QGraphicsView 中实现拖拽

开始想使用QGraphicsView结合QGraphicsScene和QGraphicsItem，做个绘制各种图形的编辑器，想使用拖拽实现快捷方式，没想到还有点波折。

已在QGraphicsVews中设置了setAcceptDrops(true);

在运行时发现：当把拖拽的图标放到QGraphicsVews上时，显示的还是不可拖拽的形状，但dragEnterEvent可以触发。但后面的dropEvent就不能触发了。

上网一查，视图接收到拖拽事件后会转交给关联的场景处理，但是在场景中重写dropevent还是没有扑捉到事件。

今天实际跟踪了一下：

1、拖拽首先进入QGraphicsView触发其dragEnterEvent事件:

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1. void QGraphicsView::dragEnterEvent(QDragEnterEvent *event)  
2. {  
3. #ifndef QT_NO_DRAGANDDROP  
4.     Q_D(QGraphicsView);  
5.     if (!d->scene || !d->sceneInteractionAllowed)  
6.         return;  
7.   
8.     // Disable replaying of mouse move events.  
9.     d->useLastMouseEvent = false;  
10.   
11.     // Generate a scene event.  
12.     QGraphicsSceneDragDropEvent sceneEvent(QEvent::GraphicsSceneDragEnter);  
13.     d->populateSceneDragDropEvent(&sceneEvent, event);  
14.   
15.     // Store it for later use.  
16.     d->storeDragDropEvent(&sceneEvent);  
17.   
18.     // Send it to the scene.   
19.     QApplication::sendEvent(d->scene, &sceneEvent);  
20.   
21.     // Accept the originating event if the scene accepted the scene event.  
22.     if (sceneEvent.isAccepted()) {  
23.         event->setAccepted(true);  
24.         event->setDropAction(sceneEvent.dropAction());  
25.     }  
26. #else   
27.     Q_UNUSED(event)  
28. #endif   
29. }  

void QGraphicsView::dragEnterEvent(QDragEnterEvent *event){#ifndef QT_NO_DRAGANDDROP    Q_D(QGraphicsView);    if (!d->scene || !d->sceneInteractionAllowed)        return;    // Disable replaying of mouse move events.    d->useLastMouseEvent = false;    // Generate a scene event.    QGraphicsSceneDragDropEvent sceneEvent(QEvent::GraphicsSceneDragEnter);    d->populateSceneDragDropEvent(&sceneEvent, event);    // Store it for later use.    d->storeDragDropEvent(&sceneEvent);    // Send it to the scene.    QApplication::sendEvent(d->scene, &sceneEvent);    // Accept the originating event if the scene accepted the scene event.    if (sceneEvent.isAccepted()) {        event->setAccepted(true);        event->setDropAction(sceneEvent.dropAction());    }#else    Q_UNUSED(event)#endif}

上述函数主要是查看有没有场景，然后将事件转交给场景处理，当然是在条件允许的情况下转交给场景处理。

2、场景的dragEnterEvent

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1. void QGraphicsScene::dragEnterEvent(QGraphicsSceneDragDropEvent *event)  
2. {  
3.     Q_D(QGraphicsScene);  
4.     d->dragDropItem = 0;  
5.     d->lastDropAction = Qt::IgnoreAction;  
6.     event->accept();  
7. }  

void QGraphicsScene::dragEnterEvent(QGraphicsSceneDragDropEvent *event){    Q_D(QGraphicsScene);    d->dragDropItem = 0;    d->lastDropAction = Qt::IgnoreAction;    event->accept();}

