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11.2 EScope4: Adding Zoom and Grab Options Using the Decorator Pattern 151
from GraphMetrics) because they emerge from an interaction between the
graph options and graph data objects and because the size of this class is yet
to become too unmanageable. (If, in the future, this class did grow to be “too
big” then we could consider moving these methods to a new, more narrowly
defined, metrics class and keep the graph mediator class to manage the class
interactions.)
The GraphMediator class is shown in Section E.5.3 on page 324. It contains
associations with objects from all of the concrete decorator classes and it
constructs and returns composite decorators using three methods:
• createDecorator
• concatenateDecorators
• getDecorator
The createDecorator method is called whenever a new graph manipulation
option is chosen, it selects the correct combination of decorator objects
to chain together and chains them together in the appropriate order.
The concatenateDecorator method performs the chaining and the
getDecorator method returns a link to the composite decorator object. (Note
that concatenateDecorator accepts a variable number of arguments which
is new to Java from Java 1.5.) Listings of all three methods follow. The
createDecorator method is a type of factory method which is a pattern
which will be discussed in Chapter 17.
public void createDecorator ()
{
/∗ Build the decorator chain .
The elements in the chain d e f i n e what
is going to be displayed .
If no data or bad data then the decorator chain
is only composed of a DrawCentredMessage object
containing an error message.∗/
if ( graphData == null || ! isGraphData)
{
if ( graphData != null && isDataError)
warningDecorator . setMessage(
”Unexpected data . See console message .”);
else
warningDecorator . setMessage(”No data”);
//Build decorator chain with a centred message
decorator = warningDecorator ;
}
else //Build the decorator chain based on the
//current display mode
{
if ( graphOptions . getDisplayMode()==
graphOptions .getCROSSHAIR())