Question

Requirement 1

Let’s say I have some view model with two DateTime properties, Start and End. I would like for End to be set to some offset from Start, and for this to be maintained even as Start changes. This is easy enough by listening to the property notification for Start and then setting End.

Requirement 2

Now, let’s add the fun part: I want to be able to stop adjusting End if the user explicitly changes it. Put another way, let’s say it’s an airline reservation system, and we want to set the end date to 3 days later (based on the average length of stay), but if the user explicitly sets the end date to a specific date, then when he/she sets the start date again, the end date stays).

The Problem

On first thought, it would seem you could just listen for End‘s notification and set some value that the handler will see when handling Start, and say “ok, I won’t adjust End. The problem is that End doesn’t know why it is being changed, so the handler for End‘s change will blithely set the stop-changing condition immediately. This violates requirement #1.

How can I satisfy both requirements?

Answer 1

Let’s say we have a flag AutoAdjust that tells you if you should adjust End when Start changes. AutoAdjust is initially true.

Whenever End is set, if the interval between Start and End is different than the “default” interval, set AutoAdjust to false.

Whenever Start is set, if AutoAdjust is true then adjust the value of End.

Note that with this scheme, it will never be possible to set AutoAdjust back to true once it flips to false. This is intentional, and IMHO more intuitive than allowing AutoAdjust to flip back — the user can easily reason that the end date has stopped auto-adjusting because they manually modified it, but it’s not as intuitive to think that if they set it back to its “auto” value it will start auto-adjusting again.