The module

<*PRAGMA LL*>

A ProperSplit.T is a type of VBT.Split that contains a circularly-linked list of its children. All of Trestle's built-in splits that are not filters are subclasses of ProperSplit.

INTERFACE ProperSplit;

IMPORT VBT, Split;

TYPE 
  T <: Public;
  Public = VBT.Split OBJECT 
    <* LL >= {SELF, VBT.mu} *>
    lastChild: Child := NIL
  END;
  Child = OBJECT 
    <* LL >= {SELF.ch.parent, VBT.mu} *>
    pred, succ: Child := NIL; 
    ch: VBT.T 
  END;

If ch is a child of a ProperSplit.T, then ch.upRef must be of type ProperSplit.Child, and ch.upRef.ch must equal ch. The succ and pred links represent a doubly-linked list of the children. The succ links are circular; the pred links are linear. The parent's lastChild field is is NIL if there are no children; otherwise it points to the last child in succ order.

The locking level comments imply that to write any of the links, a thread must have both VBT.mu and the parent locked.

If v is a T, the call v.beChild(ch) sets ch.upref to NEW(Child) if it is NIL. In any case it sets ch.upref.ch := ch and calls VBT.Split.beChild(v, ch).

A T provides replace, insert, and move methods that preserve the linked list structure of children, but allocates no screen space or event dispatching to the children. These methods use the procedures below; the replace method uses the default insert method to add the new child, which may be inappropriate for some class layout policies.

The following procedures are useful for implementing subtypes of ProperSplit.T:

PROCEDURE Insert(v: T; pred: Child; newch: VBT.T);
<* LL >= {VBT.mu, v, newch} *>

Insert newch as a new child after pred, and mark v for redisplay.

The child newch must be detached and of the appropriate screentype. The argument pred can be NIL to indicate insertion at the head of the list. Insert calls the beChild method of newCh.

PROCEDURE PreInsert(v: T; pred, ch: VBT.T): Child 
  RAISES {Split.NotAChild}; <* LL.sup = VBT.mu *>

Rescreen ch to have v's screentype (if necessary), cause a checked runtime error if ch is attached, raise Split.NotAChild if pred is non-nil and not a child of v, and finally return pred.upRef, or NIL if pred is NIL.

PROCEDURE Move(v: T; pred, ch: Child);
<* LL >= {VBT.mu, v} *>

Move ch in the list of children so that it follows pred and mark v for redisplay.

PROCEDURE Delete(v: T; ch: Child);
<* LL >= {VBT.mu} AND LL.sup < v  *>

Remove ch from the list of children, detach ch.ch, and mark v for redisplay.

END ProperSplit.