Hi,
your first solution was inefficient anyways, because every click
reloaded all Datas first, before acting on a single one only.
You should use a detachable model instead, see JpaLoadableModel here
https://ci.apache.org/projects/wicket/guide/8.x/single.html#_detachable_models
Have fun
Sven
Am 27.08.2018 um 15:00 schrieb Tobias Gierke:
Hi,
A collegue of mine just came across a rather interesting bug in our
Wicket application.
1. We have a simple page with a repeater (ListView) that displays a
table and on each row, some buttons to perform actions on the item
shown on this row (edit/delete/etc.)
2. The underlying data source (a database table) gets updated
concurrently by another process running on another machine
3. The table is supposed to always show the latest data at the very
top, so the page uses a LoadableDetachableModel to always hit the
database on every request
The bug:
Users complained that performing actions on the data items would
actually affect an item different from what they clicked.
The explanation:
Since the list model got detached at the end of the previous request,
clicking any of the action buttons would re-populate the data model,
fetching previously unseen rows from the database. Since (according to
my collegue,didn't double-check) the ListView associates the item
models only based on their list index, the action button on the very
first row now all of a sudden referred to a database row the user
didn't even know about.
His fix:
Instead of
view = new ListView<Data>("listView" , dataProvider )
{
@Override
protected void populateItem(ListItem<PCAPFile> item)
{
add(new AjaxButton( "delete , item.getModel() ) // use model
from ListItem (model gets detached after request)
{
public void onClick(AjaxRequestTarget target) {
delete( getModelObject() );
}
});
// ... more stuff
}
}
he changed the code to read:
view = new ListView<Data>("listView" , dataProvider )
{
@Override
protected void populateItem(ListItem<PCAPFile> item)
{
add(new AjaxButton( "delete , item.getModelObject() ) //
capture model object when constructing the button
{
public void onClick(AjaxRequestTarget target) {
delete( getModelObject() );
}
});
// ... more stuff
}
}
This obviously is a rather subtle issue and - depending on the size of
your model objects - also comes with a certain performance/memory cost
because of the additional serialization for the model items the
repeater components are now holding onto.
Is this the recommended approach for dealing with dynamically changing
data or is there a better way to do it ?
Thanks,
Tobias
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