场景的dragenterEvent其实很简单就是接收拖拽事件

3、场景的dragMoveEvent事件，重点就在于此

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1. <SPAN style="COLOR: #333333">void QGraphicsScene::dragMoveEvent(QGraphicsSceneDragDropEvent *event)  
2. {  
3.     Q_D(QGraphicsScene);  
4.     </SPAN><SPAN style="COLOR: #ff0000">event->ignore();</SPAN><SPAN style="COLOR: #333333">  
5.   
6.     if (!d->mouseGrabberItems.isEmpty()) {  
7.         // Mouse grabbers that start drag events lose the mouse grab.  
8.         d->clearMouseGrabber();  
9.         d->mouseGrabberButtonDownPos.clear();  
10.         d->mouseGrabberButtonDownScenePos.clear();  
11.         d->mouseGrabberButtonDownScreenPos.clear();  
12.     }  
13.   
14.     bool eventDelivered = false;  
15.   
16.     // Find the topmost enabled items under the cursor. They are all  
17.     // candidates for accepting drag & drop events.  
18.     foreach (QGraphicsItem *item, d->itemsAtPosition(event->screenPos(),  
19.                                                      event->scenePos(),  
20.                                                      event->widget())) {  
21.         if (!item->isEnabled() || !item->acceptDrops())  
22.             continue;  
23.   
24.         if (item != d->dragDropItem) {  
25.             // Enter the new drag drop item. If it accepts the event, we send  
26.             // the leave to the parent item.  
27.             QGraphicsSceneDragDropEvent dragEnter(QEvent::GraphicsSceneDragEnter);  
28.             d->cloneDragDropEvent(&dragEnter, event);  
29.             dragEnter.setDropAction(event->proposedAction());  
30.             d->sendDragDropEvent(item, &dragEnter);  
31.             event->setAccepted(dragEnter.isAccepted());  
32.             event->setDropAction(dragEnter.dropAction());  
33.             if (!event->isAccepted()) {  
34.                 // Propagate to the item under  
35.                 continue;  
36.             }  
37.   
38.             d->lastDropAction = event->dropAction();  
39.   
40.             if (d->dragDropItem) {  
41.                 // Leave the last drag drop item. A perfect implementation  
42.                 // would set the position of this event to the point where  
43.                 // this event and the last event intersect with the item's  
44.                 // shape, but that's not easy to do. :-)  
45.                 QGraphicsSceneDragDropEvent dragLeave(QEvent::GraphicsSceneDragLeave);  
46.                 d->cloneDragDropEvent(&dragLeave, event);  
47.                 d->sendDragDropEvent(d->dragDropItem, &dragLeave);  
48.             }  
49.   
50.             // We've got a new drag & drop item  
51.             d->dragDropItem = item;  
52.         }  
53.   
54.         // Send the move event.   
55.         event->setDropAction(d->lastDropAction);  
56.         event->accept();  
57.         d->sendDragDropEvent(item, event);  
58.         if (event->isAccepted())  
59.             d->lastDropAction = event->dropAction();  
60.         eventDelivered = true;  
61.         break;  
62.     }  
63.   
64.     if (!eventDelivered) {  
65.         if (d->dragDropItem) {  
66.             // Leave the last drag drop item  
67.             QGraphicsSceneDragDropEvent dragLeave(QEvent::GraphicsSceneDragLeave);  
68.             d->cloneDragDropEvent(&dragLeave, event);  
69.             d->sendDragDropEvent(d->dragDropItem, &dragLeave);  
70.             d->dragDropItem = 0;  
71.         }  
72.         // Propagate   
73.         event->setDropAction(Qt::IgnoreAction);  
74.     }  
75. }  
76. </SPAN>  

void QGraphicsScene::dragMoveEvent(QGraphicsSceneDragDropEvent *event){    Q_D(QGraphicsScene);    event->ignore();    if (!d->mouseGrabberItems.isEmpty()) {        // Mouse grabbers that start drag events lose the mouse grab.        d->clearMouseGrabber();        d->mouseGrabberButtonDownPos.clear();        d->mouseGrabberButtonDownScenePos.clear();        d->mouseGrabberButtonDownScreenPos.clear();    }    bool eventDelivered = false;    // Find the topmost enabled items under the cursor. They are all    // candidates for accepting drag & drop events.    foreach (QGraphicsItem *item, d->itemsAtPosition(event->screenPos(),                                                     event->scenePos(),                                                     event->widget())) {        if (!item->isEnabled() || !item->acceptDrops())            continue;        if (item != d->dragDropItem) {            // Enter the new drag drop item. If it accepts the event, we send            // the leave to the parent item.            QGraphicsSceneDragDropEvent dragEnter(QEvent::GraphicsSceneDragEnter);            d->cloneDragDropEvent(&dragEnter, event);            dragEnter.setDropAction(event->proposedAction());            d->sendDragDropEvent(item, &dragEnter);            event->setAccepted(dragEnter.isAccepted());            event->setDropAction(dragEnter.dropAction());            if (!event->isAccepted()) {                // Propagate to the item under                continue;            }            d->lastDropAction = event->dropAction();            if (d->dragDropItem) {                // Leave the last drag drop item. A perfect implementation                // would set the position of this event to the point where                // this event and the last event intersect with the item's                // shape, but that's not easy to do. :-)                QGraphicsSceneDragDropEvent dragLeave(QEvent::GraphicsSceneDragLeave);                d->cloneDragDropEvent(&dragLeave, event);                d->sendDragDropEvent(d->dragDropItem, &dragLeave);            }            // We've got a new drag & drop item            d->dragDropItem = item;        }        // Send the move event.        event->setDropAction(d->lastDropAction);        event->accept();        d->sendDragDropEvent(item, event);        if (event->isAccepted())            d->lastDropAction = event->dropAction();        eventDelivered = true;        break;    }    if (!eventDelivered) {        if (d->dragDropItem) {            // Leave the last drag drop item            QGraphicsSceneDragDropEvent dragLeave(QEvent::GraphicsSceneDragLeave);            d->cloneDragDropEvent(&dragLeave, event);            d->sendDragDropEvent(d->dragDropItem, &dragLeave);            d->dragDropItem = 0;        }        // Propagate        event->setDropAction(Qt::IgnoreAction);    }}

很显然，在拖拽移动过程中忽略了该事件，主要响应的是场景内可能存在的item拖拽事件

看到这，问题就有答案了，subclass场景类，重写dragMoveEvent

[cpp] view plaincopyprint?

1. void QGraphicsScene::dragMoveEvent(QGraphicsSceneDragDropEvent *event)  
2. {  
3.     event->accept();  
4. }  

void QGraphicsScene::dragMoveEvent(QGraphicsSceneDragDropEvent *event){    event->accept();}

最后重写dropEvent，做最后拖拽处理就是了。

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