Nineth Chapter-delphi Drag-and-drop programming (2)

9.1.3 drag-and-drop method

There are three drag-and-drop methods:

BeginDrag: Start a drag in manual way

EndDrag: End a drag

Dragging: Determine if a control is being dragged

These three methods are used by the source control.

When DragMode is set to dmmanual, drag must call the control's BeginDrag method to start. BeginDrag has a Boolean parameter immediate. If the input parameter is true, drag starts immediately, and the cursor changes to the Dragcursor setting. If the input parameter is false until the user moves the cursor to a certain distance (5 pixel points) before changing the cursor, start dragging. This allows the control to accept an OnClick event without starting the drag operation.

The EndDrag method aborts the dragged state of an object. It has a Boolean parameter drop. If the drop is set to True, the dragged object is dropped at the current position (whether it is accepted by the target control), and if the drop is set to False, the drag is canceled in place.

The following procedure indicates that dragging is canceled when dragged into a control panel.

Procedure Tform1.panel1dragover (Sender, Source:tobject; X, Y:integer;

State:tdragstate; var accept:boolean);

Begin

Accept: = False;

if (Source is Tlabel) and (state = Dsdragenter) Then

(Source as Tlabel). EndDrag (False);

End

The Draging method determines whether a control is being dragged. In the following example, the window changes to a different color when the user drags a different check box.

Procedure Tform1.formactivate (Sender:tobject);

Begin

Checkbox1.dragmode: = dmautomatic;

Checkbox2.dragmode: = dmautomatic;

Checkbox3.dragmode: = dmautomatic;

End

Procedure Tform1.formdragover (Sender, Source:tobject; X, Y:integer;

State:tdragstate; var accept:boolean);

Begin

If Checkbox1.dragging Then

Color: = Claqua;

If Checkbox2.dragging Then

Color: = Clyellow;

If Checkbox3.dragging Then

Color: = Cllime;

End

9.2 General steps for developing drag-and-drop functionality

Drag-and-drop as a convenient operation of Windows provides a function of the object, in Delphi can be easily developed. Depending on the process of the drag-and-drop operation, the development steps can be divided into four phases, namely:

Start the drag operation

Receive drag items

Drop Drag Item

Terminate drag operation

During the introduction we will combine a drag-and-drop operation instance of a Tabset (tab set). The interface design is as shown. At run time, when a user drags a label to the position of another label, the label moves to that location and causes the label set to be reset.

9.2.1 Start the drag operation

When drag mode (DragMode) is set to Dmautomatic, the user drags automatically when the mouse is pressed on the source control and determines whether the drag starts when set to dmmanual by handling mouse events. If you want to start dragging call the BeginDrag method.

In Tabset drag-and-drop, we use the following MouseDown event handler to start a tag drag. First, the key is to determine whether the left button, and then to determine whether the project is legal.

Procedure Tform1.tabset1mousedown (Sender:tobject; Button:tmousebutton;

Shift:tshiftstate; X, Y:integer);

Var

Dragitem:integer;

Begin

If Button = Mbleft Then

Begin

Dragitem: = Tabset1.itematpos (Point (X, Y));

if (Dragitem >-1) and (Dragitem < TabSet1.Tabs.Count) Then

Tabset1.begindrag (False);

End

End