Transformation Interface Page 1 21/08/01
Transformation Interface
-Andy Bulka, August 2001
abulka@netspace.net.au
Abstract
The Transformation Interface is both a design pattern and an architectural pattern. It is an
interface or layer, which offers services to transform an object into another format and back
again. A pair of methods on some class e.g. load/save are often the signature of a
Transformation Interface.
The ‘impedence mismatch’ between class instances in memory and relational databases can
be viewed as an object-to-table format ‘transformation’ problem. In fact, persisting an object
to disk and back again is one of the most common transformations - as is ‘transforming’ an
object to and from a human manipulable/editable format in a GUI. Other examples of object
transformations are: saving and loading an object to and from XML format, saving/restoring
or sending/receiving an object using streaming or serialization, encryption/decryption, and
perhaps even making and restoring a memento of an object.
Because there are so many implementations of such an abstract idea of ‘transformation’, I
have set up the context to be quite specific (implementing a three-tiered architecture of GUI-
to-Model mapping and Model-to-Persistent storage mapping) – in order to more concretely
explain the pattern to the reader. Consequently, this paper focuses on how the Transformation
Interface pattern resolves the forces that appear in the context of implementing such a three-
tiered architecture, rather than discussing too much the nature of the transformation interface
itself, or in studying examples of all its incarnations.
This paper includes working code to both a persistence layer and a GUI display mechanism -
both implementations of the Transformation Interface Pattern, which can then be ‘plugged
together’ to form the main hub of a complete application architecture.
Context
You are writing a number of applications, of different sizes, some in different languages and
operating systems, all of which require that require that certain objects be saved to disk and
also be displayed in a GUI. You are looking for a pattern that describes a general approach to
implementing these ‘core transformations’.
Problem
How do you design the transformation of an object into persistent, graphical and potentially
other formats, in a way that is abstract and language/class-library neutral so that you have the
flexibility to take advantage of the language facilities and class libraries available in each
particular implementation context?
You want to implement any such solution as simply as possible.
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Figure 1. How do we implementing the mapping layers
required by the above application architecture?
Forces
§ Necessity: Implementing both persistence of ‘model’ objects and the display of objects in
a GUI are core ‘transformational’ tasks - one or both are usually necessary in almost all
software applications. You want to minimise the time spent implementing these
necessary, common tasks.
§ Complexity: When implementing a presentation layer (e.g. rendering object attributes in a
visual way), a key requirement is to keep graphical knowledge out of the model,
decoupling the model data and GUI. The Model-View-Controller (MVC) pattern
achieves this using a sophisticated set of classes and behaviour. Similarly, when
implementing a persistence layer, there are numerous complexities to consider.
Thus both MVC-like and Persistence frameworks are notoriously complex to design and
implement, and even obtaining and learning third party frameworks is work that you
would rather avoid or keep to a minimum.
You are prepared to forgo the optimal efficiency of an ideal but possibly complex solution
for the simplicity of a possibly less efficient solution.
§ Data-awareness: Off-the-shelf data-aware GUI controls are easy to implement, requiring
no code - providing both persistence and GUI display. However, off-the-shelf data-aware
GUI controls usually do not map naturally to application domain objects residing in
memory, rather they map to and are restricted to displaying persisted, relational database
table data. They also may be difficult to ‘tame’ when applications and display
requirements get more complicated. Data-aware-frameworks and off-the-shelf funky data-
aware widgets are not guaranteed to be available in all your programming contexts.
Building custom data-aware controls and supporting frameworks solve some of these
limitations – allowing data-aware controls that work with your particular domain object
model. Such custom data-aware solutions usually require substantial design and
implementation overhead.
§ Transparency and Control: Code that painstakingly maps attributes of one format into
the other format is boring to write (since it is often repetitive and detailed work) however
it gives the programmer full and total control over when and how the mapping behaviour
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occurs. Data-aware control frameworks attempt to automate such specific mapping code,
but almost always introduce some mystery into the data-to-GUI synchronisation process
(unless the documentation and implementation is very good).
