unify handler grab state handling into onGrabChanged

onGrabChanged and handleGrab looked redundant.  It was also not clear
how important it is for handlers to react to passive ungrabs, overrides
or cancellations.  Rather than debating about when to call one of these
and when not to, let's centralize the responsibility in
QQuickEventPoint (because the grabber pointers are stored there, so
it's the ultimate destination of any grab change), and let's notify
all the relevant handlers about all changes, with enough information
that each handler can decide for itself what's important and what
isn't.  But so far most handlers don't need to override this virtual.
The base class QQuickPointerHandler takes care of setting the
active property to false, rejecting the eventpoint, and unsetting
keepMouseGrab and keepTouchGrab whenever grab is lost; and emitting
grabChanged or canceled as appropriate to notify any QML code which
needs to know.  Subclasses mainly care about the change of active state:
they must initiate active state themselves, and may react when it
reverts to false.

Change-Id: I6c7f29472d12564d74ae091b0c81fa08fe131ce7
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

1 files changed, 55 insertions, 44 deletions

diff --git a/src/quick/handlers/qquickpointerhandler.cpp b/src/quick/handlers/qquickpointerhandler.cpp
index 90d07b5d6d..88c6800dcc 100644
--- a/src/quick/handlers/qquickpointerhandler.cpp
+++ b/src/quick/handlers/qquickpointerhandler.cpp
@@ -76,48 +76,75 @@ QQuickPointerHandler::~QQuickPointerHandler()
     }
 }
 
+/*!
+    Notification that the grab has changed in some way which is relevant to this handler.
+    The \a grabber (subject) will be the PointerHandler whose state is changing,
+    or null if the state change regards an Item. (TODO do we have any such cases?)
+    The \a stateChange (verb) tells what happened.
+    The \a point (object) is the point that was grabbed or ungrabbed.
+    EventPoint has the sole responsibility to call this function.
+    The PointerHandler must react in whatever way is appropriate, and must
+    emit the relevant signals (for the benefit of QML code).
+    A subclass is allowed to override this virtual function, but must always
+    call its parent class's implementation in addition to (usually after)
+    whatever custom behavior it implements.
+*/
+void QQuickPointerHandler::onGrabChanged(QQuickPointerHandler *grabber, QQuickEventPoint::GrabState stateChange, QQuickEventPoint *point)
+{
+    qCDebug(lcPointerHandlerDispatch) << point << stateChange << grabber;
+    Q_ASSERT(point);
+    if (grabber == this) {
+        bool wasCanceled = false;
+        emit grabChanged(point);
+        switch (stateChange) {
+        case QQuickEventPoint::GrabPassive:
+        case QQuickEventPoint::GrabExclusive:
+            break;
+        case QQuickEventPoint::CancelGrabPassive:
+        case QQuickEventPoint::CancelGrabExclusive:
+            wasCanceled = true; // the grab was stolen by something else
+            Q_FALLTHROUGH();
+        case QQuickEventPoint::UngrabPassive:
+        case QQuickEventPoint::UngrabExclusive:
+            setActive(false);
+            point->setAccepted(false);
+            if (auto par = parentItem()) {
+                par->setKeepMouseGrab(m_hadKeepMouseGrab);
+                par->setKeepTouchGrab(m_hadKeepTouchGrab);
+            }
+        case QQuickEventPoint::OverrideGrabPassive:
+            // Passive grab is still there, but we won't receive point updates right now.
+            // No need to notify about this.
+            return;
+        }
+        if (wasCanceled)
+            emit canceled(point);
+        else
+            emit grabChanged(point);
+    }
+}
+
 void QQuickPointerHandler::setPassiveGrab(QQuickEventPoint *point, bool grab)
 {
+    qCDebug(lcPointerHandlerDispatch) << point << grab;
     if (grab) {
         point->setGrabberPointerHandler(this, false);
-        emit grabChanged(point);
-    } else if (point->grabberPointerHandler() == this) {
-        // TODO should giving up passive grab imply giving up exclusive grab too?
-        // we're being inconsistent here: check whether the exclusive grabber is this,
-        // then say that the passive grab was canceled.
-        point->cancelPassiveGrab(this);
-        emit grabChanged(point);
+    } else {
+        point->removePassiveGrabber(this);
     }
 }
 
 void QQuickPointerHandler::setExclusiveGrab(QQuickEventPoint *point, bool grab)
 {
-    QQuickPointerHandler *oldGrabber = point->grabberPointerHandler();
-    if (grab && oldGrabber != this) {
-        if (target()) {
-            m_hadKeepMouseGrab = target()->keepMouseGrab();
-            m_hadKeepTouchGrab = target()->keepTouchGrab();
-        }
-        if (oldGrabber)
-            oldGrabber->handleGrabCancel(point);
-        point->setGrabberPointerHandler(this, true);
-        onGrabChanged(point);
-//        emit grabChanged(point); // TODO maybe
-    } else if (!grab && oldGrabber == this) {
-        if (auto tgt = target()) {
-            tgt->setKeepMouseGrab(m_hadKeepMouseGrab);
-            tgt->setKeepTouchGrab(m_hadKeepTouchGrab);
-        }
-        point->setGrabberPointerHandler(nullptr, true);
-        onGrabChanged(point);
-//        emit grabChanged(point); // TODO maybe
-    }
+    // TODO m_hadKeepMouseGrab m_hadKeepTouchGrab
+    qCDebug(lcPointerHandlerDispatch) << point << grab;
+    point->setGrabberPointerHandler(grab ? this : nullptr, true);
 }
 
 void QQuickPointerHandler::cancelAllGrabs(QQuickEventPoint *point)
 {
+    qCDebug(lcPointerHandlerDispatch) << point;
     point->cancelAllGrabs(this);
-    emit grabChanged(point);
 }
 
 QPointF QQuickPointerHandler::eventPos(const QQuickEventPoint *point) const
@@ -177,22 +204,6 @@ void QQuickPointerHandler::handlePointerEvent(QQuickPointerEvent *event)
         setActive(false);
 }
 
-void QQuickPointerHandler::handleGrabCancel(QQuickEventPoint *point)
-{
-    qCDebug(lcPointerHandlerDispatch) << point;
-    Q_ASSERT(point);
-    setActive(false);
-    point->setAccepted(false);
-    emit canceled(point);
-}
-
-void QQuickPointerHandler::handleGrab(QQuickEventPoint *point, QQuickPointerHandler *grabber, bool grab)
-{
-    Q_UNUSED(point);
-    Q_UNUSED(grabber);
-    Q_UNUSED(grab);
-}
-
 bool QQuickPointerHandler::wantsPointerEvent(QQuickPointerEvent *event)
 {
     Q_UNUSED(event)