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Listbox, Grid, and Tabbox allow developers to separate the view and the model by implementing ListModel. Once the model is assigned (with Listbox.setModel(ListModel)), the display of the listbox is controlled by the model, and an optional renderer. The model is used to provide data, while the renderer is used to provide the custom look. By default, the data is shown as a single-column grid/listbox. If it is not what you want, please refer to the View section for writing a custom renderer.

Model-driven Display

DrListModelRenderer.png

As shown, the listbox retrieves elements from the specified model[1], and then invokes the renderer, if specified, to compose the listitem for the element.

The retrieval of elements is done by invoking ListModel.getSize() and ListModel.getElementAt(int).

The listbox will register itself as a data listener to the list model by invoking ListModel.addListDataListener(ListDataListener). Thus, if the list model is not mutable, the implementation has to notify all the registered data listeners. It is generally suggested to extend from AbstractListModel, or use any of the default implementations, which provide a set of utilities for handling data listeners transparently. We will talk about it later in #Notify for Data Updates.


  1. The listbox is smart enough to read the elements that are visible at the client, such the elements for the active page. It is called Live Data or Render on Demand'.'

Small Amount of Data

If your data can be represented as a list, map, set or array (java.util.List, java.util.Map, etc.), you could use one of the default implementations, such as ListModelList, ListModelMap, ListModelSet and ListModelArray. For example,

void setModel(List data) {
    listbox.setModel(new ListModelList(data));
}

If the amount of your data is small, you could load them all into a list, map, set or array. Then, you could use one of the default implementations as described above.

Alternatively, you could load all data when ListModel.getSize() is called. For example,

public class FooModel extends AbstractListModel {
    private List _data;
    public int getSize() {
        //load all data into _data
        return _data.size();
    }
    public Object getElementAt(int index) {
        return _data.get(index);
    }
}

Huge Amount of Data

If the data amount is huge, it is not a good idea to load all of them at once. Rather, you shall load only the required subset. On the other hand, it is generally not a good idea to load single elements when ListModel.getElementAt(int) is called, since the overhead loading from the database is significant.

Thus, it is suggested to use SQL LIMIT or similar feature to load only a subset of data. For example, if the total number of visible elements is about 30, you could load 30 (or more, say 60, depending on performance or memory is more important to you). If an element is not loaded, you have to discard the previous loaded data. If any. If the next invocation of ListModel.getElementAt(int) is in the subset, we could return it immediately. Here is the pseudo code:

public class FooModel extends AbstractListModel {
    public List _subset;
    public int _startAt;

    public Object getElementAt(int index) {
        if (index >= _startAt && _subset != null && index - _startAt < _subset.size())
            return _subset.get(index - _startAt); //cache hit
        _subset = new LinkedList(); //drop _subset, and load a subset of data, say, 60, to _subset
        ...

For more realist examples, please refer to Small Talks: Handling huge data using ZK.

Notify for Data Updates

If the data in the model is changed, the implementation must notify all the data listeners that are registered by ListModel.addListDataListener(ListDataListener). It can be done by invoking AbstractListModel.fireEvent(int, int, int) if your implementation is extended from AbstractListModel or derived.


Notice that if you use one of the default implementations, such as ListModelList, you don't need to worry about it. The notification is handled transparently.

For example, (pseudo code)

public void removeRange(int fromIndex, int toIndex) {
    removeElements(fromIndex, toIndex); //remove elements from fromIndex (inclusive) to toIndex (exclusive)
    fireEvent(ListDataEvent.INTERVAL_REMOVED, fromIndex, index - 1);
}
public void add(int index, Object element){
    addElements(index, element); //add an element at index
    fireEvent(ListDataEvent.INTERVAL_ADDED, index, index);
}
public void set(int index, Object element) {
    setElement(index, element); //change the element at index
    fireEvent(ListDataEvent.CONTENTS_CHANGED, index, index);
}

Once a model is assigned to a component, the component will register itself as a data listener such that any changes can be updated to UI.

Notice that you shall not update the component (such as listbox) directly. Rather, you shall update to the modal and then the model shall fire the event to notify the components to update accordingly.

Selection

Interface: Selectable
Implementation: Implemented by AbstractListModel

If your data model also provides the collection of selected elements, you shall also implement Selectable. When using with a component supporting the selection (such as Listbox), the component will invoke Selectable.isSelected(E) to display the selected elements correctly. In additions, if the end user selects or deselects an item, Selectable.addSelection(E) and Selectable.removeSelection(Object) will be called by the component to notify the model that the selection is changed. Then, you can update the selection into the persistent layer (such as database) if necessary.

On the other hand, when the model detects the selection is changed (such as Selectable.addSelection(E) is called), it has to fire the event, such as ListDataEvent.SELECTION_CHANGED to notify the component. It will cause the component to correct the selection[1].

All default implementations, including AbstractListModel, implement Selectable. Thus, your implementation generally doesn't need to handle the selection if it extends one of these classes.

It is important to note that, once a listbox is assigned with a list model, the application shall not manipulate the list items and/or change the selection of the listbox directly. Rather, the application shall access only the list model to add, remove and select data elements. Let the model notify the component what have been changed.


  1. Don't worry. The component is smart enough to prevent the dead loop, even though components invokes addSelection to notify the model while the model fire the event to notify the component.

Selection Control

since 8.0.0

With the multiple selection function in a data model, you have to implement a class for the SelectionControl to tell the data model which items are selectable and what it will perform a "select all" function with. The following implementation which extends AbstractListModel.DefaultSelectionControl is a simple example to change "selectable" items.

Please note that if your data model is much larger, you may implement on your own to get rid of the performance impact.

model.setSelectionControl(new AbstractListModel.DefaultSelectionControl(model) {
	public boolean isSelectable(Object e) {
		int i = model.indexOf(e);
		return i % 2 == 0;
	}
});

Sorting

Interface: Sortable
Implementation: You have to implement it explicitly

To support the sorting, the model must implement Sortable too. Thus, when the end user clicks the header to request the sorting, Sortable.sort(Comparator, boolean) will be called.

For example, (pseudo code)

public class FooModel extends AbstractListModel implements Sortable {
    public void sort(Comparator cmpr, final boolean ascending) {
        sortData(cmpr); //sort your data here
        fireEvent(ListDataEvent.CONTENTS_CHANGED, -1, -1); //ask component to reload all
    }
...

Notice that the ascending parameter is used only for reference and you usually don't need it, since the cmpr is already a comparator capable to sort in the order specified in the ascending parameter.

Version History

Last Update : 2016/6/17


Version Date Content
6.0.0 February 2012 All selection states are maintained in the list model. And, the application shall not access the component for the selection. Rather, the application shall invoke Selectable for retrieving or changing the selection.
6.0.0 February 2012 Sortable was introduced and replaced ListModelExt.



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



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