§ Portability and Adaptability: Any single persistence framework or GUI display solution
is unlikely to be portable to all contexts, languages, operating systems and projects faced
in a programmer’s career. However an abstract solution in the form of a design pattern
could be used in all contexts where the GUI and persistence transformation problems
occur.
Forces involved in data-aware solutions
Relational database-style data-aware controls are often compelling solutions, and may be all
that is required in certain contexts, for example, where portability and transparency forces are
weak, and you are only dealing with persistence to relational databases.
However, one day you might be writing a stockmarket simulator in the Python language,
which requires objects to be streamed to file, and requires the complex display of objects in a
Java swing GUI under Linux and then later requires you to re-implement the display of those
same objects in a Delphi GUI form under Windows. In this example, the forces of
‘adaptibility’ and ‘control’ are strong, and the ‘data-aware’ force is weak and thus traditional
data-aware controls are not going to adequately resolve the forces.
Another major issue with data-aware controls is that in such frameworks, database tables, not
objects are first class citizens - you don’t even need to define classes to use data-aware
frameworks (but of course you need to define your tables). This complicates the task of an
object oriented developer who wants to model his or her domain by defining classes with both
attributes and behaviour, and wants to make these classes first class citizens within their
overall design. Ironically the developer usually proceeds by demoting the relational database
to a mere storage facility and is forced to implement a persistence layer which transforms
(loads/saves) the objects in memory to rows in tables. Data aware controls could still
conceivably be used, as long as all relevant objects are persisted to disk immediately before
the data aware GUI form is invoked.
Solution
Define an interface featuring a pair of transformational methods. One method converts the
attributes of an object into the foreign format. The second method converts the foreign format
back into attributes of the object. Maintain a reference (e.g. pointer or integer) to the other
format in either the original object or the transformed object (e.g. a domain object in memory
might store an integer ID reference of the record in a table that persists that domain object).
The two methods of the class implementing the Transformation Interface will contain all the
(possibly detailed) mapping code required to transform an object into another format.
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Figure 2. Transformation Interface Pattern.
The object on the left can be transformed into the object on the right, and back again.
On which class to implement the Transformation Interface
You have some flexibility as to where to put the Transformation Interface mapping
implementation (the two methods and property). In our working implementations, below, the
two persistence layer transformation interface methods (save/load) for the business object are
implemented on the business object. However, the presentation layer transformation interface
methods (display/repopulate) are implemented on the form (see figure 4).
There are numerous design alternatives available, depending on your situation. For example
we can instead use an adapter class to implement transformation interface methods. Figure 5
illustrates using a mediating adaptor class to implement the GUI transformation interface
methods on behalf of the form. Even the persistence transformation interface methods of
business objects could alternatively be implemented on a separate ‘writer’ class - that knows
about the internals of the class that it is persisting/transforming. (See the related patterns
section, Serializer pattern, for an even more generic (but more complex) way of implementing
a writer class).
Regardless of this implementation flexibility, there are serious considerations to keep in mind
when deciding where to put the transformational interface. For example since you want to
keep knowledge of the GUI out of the model, you would not implement a GUI
transformational interface on a business object. However a persistence transformation
interface is often implemented on the business object - though this is not as objectionable
since how an object correctly persists itself is arguably a direct concern of the business object.
In any case, such persistence functionality can alternatively be implemented on an adapter
object, as mentioned above.
Thus one way that the mapping layers required in Figure 1. could be implemented by having
both the business object and GUI form implement Transformation Interfaces.
Class Feature Description
Id Pointer to some foreign format e.g. an integer database ID or pointer to a
GUI widget
TransfromTo()
Transformation method e.g. saveToDb () or displayObjectInGui() or
encrypt()
TransformFrom()
Transformation method e.g. loadFromDb () or
rePopulateObjectFromGUI() or decrypt()
State The data attribute(s) of the business object e.g. Name, Phone, Address etc.
