How do I work with images in a portable class library targeting Windows Store Apps and WP7,WP8,WPF?

Question

I am working on a first PCL that targets : WSA (Windows Store Application), WPF,WP7,WP8. We can say that it is a rolerdex kind of application, you have contacts , they have

Answer 1

Images are indeed quite problematic when you need to scale them across different platforms with different pixel densities and different screen sizes.

Coupled with this, there are often problems with image manipulation libraries not being portable - especially when they use hardware acceleration on each platform.

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Assuming that your rolodex app is somehow going to allow users to capture images, and then to upload them to some shared database/service for later viewing, here's how I might approach the problem. It's not the only solution - there's no "one right way" here!

** Capturing and uploading images **

1. For capturing the picture (either from a folder or from a camera) I would have to use a native hook for each platform - so NOT PCL for this part

2. If I then needed to do some on-device processing of the image (e.g. resizing or adding a thumbnail) then again this would probably be done differently on each platform - so NOT PCL.

3. Once you have the photo (e.g. as a JPEG encoded in a MemoryStream) and want to upload it to a server, then I would probably do this using the asynchronous handlers HttpWebRequest (these are the old methods, nothing to do with async/await) in common PCL code

4. What's running on the server.... well, that's almost certainly not PCL code - and I might use methods there to resize the image into device ready sizes - e.g. different size thumbnails

** Showing images **

1. When I need to display someone's image from the database, then I would probably get the server to return a list of available thumbnail URLs - this is server code so would be not PCL

2. The app code that actually asks for the list of contacts or the contact detail - that would probably be PCL code - and would probably use HttpWebRequest again.

3. The View code that takes the contact and renders it on the screen? That would probably be XAML - and I would just use the native Image control on each platform to consume and render an appropriate thumbnail for the situation.

** If you were storing the images locally **

1. If you were storing the images locally instead of using a central server, then you'll probably need to use non-PCL code for this too. Each platform has the same basic type of methods: LoadFile, SaveFile, etc - and each will provide mechanisms to create, enumerate, read and write folders and files, but each platform does this via a different API to the file system (e.g. System.IO in WPF, IsolatedStorage in WP7 Silverlight, etc).

2. Once you've got your files into a common(ish) structure, then the PCL control code will be able to treat each file as just a pair of strings - the folder it's in and the file name...

3. ... and in your UI layer (e.g. XAML) you will probably be able to reference those image files directly - probably can be done using a specific ValueConverter to generate the correct filename or filestream on each platform - e.g. in wp7 you could use the converter from Windows Phone 7 Silverlight binding image from the IsolatedStorage

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So, my summary is:

• I'd use PCL code wherever I could
• but in reality that PCL code is only going to be in the control and networking - not in the image collection, image processing or image rendering

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Some other things that might help:

• currently I find it's quite hard to mix .Net4.5 async/await code with "normal" code - so even sharing the networking code might be quite hard to do - you might actually save time if you write separate code for everything :(
• to help share the code, I would definitely try to use some form of IoC or DI to try to inject specific implementation of platform specific code where it's needed (this is what I do a lot in MvvmCross - and it's what @dsplaisted talks about in service location in http://blogs.msdn.com/b/dsplaisted/archive/2012/08/27/how-to-make-portable-class-libraries-work-for-you.aspx