MiG InfoCom MiG Calendar AShape Developer Manual - Manual (Page 16)

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Overrides
A unique feature of
AShape is that of overrides. Since every
property of an AShape is stored in a PropertyKey /value
map it is very easy to intercept the getting and setting of
every property. One such interception is the notion of
overrides.
For every retrieval of a property, e.g. Paint , PlaceRect ,
Font and such, all Interactor s are asked if they have an
overridden object to return for that particular property name.
This means that is is very simple to exchange all, a sub set
or just one of the properties for an
AShape without actually
changing it's state, and without changing properties that is
unknown at the time of writing. The only thing you change is
the
Interactor since it contains the override Map .
This is for instance how normally you would set a different
color for a mouse over:ed
AShape , you set an override for
the background paint on the corresponding Interactor .
This will work both for the one-to-one pattern as well as the
stamp pattern, as described above.
With this override pattern it is also very easy to restore the
original value, you just restore (remove) the override and the
original value is visible once again. This means that there is
no need to save the original value and late restore it.
The reason the you can't just set the new value on the
AShape itself is that it is normally used as a rubber stamp
and for instance the shape can be used to paint all Activities
in a date area (this is even the normal case). If for instance a
new color was set on the shape when it was mouse over:ed
then all activities would change color. The override is
connected to the interacted through the
Interactor so it is
overriding that property only for one paint (the mouse
over:ed one).
Interaction
An interaction is a concrete class that describes a trigger
that, when it happens, the framework should check the
validity of an
Expression . If that Expression is evaluated
to true it also contains a Command(s) that should be
executed. It isn't more complicated than that since it is very
generic, yet is is powerful just because it can describe almost
any
if-then constellation.
AShape Developer Manual Page 16 / 24