Action sheets, alerts, and modal views are temporary views that appear when something requires a user’s attention or when additional choices or functionality need to be offered.
Action SheetAn action sheet displays a set of choices related to a task the user initiates.
destructiveButtonTitle毁灭性的按钮，系统会自动用红色颜色标记该按钮
AlertAn alert gives people important information that affects their use of the app or the device.
If you supply a third button for an alert, this button is displayed above the two primary buttons at the bottom of the alert.
Modal View
presented modally—provides self-contained functionality in the context of the current task or workflow.