Figure 3. A transformation interface consists of two methods and one attribute.
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Examples of Transformations
§ From an object in memory to a record in a table on disk
e.g. load/save
§ From an object to a GUI representation of that object
e.g. display/repopulate
§ From an string attribute of an object to a text control widget in a GUI
e.g. part of the mapping code inside display/repopulate
§ A Delphi TStringList class streams its contents to and from a file
e.g. saveToFile/loadFromFile
§ A Java class is serialised to a data stream
e.g. Serialiser.store(obj, astream) / obj = Serialiser.load(astream)
§ An object into its memento
e.g. CreateMemento() / SetMemento( m )
Consequences
Resolution of Forces
All the forces are adequately resolved by this pattern (though see limitations section, below).
The necessary persistence and GUI functionality has been implemented relatively simply
compared with potentially complex MVC-like and persistence-layer schemes. We have
avoided non portable, proprietary data-aware components in favor of a flexible, homegrown
and fully OO design.
The force of total transparency and control is satisfied because the exact code that achieves
the transformation is visible to the programmer and fully encapsulated inside the two
transformation methods.
The solution is pragmatic, quick to build and easy to debug. It is a common solution used in
the real world – as opposed to theoretically optimal solutions that are never actually
implemented. For example, you might like a sophisticated MVC (Model View Controller)
framework in your next application, but you don’t have the time to build and debug it, so you
use Transformation Interface, which usually does the job (subject to various limitations,
below).
Limitations
Code Maintenance: Transformation Interface method code, whilst transparent, needs to be
created and maintained manually, which is tedious work. However it could be argued that
these ‘mappings’ need to be specificied in some way – whether by lines of code, or by
entering information into dialog boxes, or even by drawing and dragging. So as long as the
implementation code is syntactically economical (which it usually is – consisting of multiple,
simple, one line mapping statements e.g. dbrec.AGE := self.Age) then there is no
redundancy or extra work in maintaining code manually. Nevertheless, some sort of
automated code generation approach would probably be welcome where there are large
amounts of straightforward mappings between standard data types.
Granularity: A limitation of a single pair of transformational methods is that they act at a
high level of granularity – e.g. they display all the fields in a form, or they save all the fields
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of a record in a database – whether the object attributes has changed or not. More
sophisticated transformational methods could be smart enough to check for ‘changed’ and
‘dirty’ flags to avoid unecessary work. In addition, you might consider factoring the
transformation method behaviour into sub-methods which are intelligently called as required.
You could even implement Observer somewhere in your design and communicate exactly
what changed to the transformation method (e.g. as a parameter). All these enhancements
introduce additional complexity – so unless the context demands optimisation, it is usually
sufficient to use the original simple, brute force approach and map all attributes whether they
have changed or not.
Modality Issues: The above granularity considerations are closely related to issues of
‘modality’. The basic Transformation Interface solution assumes that GUI forms are
displayed modally, or that we have exclusive access to the database records we are writing to.
For example, by displaying a GUI form modally we guarantee that the information in the
form will not change whilst the user has the form open. Usually the modality requirement is
not a limitation – in fact modal forms are common, as is the requirement for exclusive access
to database records. Even other transformations like streaming and serialization usually
require exclusive access to a data streams or to the files that are being written to.
However if your application includes some sort of real-time display, or the design allows two
GUI forms representing the same model data to be active at the one time, then a fine-grained,
MVC (Model-View-Controller)-like ‘observer’ based design might be worth considering so
that every time an individual business object attribute changes, the relevant GUI control
widget will be notified and updated. This is not just a granularity issue (making
transformations as small and as efficient as possible), but a ‘timing of update’ issue (when are
the transformations performed).
Treating forms modally on the other hand keeps things very simple – opening the form causes
the transformation to the GUI, and hitting OK triggers the reverse transformation.
