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diff --git a/doc/src/examples/dragdroprobot.qdoc b/doc/src/examples/dragdroprobot.qdoc index 413f190..887b254 100644 --- a/doc/src/examples/dragdroprobot.qdoc +++ b/doc/src/examples/dragdroprobot.qdoc @@ -43,9 +43,337 @@ \example graphicsview/dragdroprobot \title Drag and Drop Robot Example - This GraphicsView example shows how to implement drag and drop in - a QGraphicsItem subclass, as well as how to animate items using - QGraphicsItemAnimation and QTimeLine. + This GraphicsView example shows how to implement Drag and Drop in a + QGraphicsItem subclass, as well as how to animate items using Qt's + \l{Animation Framework}. \image dragdroprobot-example.png + + Graphics View provides the QGraphicsScene class for managing and + interacting with a large number of custom-made 2D graphical items derived + from the QGraphicsItem class, and a QGraphicsView widget for visualizing + the items, with support for zooming and rotation. + + This example consists of a \c Robot class, a \c ColorItem class, and a main + function: the \c Robot class describes a simple robot consisting of several + \c RobotPart derived limbs, including \c RobotHead and \c RobotLimb, the \c + ColorItem class provides a draggable colored ellipse, and the \c main() + function provides the main application window. + + We will first review the \c Robot class to see how to assemble the + different parts so that they can be individually rotated and animated using + QPropertyAnimation, and we will then review the \c ColorItem class to + demonstrate how to implement Drag and Drop between items. Finally we will + review the main() function to see how we can put all the pieces together, + to form the final application. + + \section1 Robot Class Definition + + The robot consists of three main classes: the \c RobotHead, the \c + RobotTorso, and the \c RobotLimb, which is used for the upper and lower + arms and legs. All parts derive from the \c RobotPart class, which in turn + inherits \c QGraphicsObject. The \c Robot class itself has no visual + appearance and serves only as a root node for the robot. + + Let's start with the \c RobotPart class declaration. + + \snippet examples/graphicsview/dragdroprobot/robot.h 0 + + This base class inherits QGraphicsObject. QGraphicsObject provides signals + and slots through inheriting QObject, and it also declares QGraphicsItem's + properties using Q_PROPERTY, which makes the properties accessible for + QPropertyAnimation. + + RobotPart also implements the three most important event handlers for + accepting drop events: + \l{QGraphicsItem::dragEnterEvent()}{dragEnterEvent()}, + \l{QGraphicsItem::dragLeaveEvent()}{dragLeaveEvent()}, and + \l{QGraphicsItem::dropEvent()}{dropEvent()}. + + The color is stored as a member variable, along with the \c dragOver + variable, which we will use later to indicate visually that the limb can + accept colors that are is dragged onto it. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 0 + + \c RobotPart's constructor initializes the dragOver member and sets the + color to Qt::lightGray. In the constructor body we enable support for + accepting drop events by calling + \l{QGraphicsItem::setAcceptDrops()}{setAcceptDrops(true)}. + + The rest of this class's implementation is to support Drag and Drop. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 1 + + The \l{QGraphicsItem::dragEnterEvent()}{dragEnterEvent()} handler is called + when a Drag and Drop element is dragged into the robot part's area. + + The handler implementation determines whether or not this item as a whole + can accept the mime data assiciated with the incoming drag object. \c + RobotPart provides a base behavior for all parts that accepts color drops. + So if the incoming drag object contains a color, the event is accepted, we + set \c dragOver to \c true and call update() to help provide positive + visual feedback to the user; otherwise the event is ignored, which in turn + allows the event to propagate to parent elements. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 2 + + The \l{QGraphicsItem::dragLeaveEvent()}{dragLeaveEvent()} handler is called + when a Drag and Drop element is dragged away from the robot part's area. + Our implementation simply resets \e dragOver to false and calls + \l{QGraphicsItem::update()}{update()} to help provide visual feedback that + the drag has left this item. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 3 + + The \l{QGraphicsItem::dropEvent()}{dropEvent()} handler is called when a + Drag and Drop element is dropped onto an item (i.e., when the mouse button + is released over the item while dragging). + + We reset \c dragOver to false, assign the item's new color, and call + \l{QGraphicsItem::update()}{update()}. + + The declaration and implementation of \c RobotHead, \c RobotTorso, and \c + RobotLimb are practically identical. We will review \c RobotHead in detail, + as this class has one minor difference, and leave the other classes as an + exercise for the reader. + + \snippet examples/graphicsview/dragdroprobot/robot.h 1 + + The \c RobotHead class inherits \c RobotPart and provides the necessary + implementations of \l{QGraphicsItem::boundingRect()}{boundingRect()} and + \l{QGraphicsItem::paint()}{paint()}. It also reimplements + \l{QGraphicsItem::dragEnterEvent()}{dragEnterEvent()} and dropEvent() to + provide special handling of image drops. + + The class contains a private pixmap member that we can use to implement + support for accepting image drops. