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• Message dialogs (Windows.UI.Popups.MessageDialog) are used to ask questions that the user
must answer or acknowledge before the app can proceed; a message dialog disables the rest of
the app. Message dialogs are best used for errors or conditions that affect the whole app; error
messages that are specific to page content should appear inline.
• The app bar is a WinJS control on which you can place standard commands, using the
commands layout, or any HTML of your choice, using the custom layout. Custom icons are also
possible, using different fonts or custom graphics. An app can have both a top and a bottom app
bar, where the top is typically used for navigation and employs a custom layout. App bars can be
sticky to keep them visible instead of being light-dismissed.
• The app bar’s showCommands, hideCommands, and showOnlyCommands methods, along with the
extraClass property of commands, make it easy to define an app bar in a single location in the
app and to selectively show specific command sets by using querySelectorAll with a class that
represents that set.
• Command menus, as appear from an app bar command or an on-canvas control of some kind,
are implemented with the WinJS.UI.Menu control.
• As an example of using many of these features, the Here My Am! app is updated in this chapter
to greatly improve its handling of various error conditions.
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