From my understanding of it, yes and no. The calories from one timed activity is not ‘naturally’ going to be carried over to another in a data field, so if I did three runs on a particular day, I'd expect the data field not to know about the other two when I was doing my third run.
It's most likely possible to save the cumulative activity-based calories to use as a starting value each time the data field is used, but then you didn't specify having it reset every day at midnight, so then the data field may pick up the calories from 31st March when you start another run on 1st April.
If the data field is active for all of the activity profiles that you use, then yes. The algorithm would be pretty simple.
On startup use App.getProperty() to retrieve the previous total calorie count, if the value is not found, default the calorie count to 0.
When the data field gets a start notification (via onTimerStart(), capture the current calorie count from the ActivityInfo.calories field.
When the data field gets a reset notification (via onTimerReset() capture the current calorie count from the ActivityInfo.calories field, subtract the value captured in onTimerStart(), and add this to the total calorie count.
Write the total calorie count to the object store.
You'll likely want to display something (probably the total activity calories in the data field. You can do that pretty easily using some of the code from onTimerReset(). You might also want to periodically reset the total calorie count on some periodic basis (once a day at midnight, once a week on Monday, once a month on the 1st, ...). You should be able to use the object store to store information about when the last activity was started, and then use that to tell if you need to overwrite the total calorie count in the object store with zero.
I believe you could also use an app setting. You'd read the value of the setting at startup (or in App.onSettingsChanged()). If the value indicates you should clear the total calories, you'd clear out the total calories, then reset the setting that tells you to clear the total calorie count.
