Some of these widgets are configurable. When you're not in Edit mode and you move your cursor over some of the widgets' title bars, a lowercase letter i in a circle appears. Click this, and you can modify that widget--adding or removing cities from the Weather or World Clock widgets, for example.
Though many widgets are there just to be glanced at, you can interact with most of them, too. Clicking on a city in the Weather widget will expand it to show an hour-by-hour forecast, which is very much the behavior we're used to seeing in Apple's Weather app on iOS. If you click in the Calculator widget, you can click on its buttons or use the keyboard to do your math.
The base widget collection is very much in line with what we've seen on iOS and in OS X's Dashboard feature. But the great news is that the contents of the Today view can be supplemented with widgets downloaded from the Mac App Store. After you click the Edit button in Notification Center, a new App Store button appears.
Standalone apps can supply their own widgets automatically. (If you bought, say, TLA Systems's PCalc on the Mac App Store, its hypothetical widget should appear automatically in the Items list.) In addition, there will probably also be a widgets-only corner of the Mac App Store.
I'm excited to see third-party widgets, though. Apple's are nice, but they're a bit boring. The Weather widget doesn't show forecast highs or lows. The calculator widget supports only the four basic operators and percentage. World Clock doesn't offer a digital time view, nor is the list of clocks horizontally scrollable (so as you add clocks the widget just gets taller and taller). Reminders only shows items with due dates that are approaching, so undated items are ignored.
Maybe Apple's widgets will improve by the time Yosemite is released. Or maybe they'll remain light on functionality—looking at the bright side, it makes for a nice opportunity for developers to improve on them and supplant them in the Today view.
See you later, Dashboard
When Apple announced the changes to Notification Center in Yosemite, everybody seemed to have the same thought: That pretty much wraps it up for Dashboard, the interface layer for simple widgets that Apple introduced nine years ago as a part of OS X Tiger. Perhaps surprisingly, Dashboard still exists. You can enable or disable it via the Mission Control pane in System Preferences.
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