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Render

When a listbox (or grid ) uses a live data and is not in paging mold, it does not load all items at first, it pre-loads first few items. Until a user scrolls the scroll bar down, it loads and renders subsequent items. If we retrieve those un-rendered listitems in a test case, we will find its listcell containing no child components. This is one of ZK's optimizing behavior; to avoid loading large amount of data which might have not been viewed by users.

Hence, before we retrieve components inside a listcell, we should use Render operation to force listbox to load all of its content.

We use a simple case to demonstrate how to use this agent. This application contains a lisbox with 1000 listitems and has a simple logic: when a user selects an item, the label at the bottom will display the item's content. Each listcell has one child component label.

Smalltalk-mimic-render.png

RenderTest.java


	@Test
	public void testRendererAgent() {
		DesktopAgent desktop = Zats.newClient().connect("/render.zul");

		ComponentAgent listbox = desktop.query("#listbox");
		Label itemData = desktop.query("#itemData").as(Label.class);

		//selecting first item works correctly 
		listbox.getChild(1).as(SelectAgent.class).select();
		Assert.assertEquals("item0", itemData.getValue());
		
		//select a non-rendered item
		listbox.getChild(1000).as(SelectAgent.class).select();
		Assert.assertEquals("", itemData.getValue());
		//get a non-rendered listcell, check it has no child.
		Listcell listcell = listbox.getChild(1000).getChild(0).as(Listcell.class);
		Assert.assertTrue(listcell.getChildren().size()==0);

		listbox.as(RenderAgent.class).render(999, 999);

		listbox.getChild(1000).as(SelectAgent.class).select();
		Assert.assertTrue("item999".equals(itemData.getValue()));
		listcell = listbox.getChild(1000).getChild(0).as(Listcell.class);
		Assert.assertTrue(listcell.getChildren().size()>0);
	}
  • Selecting the first item, item data label displays correctly. (line 9,10)
  • Before using RenderAgent to render, label displays nothing when selecting the last item. (line 13,14)
  • We can also find that listcell has no children. (line 16,17)
  • render() could accept a range of items as parameters, and the index starts from zero. The last index of 1000 listitem is 999 but the last listitem's index of the listbox is 1000, index zero is listhead. (line 19)
  • After rendering the last item, selecting it now works properly as expected while listcell now contains child components. Remember that you should retrieve the listcell again after rendering because original listitem component is already detached and replaced with a new listitem. (line 21-24)


Supported Components

Components
Version
Note
Grid, Listbox 5, 6








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