Reference for the Mosaic class Core::View


The class Core::View provides the base functionality for all visual components. This class implements a set of methods and properties to manage the view, to redraw its content, to receive and handle any user input events and to arrange it on the screen.

The class Core::View doesn't define any particular behavior nor appearance. It exists as an empty base for all derived view classes like the Views::Text or Views::Image.

Views never appear alone. Within a GUI application they are composed to complex GUI components, like the push button or the slider. This composition is done by the class Core::Group. In this manner each view belongs to a superior group and can be managed by this group. For example, showing the group makes visible all enclosed views automatically.

method rect AdjustDrawingArea
arg rect aArea

The method AdjustDrawingArea() is invoked before the update (drawing) of a screen area has begun. The view can thereupon evaluate the affected area and eventually enlarge it. AdjustDrawingArea() is invoked automatically by the framework, you will never need to invoke this method directly.

Enlarging the update area is necessary if parts of the view surrounding this area are affected by the content within this area. For example, the contents are blurred. As consequence the surrounding area has to be taken in account during the screen update too.

The original area is passed in the parameter aArea and it is expressed in coordinates relative to the top-left corner of the view's Owner. If necessary, the view can enlarge this area and return it in this method. If the view doesn't want to enlarge the area (what is the most usual case), it returns aArea unchanged.

method point ArrangeView
arg rect aBounds,
arg Core::Formation aFormation

The method ArrangeView() is invoked automatically if the superior Owner group needs to re-arrange its views. For example, the changing of the owners size can cause the enclosed views to adapt their position and size, so all views still fit within the new owners area. This method provides the core functionality for the automatic GUI layout mechanism.

The arrangement is controlled primarily by the Layout property of the view. It specifies a set of alignment constraints and determines whether the view can change its size. The bounds area where the view should be arranged, is passed in the parameter aBounds. This is usually the current area of the views owner.

The parameter aFormation defines the desired arrangement mode. Depending on the value of this parameter, the views can be arranged in rows or columns. If aFormation == Core::Formation.None, no automatic row/column arrangement is performed and the view is moved and scaled only to fit inside the aBounds area.

ArrangeView() is invoked automatically by the framework, so you never should need to invoke it directly.

The method returns the size of the view after it has been arranged.

method void ChangeViewState
arg Core::ViewState aSetState,
arg Core::ViewState aClearState

The method ChangeViewState() modifies the current state of the view. The state is a set of switches determining whether a view is visible, whether it can react to user inputs or whether it is just performing some update operations, etc.

The modification is controlled by the both parameters. The first aSetState contains the switches to activate within the view state. The second aClearState determines all switches to disable.

Depending on the state alteration the method will perform different operations, e.g. in response to the clearing of the visible state, the method will request a screen redraw to make disappear the view from the screen.

ChangeViewState() is invoked automatically by the framework, so you never should need to invoke it directly. All relevant states are available as properties e.g. the property Visible in derived classes reflects the visible state of the view.

method Core::CursorHit CursorHitTest
arg rect aArea,
arg int32 aFinger,
arg int32 aStrikeCount,
arg Core::View aDedicatedView,
arg Core::View aStartView,
arg Core::RetargetReason aRetargetReason

The method CursorHitTest() is invoked automatically in order to determine whether the view is interested in the receipt of cursor events or not. This method will be invoked immediately after the user has tapped the visible area of the view. If the view is not interested in the cursor event, the framework repeats this procedure for the next behind view until a willing view has been found or all views are evaluated.

In the implementation of the method the view can evaluate the passed aArea parameter. It determines the place where the user has tapped the view with his fingertip expressed in the coordinates of the views Owner. The method can test e.g. whether the tapped area does intersect any touchable areas within the view, etc. The affected finger is identified in the parameter aFinger. The first finger (or the first mouse device button) has the number 0.

The parameter aStrikeCount determines how many times the same area has been tapped in series. This is useful to detect series of multiple touches, e.g. in case of the double click, aStrikeCount == 2.

The parameter aDedicatedView, if it is not 'null', restricts the event to be handled by this view only. If aDedicatedView == null, no special restriction exists.

The parameter aStartView, if it is not 'null', restricts the event to be handled by the specified view or another view lying behind it. In other words, views found in front of aStartView are not taken in account during the hit-test operation.

This method is also invoked if during an existing grab cycle the current target view has decided to resign and deflect the cursor events to another view. This is usually the case after the user has performed a gesture the current target view is not interested to process. Thereupon, the system looks for another view willing to take over the cursor event processing after the performed gesture. Which gesture has caused the operation, is specified in the parameter aRetargetReason.

If the view is willing to process the event, the method should create, initialize and return a new Core::CursorHit object. This object identify the willing view. Otherwise the method should return 'null'.

CursorHitTest() is invoked automatically by the framework, so you never should need to invoke it directly. This method is predetermined for the hit-test only. The proper processing of events should take place in the HandleEvent() method by reacting to Core::CursorEvent and Core::DragEvent events.

method void Draw
arg Graphics::Canvas aCanvas,
arg rect aClip,
arg point aOffset,
arg int32 aOpacity,
arg bool aBlend

The method Draw() is invoked automatically if parts of the view should be redrawn on the screen. This can occur when e.g. the view has been moved or the appearance of the view has changed before.

