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C H A P T E R 3
Views
About Views
3-27
Declaring a View
3
Before diving into the discussion of view instantiation, it is important to understand
the term declaring. Declaring a view is something you do during the application
development process using the Newton Toolkit (NTK). Declaring a view allows it
to be accessed symbolically from another view.
In NTK, you declare a view using the Template Info command. (Although the
phrase "declaring a view" is being used here, at development time, you're really
just dealing with the view template.) In the Template Info dialog, you declare a
view by checking the box entitled "Declare To," and then choosing another view in
which to declare the selected view. The name you give your view must be a valid
symbol, and not a reserved word or the name of a system method.
You always declare a view in its parent or in some other view farther up the parent
chain. It's best, for efficiency and speed, to declare a view in the lowest level
possible in the view hierarchy; that is, in its parent view or as close to it as possible.
If you declare a view in a view other than the parent view, it may get the wrong
parent view. Because the view's parent is wrong, its coordinates will be wrong as
well, so it will show up at the wrong position on screen.
Declaring a view simply puts the declared view in the named slot. See Appendix A,
"The Inside Story on Declare," for a complete description. The slot name is the
name of the view you are declaring. The slot value, at run time, will hold a
reference to the declared view.
The base view of your application is always declared in the system root view. Note
that the application base view is declared in a slot named with its application symbol,
specified in the Application Symbol field of the Project Settings slip in NTK.
Why would you want to declare a view? When a view is declared in another view,
it can be accessed symbolically from that other view. The NewtonScript inheritance
rules already allow access from a view to its parent view, but there is no direct
access from a parent view to its child views, or between child views of a common
parent. Declaring a view provides this access.
For example, if you have two child views of a common parent, and they need to
send messages to each other, you need to declare each of them in the common
parent view. Or, if a parent view needs to send messages to one of its child views,
the child view must be declared in the parent view.
One key situation requiring a declared view is when you want to send the
Open
message to show a nonvisible view. The
Open
message can only be sent to a
declared view.
Declaring a view has a small amount of system overhead associated with it. This is
why the system doesn't just automatically declare every view you create. You
should only declare views that you need to access from other views.
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