Implementation
Implementations of this pattern involve adding two methods and one reference (e.g. pointer
field/property) to the appropriate class (see solutions section, above, for a discussion on how
to pick this class). Typically you would rename the methods to suit the mapping situation.
Three styles of implementation are also available
§ just add the methods and property to the appropriate class
§ as above, but define the methods as implementing an abstract class or interface
§ add the methods and property to the base class and override these methods in sub-
classes
The last two implementation styles allow polymorphic iteration over descendants of the base
class e.g. a ModelManager object opens a data stream or file, then iterates through all the
business objects in its collection, calling .save on each object.
Initialization
Before displaying information in a form, the form usually needs to be created and displayed.
Before writing an object’s attribute information to a database, the database needs to be
correctly opened. Simlar issues arise with files and streams etc. These initializations can be
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placed in client code, manager objects, constructor methods or separate methods (static or
otherwise), depending on the circumstances. In the example presentation layer shown later in
this paper (figure 6), the adaptor class which implements the Transformation Interface creates
the form in its contructor method.
Attribute Mapping
The key issue in a two-way mapping is how to send the data (attributes of an object) from one
layer or format to another. This is handled by the code in the transformation methods – that is
the sole purpose of transformation methods. The implementing programmer needs to type in
the necessary code to painstakingly map each attribute of the object onto the other format.
The code in the transformation methods can be relatively straightforward or can encapsulate
and hide the possibly fiddly details of the transformation.
Encapsulating complex mappings
One of the benefits of complete transparency of implementation is the ability to encapsulate
any messy mapping code under the ultra-simplicity of a pair of transformational methods.
For example, a load transformational method, when reading an integer field from a
database, may need to validate that the value makes sense then map/cast it into an enumerated
type that the business object actually uses. The transformational method in the other direction
will map the enumerated type back into a regular integer to be stored in the database.
Similarly, a pair of GUI transformational methods would contain the either straightforward or
fiddly code to represent object attributes in widgets of some sort, and back again – the details
of which will vary depending on the GUI widgets involved.
Furthermore, complex mappings might require that aspects of a single object attribute be
displayed in separate GUI widgets. Conversely, a number of object attributes might be
combined and displayed in a single GUI widget (e.g. a series of numbers plotted on a
graph/canvas widget). Whether the code is simple or complex, it is encapsulated in the
transformational methods, and the programmer knows where to go to modify and control the
mapping behaviour.
Foreign keys and pointers
A further example of how transformation methods can encapsulating complex mappings is the
case of say, loading a composite object from a database. The .LoadFromDb transformation
method for the composite class e.g. Customer would house the code necessary to convert,
the database table record into an object. Any aggregated sub-object e.g. CustomerAddress
would be referred to using a foreign key (an integer id). So in order to fully construct the
composite domain object e.g. Customer, both the actual object (Customer ) and the object it
aggregates (CustomerAddress ) will need to be created and a pointer set from the
composite object to the aggregated sub-object. In addition, a number of database queries need
to be made in order to extract all the necessary data for both CustomerAddress and
CustomerAddress. The transformation method can encapsulate all that needs to be done.
Note that ideally, such transformational methods would typically call other transformational
methods to load in the sub-objects rather than trying to do everything themselves – this is just
common sense factoring. e.g. If class Customer aggregates the class CustomerAddress,
then Customer.LoadFromDb will at some stage call CustomerAddress.LoadFromDb.
One needs to be careful to transform/load objects in the right order, since loading one object
may rely on another object already being loaded. Some of the considerations discussed here
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are similar to those of ‘deep streaming’ (see Serializer pattern).
Introducing Observer
To get ‘real-time’ updates of changes to a foreign format’s state, (e.g. a form want to be
notified when there are changes to a business object attributes), the class implementing the
transformation interface should also implement Observer. It can then subscribe and be
notified of changes to the foreign format. To take advantage of this notification process, the
transformation method should be modified to take a parameter telling it what changed e.g.
display( somespec ), so that the display method doesn’t work hard updating all parts of
the state – only what changed..
