    if (Q_UNLIKELY(lcBindingRemoval().isInfoEnabled())) {
        if (QObject *oldObject = d->prop.object()) {
            QMetaProperty prop = oldObject->metaObject()->property(d->prop.index());
            if (prop.hasNotifySignal()) {
                QByteArray signal('2' + prop.notifySignal().methodSignature());
                QObject::disconnect(oldObject, signal.constData(),
                                    this, SLOT(targetValueChanged()));
            }
        }
        p.connectNotifySignal(this, SLOT(targetValueChanged()));
    }

    d->prop = p;
}

void QQmlBind::classBegin()
{
    Q_D(QQmlBind);
    d->componentComplete = false;
}

void QQmlBind::componentComplete()
{
    Q_D(QQmlBind);
    d->componentComplete = true;
    if (!d->prop.isValid()) {
        setTarget(QQmlProperty(d->obj, d->propName, qmlContext(this)));
        d->validate(this);
    }
    eval();
}

void QQmlBind::prepareEval()
{
    Q_D(QQmlBind);
    if (d->delayed) {
        if (!d->pendingEval)
            QTimer::singleShot(0, this, &QQmlBind::eval);
        d->pendingEval = true;
    } else {
        eval();
    }
}

void QQmlBindPrivate::clearPrev()
{
    prevBind = nullptr;
    v4Value.clear();
    prevValue.clear();
    prevIsVariant = false;
}

void QQmlBind::eval()
{
    Q_D(QQmlBind);
    d->pendingEval = false;
    if (!d->prop.isValid() || d->value.isNull || !d->componentComplete)
        return;

    if (d->when.isValid()) {
        if (!d->when) {
            //restore any previous binding
            if (d->prevBind) {
                if (d->restoreBinding) {
                    QQmlAbstractBinding::Ptr p = d->prevBind;
                    d->clearPrev(); // Do that before setBinding(), as setBinding() may recurse.
                    QQmlPropertyPrivate::setBinding(p.data());
                }
            } else if (!d->v4Value.isEmpty()) {
                if (d->restoreValue) {
                    auto propPriv = QQmlPropertyPrivate::get(d->prop);
                    QQmlVMEMetaObject *vmemo = QQmlVMEMetaObject::get(propPriv->object);
                    Q_ASSERT(vmemo);
                    vmemo->setVMEProperty(propPriv->core.coreIndex(), *d->v4Value.valueRef());
                    d->clearPrev();
                } else if (!d->restoreModeExplicit && lcQmlBindingRestoreMode().isWarningEnabled()) {
                    qmlWarning(this)
                            << "Not restoring previous value because restoreMode has not been set.\n"
                            << "This behavior is deprecated.\n"
                            << "You have to import QtQml 2.15 after any QtQuick imports and set\n"
                            << "the restoreMode of the binding to fix this warning.\n"
                            << "In Qt < 6.0 the default is Binding.RestoreBinding.\n"
                            << "In Qt >= 6.0 the default is Binding.RestoreBindingOrValue.";
                }
            } else if (d->prevIsVariant) {
                if (d->restoreValue) {
                    d->prop.write(d->prevValue);
                    d->clearPrev();
                } else if (!d->restoreModeExplicit  && lcQmlBindingRestoreMode().isWarningEnabled()) {
                    qmlWarning(this)
                            << "Not restoring previous value because restoreMode has not been set.\n"
                            << "This behavior is deprecated.\n"
                            << "You have to import QtQml 2.15 after any QtQuick imports and set\n"
                            << "the restoreMode of the binding to fix this warning.\n"
                            << "In Qt < 6.0 the default is Binding.RestoreBinding.\n"
                            << "In Qt >= 6.0 the default is Binding.RestoreBindingOrValue.\n";
                }
            }
            return;
        }

        //save any set binding for restoration
        if (!d->prevBind && d->v4Value.isEmpty() && !d->prevIsVariant) {
            // try binding first
            d->prevBind = QQmlPropertyPrivate::binding(d->prop);

            if (!d->prevBind) { // nope, try a V4 value next
                auto propPriv = QQmlPropertyPrivate::get(d->prop);
                auto propData = propPriv->core;
                if (!propPriv->valueTypeData.isValid() && propData.isVarProperty()) {
                    QQmlVMEMetaObject *vmemo = QQmlVMEMetaObject::get(propPriv->object);
                    Q_ASSERT(vmemo);
                    auto retVal = vmemo->vmeProperty(propData.coreIndex());
                    d->v4Value = QV4::PersistentValue(vmemo->engine, retVal);
                } else { // nope, use the meta object to get a QVariant
                    d->prevValue = d->prop.read();
                    d->prevIsVariant = true;
                }
            }
        }

        QQmlPropertyPrivate::removeBinding(d->prop);
    }

    d->writingProperty = true;
    d->prop.write(d->value.value.toVariant());
    d->writingProperty = false;
}

void QQmlBind::targetValueChanged()
{
    Q_D(QQmlBind);
    if (d->writingProperty)
        return;

    if (d->when.isValid() && !d->when)
        return;

    QUrl url;
    quint16 line = 0;

    const QQmlData *ddata = QQmlData::get(this, false);
    if (ddata && ddata->outerContext) {
        url = ddata->outerContext->url();
        line = ddata->lineNumber;
    }

    qCInfo(lcBindingRemoval,
           "The target property of the Binding element created at %s:%d was changed from "
           "elsewhere. This does not overwrite the binding. The target property will still be "
           "updated when the value of the Binding element changes.",
           qPrintable(url.toString()), line);
}

QT_END_NAMESPACE

#include "moc_qqmlbind_p.cpp"