为多个事件显示单个加载屏幕

Question

My app downloads from several URLs at the same time. Non of them finishes at the same time, but i only want one loading screen letting the user see that something is going on :-)

I've tried a "ticket" system, where i have a List<>. Each event that triggers a download add a ticket to the list. When they are done they remove one. I've then have a BackgroundWorker running every 200ms that checks this list. If the list count is 0 and the loadingscreen (UserControl) exists (busyscreen.GetType().Name comparison to every object in this.LayoutRoot.Children) it removes the child. If the list isnt 0 and there is no child name busyscreen it gets added.

What's wrong with this logic? I cant seem to get it to work, is there a better way to do this kind of work?

Answer 1

Ideally you want to use blocking and something like WaitHandle- you can make your code block until signalled to proceed

http://msdn.microsoft.com/en-us/library/system.threading.waithandle.aspx

Basically, you create an array of ManualResetEvent objects - these objects will signal to the main thread when the work is done

Then you call

WaitHandle.WaitAll(arrayOfManualResetEvents) 

which will block until the reset events are signalled

ManualResetEvent.Set()

So in pseudocode:

> Create each URL loader
> Create a ManualResetEvent for each loader
> Start each loader loading
> Call WaitHandle.WaitAll(arrayOfManualResets) which will block on your main thread
> In each URL LoadComplete handler call the associated ManualResetEvent.Set()
> The main thread will continue when all are signalled

Edit: As has been pointed out - WaitHandle.WaitAll doesn't work on Windows Phone. There's an alternative to it here:

Alternatives for WaitHandle.WaitAll on Windows Phone?

Which seems like it would do the job. Consider using it because I always find that code that ideally should block, when used with timers can work but is more fiddly and prone to bugs

Answer 2

You can create Singleton object that will contain LoadCount property and loadCount field (of course do not forget about INotifyProeprtyChanged ) which you can use further as you need. When you start downloading from new URL do something like Interlocked.Increment(ref loadCount); and on callback - Interlocked.Decrement(ref loadCount); .
Depending on LoadCount you can show "loading screen".