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 4 + + \c RobotHead has a rather plain constructor that simply forwards to + \c RobotPart's constructor. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 5 + + The \l{QGraphicsItem::boundingRect()}{boundingRect()} reimplementation + returns the extents for the head. Because we want the center of rotation to + be the bottom center of the item, we have chosen a bounding rectangle that + starts at (-15, -50) and extends to 30 units wide and 50 units tall. When + rotating the head, the "neck" will stay still while the top of the head + tilts from side to side. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 6 + + In \l{QGraphicsItem::paint()}{paint()} we draw the actual head. The + implementation is split into two sections; if an image has been dropped + onto the head, we draw the image, otherwise we draw a round rectangular + robot head with simple vector graphics. + + For performance reasons, depending on the complexity of what is painted, it + can often be faster to draw the head as an image rather than using a + sequence of vector operations. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 7 + + The robot head can accept image drops. In order to support this, its + reimplementation of \l{QGraphicsItem::dragEnterEvent()}{dragEnterEvent()} + checks if the drag object contains image data, and if it does, then the + event is accepted. Otherwise we fall back to the base \c RobotPart + implementation. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 8 + + To follow up on image support, we must also implement + \l{QGraphicsItem::dropEvent()}{dropEvent()}. We check if the drag object + contains image data, and if it does, we store this data as a member pixmap + and call \l{QGraphicsItem::update()}{update()}. This pixmap is used inside + the \l{QGraphicsItem::paint()}{paint()} implementation that we reviewed + before. + + \c RobotTorso and \c RobotLimb are similar to \c RobotHead, so let's + skip directly to the \c Robot class. + + \snippet examples/graphicsview/dragdroprobot/robot.h 4 + + The \c Robot class also inherits \c RobotPart, and like the other parts it + also implements \l{QGraphicsItem::boundingRect()}{boundingRect()} and + \l{QGraphicsItem::paint()}{paint()}. It provides a rather special + implementation, though: + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 9 + + Because the \c Robot class is only used as a base node for the rest of the + robot, it has no visual representation. Its + \l{QGraphicsItem::boundingRect()}{boundingRect()} implementation can + therefore return a null QRectF, and its paint() function does nothing. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 10 + + The constuctor starts by setting the flag + \l{QGraphicsItem::ItemHasNoContents}{ItemHasNoContents}, which is a minor + optimization for items that have no visual appearance. + + We then construct all the robot parts (head, torso, and upper/lower arms + and legs). The stacking order is very important, and we use the + parent-child hierarchy to ensure the elements rotate and move properly. We + construct the torso first, as this is the root element. We then construct + the head and pass the torso to \c HeadItem's constructor. This will make + the head a child of the torso; if you rotate the torso, the head will + follow. The same pattern is applied to the rest of the limbs. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 11 + + Each robot part is carefully positioned. For example, the upper left arm is + moved precisely to the top-left area of the torso, and the upper right arm + is moved to the top-right area. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 12 + + The next section creates all animation objects. This snippet shows the two + animations that operate on the head's scale and rotation. The two + QPropertyAnimation instances simply set the object, property, and + respective start and end values. + + All animations are controlled by one top-level parallel animation group. + The scale and rotation animations are added to this group. + + The rest of the animations are defined in a similar way. + + \snippet examples/graphicsview/dragdroprobot/robot.cpp 13 + + Finally we set an easing curve and duration on each animation, ensure the + toplevel animation group loops forever, and start the toplevel animation. + + \section1 ColorItem Class Definition + + The \c ColorItem class represents a circular item that can be pressed to + drag colors onto robot parts. + + \snippet examples/graphicsview/dragdroprobot/coloritem.h 0 + + This class is very simple. It does not use animations, and has no need for + properties nor signals and slots, so to save resources, it's most natural + that it inherits QGraphicsItem (as opposed to QGraphicsObject). + + It declares the mandatory \l{QGraphicsItem::boundingRect()}{boundingRect()} + and \l{QGraphicsItem::paint()}{paint()} functions, and adds + reimplementations of + \l{QGraphicsItem::mousePressEvent()}{mousePressEvent()}, + \l{QGraphicsItem::mouseMoveEvent()}{mouseMoveEvent()}, and + \l{QGraphicsItem::mouseReleaseEvent()}{mouseReleaseEvent()}. It contains a + single private color member. + + Let's take a look at its implementation. