UI icon - should it be A-Z or Z-A when the current list is sorted in ascending order?

I have a "sort by alphabet" icon in the application bar. If the list of items is currently listing:

1
a
b

Then should the icon show what the listing is currently sorted by (A-Z) or what it WILL be sorted by if you press the button (Z开发者_运维百科-A)?

The only example I can find on the phone is setting the volume to ring or vibrate (If it is set to ring, it shows a bell) but would like more examples / confirmation from people here.

If it's clear how the list is currently sorted by looking at it then I'd recommend displaying what the list will be changed to by pressing the button.

If the list can be sorted in any ways other than just ascending or descending alphabetical order you will probably need a button for setting each of the sort orders.
If it's not clear how the list is sorted by looking at it then this indicator should be separate.

Overall I'd question the value of having this sorting functionality. If you have a long list I'd use the LongListSelector from the Toolkit and then always have it sorted A-Z. This will simplify your UI (by not requiring a button to change the sort order) and will require less code.

If the button is closely associated visually with the list, like a column header is with a column in a grid, then I would say you could show what the current state is just like headers in columns of grids tend to do that offer this capability.

In this case, whichever way you do it, I would say the user would adapt fairly quickly by looking at the list to see how it works. There will be some who initially expect one way, and some the other I expect. Arguably there isn't a correct way to do it. It's a good idea to look at what other apps on the platform are doing, but there may not really be much to go on just yet.

The visual cue in this case is the tight visual association of the list and the button, suggesting to me that that button reflects the lists current state and that you can press to change it.

If however you had some form of a label that the button was associated visually to, then that button's text could provide the necessary context and your decision could be driven by that. For example your label could read "change sort to" or it could read "current sort is". This probably offers the most clarity, alleviating unsure users of the need to press the button a couple of times to learn how it works, but comes at the expense of valuable screen real estate.

If you walk into a hotel and your room number is 301, you want to go to the 3rd floor.

So you step inside the elevator and press the '3rd floor' button.

Therefore I think buttons should always show possible options, not the current state (i.e. ground floor).

So I think the button should show 'Z-A'.