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The default implementation of models, such as ListModelList and DefaultTreeModel assumes all data are available in the memory. It is not practical if a model has a lot of data. For huge amount of data, it is suggested to implement your own model by loading and caching only one portion of data at a time.

To implement your own model for a component it is recommended that you extend the correct abstract model type. For a ListModel extend from AbstractListModel, for a GroupsModel extend AbstractGroupsModel and for a TreeModel extend AbstractTreeModel as described in the Model section. To implement a model that supports sorting, you have to implement Sortable too. Each time an user requires sorting, Sortable.sort(Comparator, boolean) will be called and the implementation usually clears the cache and re-generate the SQL statement accordingly.

Here is some pseudo for a custom ListModel:

public class FooListModel extends AbstractListModel implements Sortable {
    private int _size = -1;
    private Object[] _cache;
    private int _beginOffset;
    private String _orderBy;
    private boolean _ascending, _descending;
    private Comparator _sorting;
 
    public int getSize() {
        if (_size < 0)
            _size = /**SELECT COUNT(*) FROM ...*/
        return _size;
    }
    public Object getElementAt(int index) {
        if (_cache == null || index < _beginOffset || index >= _beginOffset + _cache.length) {
           loadToCache(index, 100); //SELECT ... FROM .... OFFSET index LIMIT 100
                //if _ascending, ORDER BY _orderBy ASC
                //if _descending, ORDER BY _orderBy DSC
        }
        return _cache[index - _beginOffset];
    }
    @Override
    public void sort(Comparator cmpr, boolean ascending) {
        _cache = null; //purge cache
        _size = -1; //so size will be reloaded
        _descending = !(_ascending = ascending);
        _orderBy = ((FieldComparator)cmpr).getRawOrderBy();
        _sorting = cmpr;
             //Here we assume sort="auto(fieldName)" is specified in ZUML, so cmpr is FieldComparator
             //On other hand, if you specifies your own comparator, such as sortAscending="${mycmpr}",
             //then, cmpr will be the comparator you assigned
        fireEvent(ListDataEvent.CONTENTS_CHANGED, -1, -1);
    }
    @Override
    public String getSortDirection(Comparator cmpr) {
        if (Objects.equals(_sorting, cmpr))
            return _ascending ?  "ascending" : "descending";
        return "natural";	
    }
}

The implementation of Sortable.sort(Comparator, boolean) generally has to purge the cache, store the sorting direction and field, and then fire ListDataEvent.CONTENTS_CHANGED to reload the content.

The field to sort against has to be retrieved from the given comparator. If you specify "auto(fieldName)" to Listheader.setSort(String), then the comparator is an instance of FieldComparator, and you could retrieve the field's name from FieldComparator.getRawOrderBy().

If you'd like to use your own comparator, you have to carry the information in it and then retrieve it back when Sortable.sort(Comparator, boolean) is called.

Also notice that we cache the size to improve the performance, since ListModel.getSize() might be called multiple times.

Here is some pseudo for a custom TreeModel which renders an Apache Commons VFS FileObject to be able to browse a filesystem:

public class VfsTreeModel extends AbstractTreeModel<FileObject> {
	public VfsTreeModel(FileObject root){
		super(root);
	}
	
	@Override
	public FileObject getChild(FileObject parent, int index) {
		FileObject child = null;
		try {
			FileObject[] children = parent.getChildren();
			child = children[index];
		} catch (FileSystemException e) {
			throw new IllegalArgumentException(e);
		}
		return child;
	}

	@Override
	public int getChildCount(FileObject node) {
		int childCount = 0;
		try {
			FileType type = node.getType();
			if( type == FileType.FOLDER ){
				childCount = node.getChildren().length;
			}
		} catch (FileSystemException e) {
			throw new IllegalArgumentException(e);
		}
		return childCount;
	}

	@Override
	public boolean isLeaf(FileObject node) {
		boolean isLeaf = false;
		try {
			FileType type = node.getType();
			isLeaf = (type == FileType.FILE );
		} catch (FileSystemException e) {
			throw new IllegalArgumentException(e);
		}
		return isLeaf;
	}
	/*
	 * Return the sibling index of each node in walk down from the root. 
	 */
	@Override
	public int[] getPath(FileObject node) {
		List<Integer> paths = new ArrayList<Integer>();
		try {
			// walk upwards to root getting sibling index of each child in each parent
			FileObject parent = node.getParent(); 
			while (parent != null && parent.getType().equals(FileType.FOLDER)) {
				FileObject[] children = parent.getChildren();
				for( int index = 0; index < children.length; index++){
					FileObject c = children[index];
					if( node.equals(c)){
						paths.add(index);
						break;
					}
				}
				node = parent;
				parent = node.getParent();
			}
		} catch (FileSystemException e) {
			throw new IllegalArgumentException(e);
		}
		int[] p = new int[paths.size()];
		for( int index = 0; index < paths.size(); index++){
			p[index] = paths.get(p.length - 1 - index); // reverse
		}
		return p;
	}
}

When many treerows are open and further rows are expanded the re-rendering of the tree may visit many of the open rows. It is therefore recommended that you cache the results of any expensive calls where possible with a suitable eviction strategy.

For a real example, please refer to Small Talk: Handling sortable huge data using ZK and/or Small Talk: Handling huge data using ZK.

Version History

Last Update : 2012/12/28


Version Date Content
6.0.0 02/03/2012 Sortable interface



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



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