Computations

A computation is used to set the value of a page item either during page rendering or during page processing.  There is also the ability to create Application Computations to set page item values when a new session is created during login.  When creating computations, properties are set to define what page item(s) will be set, when the computation will be executed, and how it will be done.

• Computation Location

• Computation Point

• Computation Type

• Conditions

Computation Locations

Computation locations identify where the page item is located that will be set by the computation.

Item on This Page:   This type of computation is used to set the value of a single item on the current application page.

Item on Another Page:   This type of computation is useful when navigating to another page and wanting to set page items on that page.  Using this method, the values can be hidden by not having to send them in the URL.  The limitation of using this method is the page will not provide a URL for the user to save in the browser as a Favorite. A common example would be saving a sale item on the web site.

Application Level Item:  This type of computation is used to set the value of an item defined at the Application Level.  Application level items act as global items.  A benefit of using an application level item is when the page cache is cleared, the application level items are not cleared.  A disadvantage of using application level items as place holders for values is another page may overwrite the value.

Computation Points

A Computation Point defines when the logic control will be considered in the order of execution.  This happens for both page rendering and page processing.

Application Level Computation Points

On New Instance is an application level computation and should be created as such.  It can be a little confusing since this computation point is available when creating a computation at the page level.  However, do not use the On New Instance display point on application pages.  These should be defined as an Application Computation in Shared Components.  When an On New Instance computation is created at the application level, it will be executed during login when the session is created in HTML DB.  A good use of this type of computation would be to read a cookie from the user’s browser to set some application level items.

Page Rendering Computations

The computation points covered in this section are executed during page rendering.  This may be confusing when first starting to build web applications using HTML DB, but this chapter will show how they are used and reveal their inherent value.

•  Before Header

• After Header

• Before Region(s)

• After Region(s)

• Before Footer

• After Footer

The explanation of these execution points will be delayed until the section on Computation and Processing Points in Detail is covered in this chapter.  There, a thorough explanation of the execution points during page rendering will be presented.

Page Processing Computations

The final computation point is After Submit.  An After Submit Computation Point is considered for execution during Page Processing.

An After Submit computation would be used to calculate a person’s age based on a birth date page item, to calculate the number of days between two dates, or to set a page item with the total amount from a list of numbers.  As mentioned earlier, it could also be used to set values on another page so as to not pass the values in the URL.