Are there any guidelines as to whether the Cancel button in iOS pop-ups should be at the very top of the popup menu or bottom?
Which is the correct one from below examples:
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According to the latest human interface guideline, for an action sheet (as given by you), the cancel button should be at the bottom so as to encourage users to read through all the alternatives before making a choice.
For an alert box:
When the most likely button performs a nondestructive action, it should be on the right in a two-button alert. The button that cancels this action should be on the left.
When the most likely button performs a destructive action, it should be on the left in a two-button alert. The button that cancels this action should be on the right.
Looking at the two examples given, I'm fairly certain that the top option in each list is the one deemed most useful in the given situation. The first example asks the user to sign in so the first button offers exactly that while the cancel option is the least likely action and so is bottom of the list. In the second example the user is being informed of a process failure so the most likely action is to end or cancel the process so the cancel button is first on the list.
I agree with Andrew Martin above. The most important (or primary actions) should top the list. The other actions should follow. And the least favourable option should be place at the bottom. I believe this is even further exaggerated in the bottom example, where there is a noticable 'gap' between the primary action(s) and cancel. :)