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ZK allows a user to drag particular components around the user interface. For example, dragging an image representing a file onto a tree represents a directory, or dragging a listitem representing a product onto a listbox represents a shopping cart.

A component is droppable, if a user could drop a draggable component to it.

Notice that ZK does not assume any behavior about what should take place after dropping. It is up to application developers to implement the onDrop event listener.

If an application doesn't do anything, the dragged component is simply moved back where it is originated from.

The draggable and droppable Properties

With ZK, you could make a component draggable by assigning any value, other than "false", to the draggable property by the use of HtmlBasedComponent.setDraggable(String). To disable it, assign it with "false".

<image draggable="true"/>

Similarly, you could make a component droppable by assigning "true" to the droppable property by the use of HtmlBasedComponent.setDroppable(String).

<hbox droppable="true"/>

Then, the user could drag a draggable component, and then drop it to a droppable component.

Since the draggable and droppable properties are implemented in HtmlBasedComponent, almost all the components can become draggable or droppable.

The onDrop Event

Once a user has dragged a component and dropped it to another component, the component that the user dropped the component to will be notified by the onDrop event. The onDrop event is an instance of DropEvent. By calling DropEvent.getDragged(), you could retrieve what has been dragged (and dropped).

Notice that the target of the onDrop event is the droppable component, not the component being dragged.

The following is a simple example that allows users to reorder list items by drag-and-drop.

<window title="Reorder by Drag-and-Drop" border="normal">
    Unique Visitors of ZK:
    <listbox id="src" multiple="true" width="300px">
            <listheader label="Country/Area"/>
            <listheader align="right" label="Visits"/>
            <listheader align="right" label="%"/>
        <listitem draggable="true" droppable="true" onDrop="move(event.dragged)">
            <listcell label="United States"/>
            <listcell label="5,093"/>
            <listcell label="19.39%"/>
        <listitem draggable="true" droppable="true" onDrop="move(event.dragged)">
            <listcell label="China"/>
            <listcell label="4,274"/>
            <listcell label="16.27%"/>
        <listitem draggable="true" droppable="true" onDrop="move(event.dragged)">
            <listcell label="France"/>
            <listcell label="1,892"/>
            <listcell label="7.20%"/>
        <listitem draggable="true" droppable="true" onDrop="move(event.dragged)">
            <listcell label="Germany"/>
            <listcell label="1,846"/>
            <listcell label="7.03%"/>
        <listitem draggable="true" droppable="true" onDrop="move(event.dragged)">
            <listcell label="(other)"/>
            <listcell label="13,162"/>
            <listcell label="50.01%"/>
            <listfooter label="Total 132"/>
            <listfooter label="26,267"/>
            <listfooter label="100.00%"/>
		void move(Component dragged) {
			self.parent.insertBefore(dragged, self);

Dragging with Multiple Selections

When a user drag-and-drops a list item or a tree item, the selection status of these items won't be changed. Usually only the dragged item is moved, but you can handle all the selected items at once by looking up the set of all selected items as depicted below.

public void onDrop(DropEvent evt) {
	Set selected = ((Listitem)evt.getDragged()).getListbox().getSelectedItems();
	//then, you can handle the whole set at once

Notice that the dragged item may not be selected. Thus, you may prefer to change the selection to the dragged item for this case, as shown below.

Listitem li = (Listitem)evt.getDragged();
	if (li.isSelected()) {
		Set selected = ((Listitem)evt.getDragged()).getListbox().getSelectedItems();
		//then, you can handle the whole set at once
	} else {
		//handle li only

Multiple Types of Draggable Components

It is common that a droppable component doesn't accept all draggable components. For example, an e-mail folder accepts only e-mails and it rejects contacts or others. You could silently ignore non-acceptable components or alert an message, when onDrop is invoked.

To have better visual effect, you could identify each type of draggable components with an identifier, and then assign the identifier to the draggable property.

<listitem draggable="email"/>
<listitem draggable="contact"/>

Then, you could specify a list of identifiers to the droppable property to limit what can be dropped. For example, the following image accepts only email and contact.

<image src="/img/send.png" droppable="email, contact" onDrop="send(event.dragged)"/>

To accept any kind of draggable components, you could specify "true" to the droppable property. For example, the following image accepts any kind of draggable components.

<image src="/img/trash.png" droppable="true" onDrop="remove(event.dragged)"/>

On the other hand, if the draggable property is "true", it means the component belongs to anonymous type. Furthermore, only components with the droppable property assigned to "true" could accept it.

Drag-and-Drop Effect Customization

The effects of drag-and-drop can be customized. It requires some client-side programming. Please refer to ZK Client-side Reference/Customization/Drag-and-Drop Effects for more information.

Version History

Last Update : 2011/7/25

Version Date Content

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.

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