HasLoad

January 21, 1994
February 26, 1996

Defined in Object.Def
Mixes in with Object

Class Description

Many classes have an Init method that the system calls when a new class member is created with New. The system doesn't call Init when it loads in static objects defined in an object definition file, however. If you need to initialize those objects, you will find that the HasLoad mixin comes in handy. The HasLoad mixin provides you with just one method: the Load method. The system calls Load when those pesky predefined objects are created. Override Load to do any intialization processing or checking.

Note: Class HasLoad has been declared obsolete except for cases of per instance initialization.

Using the HasLoad Mixin

The HasLoad class mixes in with any subclass of the Object class.

Here is a quick list detailing the differences among various initialization methods:

Method	Description
Object_Install	Called on each class which overrides install, every time the device is turned on.
Object_Reset	Called on each class which overrides reset, every time the device is reset.
HasLoad_Load	Called on static instances (from object definition files) of a class descending from HasLoad, every time the device is turned on.
Object_Init	Called when an object is created with new.

Software packages can't tell the difference between a cold boot and a warm boot. You should ensure that your Load methods will do the right thing if they're called a second time (such as after a warm boot).

GoTo methods have no effect when called during the boot process.

The system calls the Load method only on static objects, that is, objects defined in an Objects.Def file. The system will not call Load on objects created with the New or Copy methods.

Programming Information

Instantiate: never
Subclass: always
Call its methods: sometimes

The actor demo in the Miscellany sample in the MagicDeveloper kit contains an example of a class which inherits from HasLoad. Here is the class definition:

Define Class ActorDemo;
   inherits from Viewable, HasLoad;

   field actors: Object, getter, setter;
   field centerKey: Object, getter, setter;
   field centerBox: Box;
   operation GetCenterBox(center : Box);
   Overrides Draw;
   operation LaunchActors(), safe, common;
   Overrides Load;
   attribute CenterKey: Object;
End Class;

And here is the definition of the Load method:

Method void 
ActorDemo_Load(ObjectID self)
{
   ObjectID             actors;
   Box                  aBox;
   ActorDemo_Fields     rec;
   
   actors = NewTransient(ObjectList_, NULL);
   SetActors(self, actors);
   
   /*  Calculate the center area. */
   ContentBox(self, &aBox);
   InsetBox(&aBox, 100 * onePixel, 50 * onePixel);
   ReadFields(self, &rec);
   rec.centerBox = aBox;
   WriteFields(self, &rec);
   /* Create the semaphore that controls access to the 
      center area */
   SetCenterKey(self, NewTransient(Semaphore_, nil));
}

Methods you might override

Whenever you create a subclass using the HasLoad mixin, you must override the following method:

Method	Description
Load	Process each object of this class from the objects.Def file

Description of fields

The HasLoad mixin defines no fields.

Method Descriptions

Load

operation Load(), noMethod

Call: rarely
Override: always

You will probably never call the Load method yourself. The system calls it instead. See the discussion above for more information on when the method is called.