This article is about the unload UDB event handler.
unload
The unload event may be called from web pages developed in Web Designer and processed by a USoft page engine service.
| Event | Applies to | Occurs when |
|---|---|---|
| Page objects | The page is "swapped" for a different page |
This event is similar, but not identical, to the HTML onunload event.
The HTML onunload event :
- Is usually attached to the <body> DOM element.
- Occurs when the browser is about to unload the current page because it is instructed to navigate to a different page.
- Occurs after the HTML beforeunload event occurs. This beforeunload event may be used to prevent the browser from leaving the page.
The USoft unload event is usually attached to the top-level Page object.
The USoft unload event is different from HTML onunload, because in a USoft context you usually 'swap' displayed content dynamically, rather than to leave the current page completely. To explain when it occurs, we need a closer look at what happens during this 'swap'.
When opening a new page (stack) via menu navigation:
- The existing controls are unrendered and any resources they use are cleaned up;
- The existing data sources are destroyed;
- The new page controls are loaded and displayed following the procedure described for the USoft load event.
When opening a lookup or related page:
- The existing controls are hidden;
- The current data sources are "archived";
- The page stack index is increased;
- The new page is loaded following the procedure described for the USoft load event.
When a lookup or related page is closed and the application navigates back to the original page (.closePage() behaviour):
- The existing controls are unrendered;
- Information from current data sources that is needed on the original page is sent back, and then the current data sources are destroyed;
- The original page is re-displayed, previous data sources are "unarchived", and updated with info from the lookup or related page.
In these scenarios, the USoft unload event occurs when existing controls are unrendered, or in the case of a lookup or related page being opened, directly before the previous page content is hidden.
How to use
Find or create an Event Listener object with Event Type = unload. Event Listeners are in the Web Designer Controls catalog:
Insert the event listener into the Page object. Insert a callClientScript action into the event listener. Use this action's Script property to code the behaviour that you want to see when the event occurs.
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