Last updated at 9:30 am UTC on 2 July 2018
A DiskProxy is an externalized form of an object to write on a DataStream.
It contains a "constructor" message to regenerate the object, in context, when sent a comeFullyUpOnReload message (i.e. "internalize").
We are now using DiskProxy for shared system objects like StrikeFonts.
The idea is to define, for each kind of object that needs special externalization, a class method that will internalize the object by reconstructing it from its defining state. We call this a "constructor" method.
Then externalize such an object as a frozen message that invokes this method–a DiskProxy.
(Here is the old comment: Constructing a new object is good for any object that
(1) can not be externalized simply by snapshotting and reloading its instance variables (like a CompiledMethod or a Picture), or
(2) wants to be free to evolve its internal representation without making stored instances obsolete (and dangerous). Snapshotting and reloading an object"s instance variables is a dangerous breach of encapsulation.
The internal structure of the class is then free to evolve. All externalized instances will be useful as long as the constructor methods are maintained with the same semantics.
There may be several constructor methods for a particular class. This
is useful for (1) instances with characteristically different
defining state, and (2) newer, evolved forms of an object and its
constructors, with the old constructor methods kept around so old
data can still be properly loaded.)
Create one like this example from class Picture
DiskProxy global: #Picture
args: (Array with: self storage asByteArray)
send to the new object.
- See also subclass DiskProxyQ that will construct an object in the above manner and then send it a sequence of messages. This may save creating a wide variety of constructor methods. It is also useful because the newly read-in DiskProxyQ can catch messages like #objectContainedIn: (via #doesNotUnderstand:) and add them to the queue of messages to
expression to compute the receiver of the constructor message.
- We may also want a subclass of DiskProxy that evaluates a string
My instance variables:
- globalObjectName – the Symbol name of a global object in the System dictionary (usually a class).
- constructorSelector – the constructor message selector Symbol to send to the global object (perform:withArguments:), typically a variation on newFrom:.
- constructorArgs – the Array of arguments to pass in the constructor message.