In a GUI display scenario, another alternative way of implementing more granular and
efficient transformations (mappings / screen updates) is to create an adapter object for each
business object or even for every business object attribute, and create appropriate manager
objects to coordinate all the adaptors. Here we start to equal fine grained MVC-like
efficiency, but begin to take on some of its complexity. On the positive side, by using
Transformation Interface throughout these more complex composite designs, you help to
reduce their complexity a little - for example, by insisting that all relevant classes implement
standard transformation method pairs.
The “Persistence Layer” Transformation
Running Example of a Persistence Layer
The following is a small but fully working implementation of a persistence layer. The
Transformation Interface is implemented on the business class as a pair of load/save methods,
which are overloaded in the Customer and Supplier classes. Written in the pseudo-code-
like, syntactically minimalist language Python: (See Appendix for easy tips on reading
Python)
import shelve, string
class PersistenceTransformationInterface: # Abstract
def save(self):
pass
def load(self):
pass
def getId(self):
pass
class DbPersistenceTI( PersistenceTransformationInterface ):
NextKeyId = 1 # class variable
def AllocateNextId(self):
result = DbPersistenceTI.NextKeyId
DbPersistenceTI.NextKeyId += 1
return str(result)
def getId(self):
return self.dbKeyId
class BusinessObject( DbPersistenceTI ):
def __init__(self): # Constructor
self.dbKeyId = self.AllocateNextId() + ',' + \
self.__class__.__name__
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class Model:
def __init__(self, file):
if not file:
raise 'NoFileSpecified'
self.Filename = file
self.BOlist = []
def Add(self, obj):
self.BOlist.append(obj)
return self
def SaveAll(self):
db = shelve.open(self.Filename,'c')
for obj in self.BOlist:
obj.save(db)
db.close()
def LoadAll(self):
db = shelve.open(self.Filename,'r')
self.BOlist = []
keyz = db.keys()
for dbkey in keyz:
classref = string.split(dbkey,',')[1]
obj = eval(classref)() # Create appropriate object
obj.dbKeyId = dbkey
obj.load(db)
self.BOlist.append(obj)
db.close()
return self
class Customer( BusinessObject ):
def __init__(self, name=''):
BusinessObject.__init__(self) # Call inherited
constructor
if name:
self.FirstName, self.Surname = string.split(name)
def save(self,db):
db[ self.dbKeyId ] = (self.FirstName, self.Surname)
def load(self,db):
self.FirstName, self.Surname = db[ self.dbKeyId ]
To test drive the above code, we run the following:
model = Model('mydata.dat')
cust1 = model.Add( Customer('Fred Flinstone') )
cust2 = model.Add( Customer('Anastasia Jones') )
model.SaveAll()
then in a later session ....
m = Model('mydata.dat').LoadAll()
for businessObject in m.BOlist:
print businessObject
which yeilds:
<__main__.Customer instance at 01A10B9C> Anastasia Jones
<__main__.Customer instance at 01A10B7C> Fred Flinstone
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Success! We have just implemented a working persistence layer/framework using about
forty lines of code (To aid clarity, Model.Remove is not listed, and the Model.LoadAll()
lacks a few lines of code which sync the DbPersistentTI.NextKeyId to the maximum id
read in, plus one). The above implementation includes an additional ten lines of code for the
Customer class which is used in our test code, directly above).
