[Universal Apps] Working around XamlRenderingBackgroundTask restrictions

Question

Hi,
I'm still trying to wrap my head around the introduction of the XamlRenderingBackgroundTask for Universal apps. I'm currently planning my next app and I want it to have custom XAML bitmaps on tiles. Now I have a few questions on this:
All samples and references recommend to write the XamlRenderingBackgroundTask in C++. While I'm generally not opposed to that, I still have a few problems regarding to accessing the app's functionality while rendering the tile. E.g. I want to get
the newest updates from a server and calculate locally on what I should show on the tile. The same functionality is used within the full app and is written in C#, and I think implementing the whole functionality in C++ would be a lot of work (server
communication, SQLite and Linq usage). This
sample here shows one implementation of the XamlRenderingBackgroundTask but it is fairly limited in functionality because it only displays in time.
I have thought of a two possibilites to work around the XamlRenderingBackgroundTask restrictions and I wanted to get some opinions on what the best way would be:
Create two background tasks, one in C# and one in C++. The C# background task calculates the data that will be used and stores it locally for the C++ background task.
The problem here is: Will the memory/CPU quotas hit the limit here? How can I guarantee the sequence (First C#, then C++)?
Store the C# logic in a WinRT component and reference it from the C++ background task.
I can't imagine this works, because the C# component will load the CLR runtime and I will hit the memory limit.
Also I have another question regarding background tasks:
Are the memory restrictions cummulative or is 16 MB on a 512 MB RAM phone device an upper limit that shall never be exceeded? E.g.:
I create an image and update the tile which takes 14 MB. If I flush the tile correclty etc. do I again have 16 MB to work with or only the remaining 2 MB?
The reason I ask is, because I wanted to allow the user to pin multiple (secondary) tiles to the start screen. If I only can create a tile once, it could never update (with XAML images) the other tiles.
I'd be thankful for any thoughts on this.

Answer 1

Hey, thanks for the replies so far!
I don't control the server side, so I can't create the image there and push it to the device.
I definitely want to create it in a BackgroundTask, otherwise there would be no value to these Live Tiles. Now, I did some digging around and played with the XAMLRenderingBackgroundTask types in both C# and C++. I created a sample app, which monitors
the memory usage of a C++ and a C# background task and saves the results to files in the pictures library whenever the time zone is changed. You can download the source code here:
Download solution
My findings:
1. If you have two separate Background tasks registered (e.g. one C# and one C++) and both tasks are run at the same time the limit will sum up, e.g. I got 60 MB as a limit and not 30, which is nice :) This means, that I can go ahead with the two
Background tasks mailbox scenario, which will only result in a small time delay.
2. Weirdly, it appears that the memory usage of the C# Background task is sometimes lower than the C++ one. Also, I never really got the C++ one below the C# Background task. Both Background tasks inherit from XamlRenderingBackgroundTask,
but I never really try to render an image, is this the problem? Otherwise, I fail to see the reason why I can't use the XamlRenderingBackgroundTask on low memory devices ...
Sample results from the tasks:
C#
Limit: 40
Current usage: 3.296875
Usage after memory allocation: 8.59375
Concurrent: false
C++
Limit: 40
Current usage: 4.50781
Usage after memory allocation: 9.63672
Concurrent: false
C#
Limit: 60
Current usage: 6.0703125
Usage after memory allocation: 6.4921875
Concurrent: true
C++
Limit: 60
Current usage: 6.12109
Usage after memory allocation: 11.6563
Concurrent: true
Limit 40 means the debugger is still attached, otherwise the limit is 30 because I have a Lumia 925.
Any ideas?