Description
The WidgetModel.on_some_change uses this.hasChanged, but if a previous change handler has called this.touch() or has saved a value (or maybe even just set a value??), the changed dictionary is wiped out, and on_some_change then does not fire.
We see this in bqplot, in the last few cells of the https://github.com/bloomberg/bqplot/blob/e3501c367af3f91559d702896ec49ba73189a614/examples/Advanced%20Plotting/Axis%20Properties.ipynb demo. If the cells changing the orientation of the color axis, sometimes the axis does not update. It turns out that there is a validation change handler (https://github.com/bloomberg/bqplot/blob/e3501c367af3f91559d702896ec49ba73189a614/js/src/AxisModel.js#L56, for example) that runs and saves changes, which then wipes out the model's last changes.
I think a much simpler state change model is one where state changes only occur after the current processing is done (i.e., saving is an asynchronous command, much more like the CQRS system like React). Also, if we're going to have validation methods, we might as well use backbone's real validation methods, rather than making change handlers also to validation.