In the case of UIAlertController(), there are two kinds of style: .Alert, which pops up a message box over the center of the screen, and .ActionSheet, which slides options up from the bottom. They are similar, but Apple recommends you use .Alert when telling users about a situation change, and .ActionSheet when asking them to choose from a set of options.
UIAlerAction: There are three possible styles: .Default, .Cancel, and .Destructive. What these look like depends on iOS, but it's important you use them appropriately because they provide subtle hints to users.
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