custom database paging listbox

Author
Cossaer Filip

Date
December 01, 2015

Version
ZK 7 and above

This paragraph is under construction.

Introduction: Database paging listbox

While ZK has a lot of good components easy to use, I missed out some important one for me, and that's an easy to use database paging listbox.
You can do automatically paging but then your whole list is actually in your model.
If you wanted to do database paging you needed to set a <paging> and bind it to your viewmodel.
The viewmodel needed then methods for activePage, totalSize, pageSize and when I work with Spring this is already contained in mine Page I get from the repository.
I did some attempts on creating this with custom implementations of ListModel and the use of the autopaging of the listbox.
I failed here because the listbox inner paging doesn't make a difference between totalSize and pageSize.
If I used pageSize, I wasn't able to page to other pages.
If I used totalSize, everything worked BUT underlying the listbox generated totalsize minus pagesize empty objects.
For small queries it isn't so bad, but I had queries what have results of over 1000000 results while page was size 20.
The query was fast enough, but still the listbox takes a long time to load because it was creating the empty objects.
It's here where I decided to go for a custom listbox, where I add mine own paging above and handle it by myself.

Requirements

Now it was time to think about the requirements of the listbox.
What should it do and how to make it easy for usage.
The first thing what popped in mine head is MVVM. We all like it and let's be honest, it's ZK strongest point.
Then I was looking at the listbox and paging component.
The following attributes where important for me that I could set in the zul :

• pageSize
• model
• selectedItem
• selectedItems
• pagingPosition
• checkmark
• multiple
• detailed

Then it's up to how do I want to code it in the zul.
The first idea was to use it like this :

<paginglistbox>
      <auxhead>
        <auxheader/>
      </auxhead>
      <listhead>
        <listheader  />
        <listheader  />
      </listhead>
      <template name="model">
        <listitem>
          <listcell>
            <label value="${each.id}" />
          </listcell>
         </listitem>
      </template>
  </paginglistbox>

But then we have the problem because paginglistbox isn't a listbox but it's mine own component who has a listbox inside.
In a zul page, the paginglistbox should look like this :

  <paging/>
  <listbox/>
  <paging/>

Luckily there are templates in ZK. With the templates I could retrieve it and inject it in the listbox.
The usage of the paginglistbox would be changing to this :

<paginglistbox>
    <template>
      <auxhead>
       ...
      </template>
    </template>
  </paginglistbox>

Now we have 2 possible solutions what we could do :

• Define a constant name to the template like you use <template name="model"> for your model.
• Don't define a name but add the name of the template in the attribute of paginglistbox.

As I'm pretty lazy, I don't want to add the name of the template in the attributes.
But let's think ahead, if I add the name in attribute I can even use @load for that.
This means I can change the template name in runtime, witch means that if I create multiple templates I can easily switch between them.
This behavior does attract me more then mine laziness of not writing an extra attribute.

Creating the PagingListbox class

Here I am writing mine PagingListbox and everything is going great.
At that point I realize I'm forgetting 1 big thing namely sorting.
If we use sort="auto(property)" in the listheader we don't get database sorting but we sort only the page.
After some investigation of the Listheader class, I saw that if we use sort="client(property)" the sorting is enabled on the columns.
The next problem was getting that value back from the listheader. There is a setter but no getter for it.
At this point I'm left with one very dangerous solution called reflection.
I don't like to use reflection because if ZK decide to change the variable name the effect would be in best case that sorting don't work anymore, in worst case we are getting errors.