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 0 + + \c ColorItem's constructor assigns an opaque random color to its color + member by making use of qrand(). For improved usability, it assigns a + tooltip that provides a useful hint to the user, and it also sets a + suitable cursor. This ensures that the cursor will chance to + Qt::OpenHandCursor when the mouse pointer hovers over the item. + + Finally, we call + \l{QGraphicsItem::setAcceptedMouseButtons()}{setAcceptedMouseButtons()} to + ensure that this item can only process Qt::LeftButton. This simplifies the + mouse event handlers greatly, as we can always assume that only the left + mouse button is pressed and released. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 1 + + The item's bounding rect is a fixed 30x30 units centered around the item's + origin (0, 0), and adjusted by 0.5 units in all directions to allow a + scalable pen to draw its outline. For a final visual touch the bounds + also compensate with a few units down and to the right to make room + for a simple dropshadow. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 2 + + The \l{QGraphicsItem::paint()}{paint()} implementation draws an ellipse + with a 1-unit black outline, a plain color fill, and a dark gray + dropshadow. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 3 + + The \l{QGraphicsItem::mousePressEvent()}{mousePressEvent()} handler is + called when you press the mouse button inside the item's area. Our + implementation simply sets the cursor to Qt::ClosedHandCursor. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 4 + + The \l{QGraphicsItem::mouseReleaseEvent()}{mouseReleaseEvent()} handler is + called when you release the mouse button after having pressed it inside an + item's area. Our implementation sets the cursor back to Qt::OpenHandCursor. + The mouse press and release event handlers together provide useful visual + feedback to the user: when you move the mouse pointer over a \c CircleItem, + the cursor changes to an open hand. Pressing the item will show a closed + hand cursor. Releasing will restore to an open hand cursor again. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 5 + + The \l{QGraphicsItem::mouseMoveEvent()}{mouseMoveEvent()} handler is called + when you move the mouse around after pressing the mouse button inside the + \c ColorItem's area. This implementation provides the most important piece + of logic for \c CircleItem: the code that starts and manages drags. + + The implementation starts by checking if the mouse has been dragged far + enough to eliminate mouse jitter noise. We only want to start a drag if the + mouse has been dragged farther than the application start drag distance. + + Continuing, we create a QDrag object, passing the event + \l{QGraphicsSceneEvent::widget()}{widget} (i.e., the QGraphicsView + viewport) to its constructor. Qt will ensure that this object is deleted at + the right time. We also create a QMimeData instance that can contain our + color or image data, and assign this to the drag object. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 6 + + This snippet has a somewhat random outcome: once in a while, a special + image is assigned to the drag object's mime data. The pixmap is also + assiged as the drag object's pixmap. This will ensure that you can see the + image that is being dragged as a pixmap under the mouse cursor. + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 7 + + Otherwise, and this is the most common outcome, a simple color is assigned + to the drag object's mime data. We render this \c ColorItem into a new + pixmap to give the user visual feedback that the color is being "dragged". + + \snippet examples/graphicsview/dragdroprobot/coloritem.cpp 8 + + Finally we execute the drag. QDrag::exec() will reenter the event loop, and + only exit if the drag has either been dropped, or canceled. In any case we + reset the cursor to Qt::OpenHandCursor. + + \section1 The main() Function + + Now that the \c Robot and \c ColorItem classes are complete, we can put all + the pieces together inside the main() function. + + \snippet examples/graphicsview/dragdroprobot/main.cpp 0 + + We start off by constructing QApplication, and initializing the random + number generator. This ensures that the color items have different colors + every time the application starts. + + \snippet examples/graphicsview/dragdroprobot/main.cpp 1 + + We construct a fixed size scene, and create 10 \c ColorItem instances + arranged in a circle. Each item is added to the scene. + + In the center of this circle we create one \c Robot instance. The + robot is scaled and moved up a few units. It is then added to the scene. + + \snippet examples/graphicsview/dragdroprobot/main.cpp 2 + + Finally we create a QGraphicsView window, and assign the scene to it. + + For increased visual quality, we enable antialiasing. We also choose to use + bounding rectangle updates to simplify visual update handling. + The view is given a fixed sand-colored background, and a window title. + + We then show the view. The animations start immediately after + control enters the event loop. */ + |