Handling Errors in the Future
Until now, we have conveniently ignored the question of how to handle exceptions. What happens if our POST to the back end results in a 400-class error? How can we design Hipster MVC so that developers can handle exceptions appropriately?
Had we stuck with options, the answer would have been to add yet another callback inside the Map of options. Luckily for us, Completer and Future have a formal mechanism for dealing with just such a situation. A Completer invokes completeError to signal a problem and Future deals with problems with handleException to do something with that exception.
The default data sync behavior in HipsterSync would signal an exceptional condition to the future by calling completeError when ...
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