How to Properly Declare NewtApp Views
One of the Newton 2.x OS Q&As
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This document was exported on 7/23/97.
How to Properly Declare NewtApp Views (1/6/97)
- Q: I have a
newtEntryPageHeader which is declared to my newtLayout view. Each time I change entries in my application, the header does not get properly updated. What's going wrong?
A: If you declare your newtApplication views, they need to be declared to their parent. Declaring newtApplication views to a grandparent can cause undefined behavior.
Because of how the declare mechanism works, you must be careful when you declare a view to a grandparent view. In some circumstances, you could try to access a view which has been closed.
As an example, pretend you have three views called viewA, viewB, and viewC. They have the following heirarchy.:
ViewA (grandparent)
ViewB (parent)
ViewC (child)
ViewC is a child of viewB and viewB is a child of viewA; ViewC is declared to viewA. If you close viewB, viewC will also be closed because it is a child of viewB. Since ViewC was declared to ViewA, ViewA will still have a reference to viewC which has been closed. Sending view messages to viewC will throw.
For more information on the Newton OS declare mechanism, see the "Declaring Multiple Levels" Q&A, and the "The Inside Story on Declare" appendix in the Newton Programmer's Guide.