Name: dbT83986 Date: 02/17/99


The java api states the notifyObservers() checks to see
if hasChanged(), notifies the observers, and then calls
clearChanged(). However, in reality, it constructs an
array of Observers to notify, then calls clearChanged,
then notifies the controllers.

This is a codelet from java.util.notifyObservers():

   synchronized (this) {
/* We don't want the Observer doing callbacks into
* into arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not). The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
* notification in progress
* 2) a recently unregistered Observer will be
* wrongly notified when it doesn't care
*/
if (!hasChanged())
return;
size = obs.size();
if (size > arr.length) {
arr = new Observer[size];
}
obs.copyInto(arr);
clearChanged();
}

for (int i = size -1; i>=0; i--) {
if (arr[i] != null) {
arr[i].update(this, arg);
}
}

In my program, I needed to know exactly what changed so I have
private variables to indicate what changed. Each observer
checks these statuses if they are notified. I overrode
clearChanged() to set these statuses back assuming that they
would be cleared AFTER the controllers were actually notified
and not before.

public void clearChanged()
{
super.clearChanged();
mSectorDisplayChanged = false;
mAccountDisplayChanged = false;
mGroupDisplayChanged = false;
}
(Review ID: 48263)
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