26 April 2007

I’m confused…

Reading through the WPF docs it is pretty clear that, when building a page-based WPF app, the WPF container (which looks kind of like a stripped down IE) doesn’t keep old page instances in memory. So when you click the back button, previous pages are reloaded.

I found this to be the case with one of my pages, where not only did the page reload, but all the data binding was broken when that occurred. So I had to set KeepAlive to true for that page to make it work. That made some sense to me (though I still don’t understand why the data binding within the page was all messed up…)

But now I’m finding that other pages are not reloaded. In fact, they are cached in memory, so when I use the Back button to get to them they are showing outdated data.

What a serious pain. I’m sure there are some rules at work behind the scenes here - but I wish the WPF docs were more clear as to what those rules actually were… It appears that the rules are designed to frustrate me: when I want a page to stay in memory it gets reloaded, but when I want a page to reload it stays in memory. Perhaps that’s the rule? :)

So the problem I’m trying to resolve right now is this:

I have a page showing Project 123. I then navigate to another page, where I’m able to alter some of the data for Project 123. I then use the Back button to get back to the Project 123 page, but it still shows the old data - making it very clear that the page was not reloaded, but rather was cached.

Worse, there doesn’t appear to be any nice event raised in a Page object to indicate that it was just navigated to, so I have no way of forcing a refresh of the data. That’d be too easy I guess…

Explicitly setting KeepAlive to false has no affect. And I didn’t think it would - false is supposed to be the default value.

And I have double-checked to make sure my code doesn’t keep any reference to any pages. But I don’t: I just create a page object in a method using a local variable and navigate to it using the WPF navigation service.

To be very clear, I actually have a single main page that has a Frame control. My content pages are displayed in this Frame, and my navigation is like this:

_mainFrame.NavigationService.Navigate(page);

Which does appear to work in all respects, except that page objects are and aren’t cached in memory in ways that defy the docs and any rational logic I can see…

Ideas anyone?