Tersus: Demo Stage 9 – Paging: Allowing for paging in the list

Demo Stage 9 – Paging: Allowing for paging in the listDate: Jul 25, 2012Enabling PagingThe number of items in each category can become rather big (think of the list of drinks before a big party).In the desktop view, displaying a couple of dozens of items and scrolling among them may be acceptable, but on the iPhone, this looks rather ugly.An alternative to scrolling within a long page is to enable paging (version 1.3.28):

In '<Desktop View>', right click on

'Items' and select Apply Table Wizard.

Clear the 'Filtering' column by choosing the blank option for all fields. This will discard the filtering rows, to save space and enable the display of more item lines in each page.

Press the Advanced>> button.

Set Number of pages to display to 6.

Set Number of records per page to 4.

Press 'Finish'.

Save the model and start the application. If you now add a few more items, you will be able to page in the item list by clicking the page numbers and the paging symbols at the bottom of the page.Paging works well with sorting - try sorting by category and paging along the list of items:

Publish-Subscribe ImprovementsAt the end of stage 4 we noted that the updater experiences two refreshes of the table upon any update – one due to the built-in refresh process in each of the buttons ('New', 'Update' or 'Delete'), and one due to the subscription to updates. We said that to overcome this problem, users should ignore updates made by themselves.Now that we have paging, we may want to improve the subscription refresh even further, and for other users (not the updater) avoid refresh altogether if the relevant item is not included in the currently displayed page.Ignore Your Own UpdatesWe'll use the 'Source' field of the publish-subscribe message to identify the publisher of each change. The publisher will "sign" any change message it publishes, and the subscribers will ignore any message signed by themselves.Here is a possible way to model this behavior:

Create an identifier for your application session:

In 'Items', add a Text data item 'App Session Id'.
Zoom into 'Init' and add a process 'Init App Session Id' (to model it, drag from the palette the templates 'Sequence Number' and 'Convert Number to Text'):

Note 1: If you don't know in which section of the palette to find a template you need, press the 'Search' icon (the located at the top of the palette) and type the template's name (or a prefix of its name).

Note 2: 'App Session Id' is initialized with the view, and won't change until a refresh is performed in the browser. Therefore it is a good way to identify your application session.

Sign any change message you publish with your identifier:

Zoom into the 'Publish Change' process (from stage 4).
Right click, select Add Ancestor Reference, and select 'Items'.
Add a flow from the 'App Session Id' element of 'Items' to the 'Source' element of 'Message' (if the 'Message' data structure does not contain a 'Source' element, upgrade to version 1.3.29):

When a message arrives, ignore it if it is signed by your own identifier:

Return to the 'Init' of 'Items' and zoom into 'Subscribe to Shopping List Changes'. Further zoom into 'Subscribe' and then into '<On Message>'.
Right click, select Add Ancestor Reference, and select 'Items'.
Compare the 'App Session Id' element of 'Items' with the 'Source' element of 'Message'. The 'Refresh Current Table Display' process should be activated only if the source is different from our own identifier:

Refresh Only Relevant Pages
This is beyond the scope of this tutorial. If you want to implement the improvement, you may choose one of two approaches:

Check whether the updated item is one of the items currently displayed. This can be done by comparing the published item (identified by the 'Content' element of 'Message') with the 'Product' element of each displayed item.

Improve the refresh mechanism to get an additional optional parameter - the primary key of a database record. If this parameter is provided, the display is refreshed only if a row with this primary key is currently displayed.

Download Model: Stage 9 (version 1.3.29) See Also:

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