I have a long list of cells which each contain an image.
The images are large on disk, as they are used for other things in the app like wallpapers etc.
I am familiar with the normal Android process for resampling large bitmaps and disposing of them when no longer needed.
However, I feel like trying to resample the images on the fly in a list adapter would be inefficient without caching them once decoded, otherwise a fling would spawn many threads and I will have to manage cancelling unneeded images etc etc.
The app is built making extensive use of the fantastic MVVMCross framework. I was thinking about using the MvxImageViews as these can load images from disk and cache them easily. The thing is, I need to resample them before they are cached.
My question is, does anybody know of an established pattern to do this in MVVMCross, or have any suggestions as to how I might go about achieving it? Do I need to customise the Download Cache plugin? Any suggestions would be great :)
OK, I think I have found my answer. I had been accidentally looking at the old MVVMCross 3.1 version of the DownloadCache Plugin / MvxLocalFileImageLoader.
After cloning the up to date (v3.5) repo I found that this functionality has been added. Local files are now cached and can be resampled on first load :)
The MvxImageView has a Max Height / Width setter method that propagates out to its MvxImageHelper, which in turn sends it to the MvxLocalFileImageLoader.
One thing to note is that the resampling only happens if you are loading from a file, not if you are using a resource id.
Source is here: https://github.com/MvvmCross/MvvmCross/blob/3.5/Plugins/Cirrious/DownloadCache/Cirrious.MvvmCross.Plugins.DownloadCache.Droid/MvxAndroidLocalFileImageLoader.cs
Once again MVVMCross saves my day ^_^
UPDATE: Now I actually have it all working, here are some pointers:
As I noted in the comments, the local image caching is only currently available on the 3.5.2 alpha MVVMCross. This was incompatible with my project, so using 3.5.1 I created my own copies of the 3.5.2a MvxImageView, MvxImageHelper and MvxAndroidLocalFileImageLoader, along with their interfaces, and registered them in the Setup class.
I modified the MvxAndroidLocalFileImageLoader to also resample resources, not just files.
You have to bind to the MvxImageView's ImageUrl property using the "res:" prefix as documented here (Struggling to bind local images in an MvxImageView with MvvmCross); If you bind to 'DrawableId' this assigns the image directly to the underlying ImageView and no caching / resampling happens.
I needed to be able to set the customised MvxImageview's Max Height / Width for resampling after the layout was inflated/bound, but before the images were retrieved (I wanted to set them during 'OnMeasure', but the images had already been loaded by then). There is probably a better way but I hacked in a bool flag 'SizeSet'. The image url is temporarily stored if this is false (i.e. during the initial binding). Once this is set to true (after OnMeasure), the stored url is passed to the underlying ImageHelper to be loaded.
One section of the app uses full screen images as the background of fragments in a pager adapter. The bitmaps are not getting garbage collected quick enough, leading to eventual OOMs when trying to load the next large image. Manually calling GC.Collect() when the fragments are destroyed frees up the memory but causes a UI stutter and also wipes the cache as it uses weak refs.
I was getting frequent SIGSEGV crashes on Lollipop when moving between fragments in the pager adapter (they never happened on KitKat). I managed to work around the issue by adding a SetImageBitmap(null) to the ImageView's Dispose method. I then call Dispose() on the ImageView in its containing fragment's OnDestroyView().
Hope this helps someone, as it took me a while!