How additional classes participate in persistence
To add new business classes that are capable of persisting themselves, descend a new class
from class BusinessObject and add the four lines of code implementing the Transformation
Interface methods (e.g. the load/save method pair). This is all that is required to participate in
this persistence framework. Here is an implementation of the Supplier class:
class Supplier( BusinessObject ):
def __init__(self, name='', address='', phone=''):
BusinessObject.__init__(self)
self.Companyname = name
self.Address = address
self.Phone = phone
def save(self,db):
db[ self.dbKeyId ] = (self.Companyname, self.Address, \
self.Phone)
def load(self,db):
self.Companyname, self.Address, self.Phone = \
db[ self.dbKeyId ]
Note that the Python shelve module provides database like access using language native
dictionary like syntax e.g. shelfFileObject[ stringkey ] = data. Examine the
save/load methods - above.
Test drive creating and persisting Customers and Suppliers with the following code:
model = Model('mydata.dat')
cust1 = model.Add( Customer('Fred Flinstone') )
cust2 = model.Add( Customer('Anastasia Jones') )
supplier = model.Add( Supplier('McDonald Farms Ltd.') )
model.SaveAll()
Ok, our customers and supplier are created and have persisted to the file 'mydata.dat'. Later,
to retrieve all customers out of the database and back into being objects in memory, we:
model = Model('mydata.dat').LoadAll()
Then we can display all the customers in the model using a loop:
for businessObject in model.BOlist:
print businessObject
which yields:
<__main__.Customer instance at 01A10A9C> Anastasia Jones
<__main__.Customer instance at 01A1097C> Fred Flinstone
<__main__.Supplier instance at 01A10A6C> McDonald Farms Ltd.
Two customers and one supplier object – correct!
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The “Presentation Layer” GUI Transformation
Now we will implement the ‘mirror transformational task’ of providing a GUI for a business
object. The object or model is ‘transformed’ to and from a human manipulable/editable
format in a GUI (Graphical User Interface).
Sample Implementation #1 - Put the transformational methods on the form class
A common way to implement the GUI Transformation Interface is to add the required two
methods and attribute to a GUI form class:
Figure 4. A GUI form implementing the transformation interface.
Typically you would also create a static class method for the form class that takes care of all
the initialization details including the invocation/showing and disposal of the form. This
initialization code might even be implemented inside the form’s constructor, taking as a
parameter the business object(s) or model to display.
After creating/initializing the form, the startup method or constructor must eventually call the
transformational method
displayObjectInGui( businessobject )
in order to actually map the object attributes to particular widgets on the GUI form.
…
.
Figure 5. A GUI form class implementing the transformation interface.
After the user has edited the widgets on the form and clicks OK, the
repopulateObjectFromGUI
transformational method should be called. This method will iterate through all the relevant
widgets on the form, and write their values back into attributes of the business object. Then
the form is closed.
If Cancel is clicked, then repopulateObjectFromGUI is not called, and the form is simply
closed
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Form as mediator
In this first, sample implementation of a persistence layer transformation, the form acts as a
handy central mediator, providing a shared namespace, housing the transformation methods
(usually implemented as methods of the form class) and also owning all the form widgets.
The form usually also houses the event methods that respond to user interactions.
Alternatively, some of these roles can be taken over by an adaptor class (see next working
example, implementation #2).
Working Example Implementation #2 - Put the transformational methods on mediating adapter
class
Sometimes it is more convenient to place the transformation interface on a separate mediating
adapter object rather than on the form itself:
Figure 6. Rather than implementing the transformation interface on the GUI form itself, the
transformation interface can be implemented by a mediating adapter class.
Use an adapter class when you cannot or choose not to sub-class forms. For example, your
forms or GUI widgets may be difficult to subclass. Perhaps you want the flexibility of being
able to change adapters without affecting forms.
The ModelToGuiMediatingAdapter class implements the Transformation Interface, and thus
features two transformational methods which encapsulate all the (possibly complex) code for
mapping attributes of the business object to particular widgets on the GUI form – and back
again.
Optionally, if the adaptor class also implements Observer, it can be notified of specific
attribute changes in the business object or model (See MGM pattern). The following
implementation code works without the need for Observer, using only the simple
Transformation Interface pattern. