I have what I thought would be a simple requirement: sum up the features in the map across all featurelayers.
I see that I'm able to have a statistics widget that gives me a count for all features in a featureservice.
It would be great if there were a widget where I'd configure it with a list of featurelayers in the map and have it display a number reflecting the total across all those feature services.
If there is no such widget, is there a sample custom widget that might be a good starting point for development?
You can limit the results you get on the summary widget to the map extent if you didn't want all features. Jake Skinner posted a walk through on setting that up here: Limit Widget Results to Current Map Extent
But this will only work for one layer at a time. You would need to use a custom built widget to get those results across all layers.