This event is triggered when the page is about to show. It is usually used to do some initialize work before the page is showing up.
For example, if you switch page from Page 1 to Page 2, the following events will occur successively:
|1.||"Page Unloaded" on Page 1|
|2.||"Page Loaded" on Page 2|
Remarks: this event could be triggered multiple times if you switch pages again and again.
This event is available for page only.