Draw() is invoked automatically by the framework, you will never need to invoke this method directly. However you can request an invocation of this method by calling the method InvalidateArea() of the views Owner. Usually this is also unnecessary unless you are developing your own view.

The passed parameters determine the drawing destination aCanvas and the area to redraw aClip in the coordinate space of the canvas. The parameter aOffset contains the displacement between the origin of the views owner and the origin of the canvas. You will need it to convert views coordinates into these of the canvas.

The parameter aOpacity contains the opacity descended from this view's Owner. It lies in range 0 .. 255. If the view implements its own 'Opacity', 'Color', etc. properties, the Draw() method should calculate the resulting real opacity by mixing the values of these properties with the one passed in aOpacity parameter.

The parameter aBlend contains the blending mode descended from this view's Owner. It determines, whether the view should be drawn with alpha-blending active or not. If aBlend is false, the outputs of the view should overwrite the corresponding pixel in the drawing destination aCanvas. If aBlend is true, the outputs should be mixed with the pixel already stored in aCanvas. For this purpose all Graphics Engine functions provide a parameter to specify the mode for the respective drawing operation. If the view implements its own 'Blend' property, the Draw() method should calculate the resulting real blend mode by using logical AND operation of the value of the property and the one passed in aBlend parameter.

method rect GetClipping();

The method GetClipping() returns the clipping area of the view as rectangle relative to the origin of the view's Owner. With the returned clipping area the view limits its own visibility. Contents belonging to the view and lying outside this area will not be displayed - they are clipped during the screen update process.

Per default the clipping area corresponds to the boundary area of the view (see also GetExtent()). Derived views can override GetClipping() and enlarge the area so the view can display additional contents (e.g. shadows or decoration) outside of its original boundary area.

method rect GetExtent();

The method GetExtent() returns the position and the size of the view relative to the origin of its Owner. In case of views with a non rectangular shape the method returns the rectangular boundary area enclosing the entire shape.

method Core::Root GetRoot();

The method GetRoot() delivers the application object, this view belongs to. The application object represents the entire screen of the GUI application. Thus in the views hierarchy, the application object serves as the root view.

This method can fail and return null if the view still doesn't belong to any owner group.

method object HandleEvent
arg Core::Event aEvent

The method HandleEvent() is invoked automatically if the view has received an event. For example, touching the view on the touch screen can cause the view to receive a Core::CursorEvent event. Within this method the view can evaluate the event and react to it.

Whether the event has been handled by the view or not is determined by the return value. To sign an event as handled HandleEvent() should return a valid object (e.g. 'this'). If the event has not been handled, 'null' should be returned.

Depending on the kind of the event, the framework can continue dispatching of still unhandled events. For example, keyboard events (Core::KeyEvent class) are automatically delivered to the superior Owner of the receiver view if this view has ignored the event.

HandleEvent() is invoked automatically by the framework, so you never should need to invoke it directly. However you can prepare and post new events by using the methods DispatchEvent() and BroadcastEvent() of the application class Core::Root.

property Core::Layout Layout = Core::Layout[AlignToLeft, AlignToTop];

The property 'Layout' controls the automatic arrangement of the view. It specifies a set of alignment constraints and determines whether the view can change its size. It is used primarily to re-arrange the view after its Owner has been resized. It also controls the arrangement of the view if it has been embedded within a Core::Outline.

The constraints AlignToLeft, AlignToRight, AlignToTop and AlignToBottom determine whether the view is left, right, top or bottom aligned. If active, the view will try to remain a constant distance to the corresponding edge of its owner. In contrast all not explicitly aligned edges are moved proportionally to the size modification of the views owner.

The constraints ResizeVert and ResizeHorz determine the resize behavior of the view in response to the size modification of its owner. Views with these constraints disabled always will keep their size unchanged. In conflict cases when the alignment and resize constraints do contradict, the view will maintain its size and will be pulled into the middle area resulting by the alignment constraints.

In case of views automatically arranged in rows or columns within a Core::Outline, the alignment constraints determine the vertical or horizontal alignment of the view within the corresponding row or column. The resize constraints in this case enable the view to fill the entire height of a column or the entire width of a row.

method void MoveView
arg point aOffset,
arg bool aFastMove

The method MoveView() changes the position of the view by adding the value aOffset to all corners of the view. For example, in case of a line view the value is added to the both line end points.

The parameter aFastMove serves for the optimization purpose. If it is 'true', the corners are modified without performing any updates of the view and without redrawing the screen. This is useful, when you wish to move or arrange a lot of views at once. In this case it's up to you to request the finally screen update. To do this you can use the method InvalidateArea() of the views Owner.

In the case aFastMove == false, the operation automatically requests the screen redraw of the view areas before and after the movement. You don't need to take care about it.

var Core::Group Owner = null;

The variable 'Owner' refers to the superior component, where this view is enclosed. This is the unique owner of the view. It determines the visibility of the view, controls the order in which one view does overlap other views, etc.

property int32 StackingPriority = 0;

The property 'StackingPriority' determines the constraints how to arrange views one above the other. Concrete, views having higher StackingPriority value will automatically appear in front of views with lower StackingPriority value. As such, this property is very convenient if you want some views to always appear in front of other views (so-called top-most views). Just configure the top-most views with 'StackingPriority' value greater than the value of other views.

This property can be configured with positive and negative values. The lower the value, the lower the priority. The value 0 (zero) determines the default priority.