LLDB display of NSDate is inconsistent?

问题:

I am puzzled by the differences underlined in red below:

How come this very same NSDate object is displayed in BST in the debug pane, but in GMT in the LLDB terminal when asked to '"Print description of "date"' ?

This is with Xcode 4.6.1

回答1:

An NSDate represents a specific moment in time, without any consideration of what human beings call that moment. If you look at NSDate, you'll notice that there aren't even hour, minute, or second properties, let alone a timeZone property. The time zone is a feature of the NSCalendar used to interpret that NSDate for display. (You may be more familiar with NSDateFormatter; it internally uses an NSCalendar to interpret the date.)

In this case, Xcode happens to configure the calendar for the variables panel a little differently from how LLDB configures the one for the debug console. I'd have to guess that the debug console is calling -description, which always uses UTC, while the variables panel is using a date formatter that respects the current time zone. (Your Mac is configured to use BST, right? If not, that's an odd choice...)

回答2:

Brent's reply is fine - but I wanted to address one detail specifically. lldb has built in type formatters for many common types including NSDate. If you did p date in the debugger console, you would get the same output as you see in the Locals window. When you right-click/control-clicked on the variable and did 'Print description', it is equivalent to writing po date in the console -- as Brent says, it calls the -description method.

This isn't a console vrs. Locals window difference, or an Xcode vrs. lldb difference. One access method is using lldb's built in data formatters and one is calling -description.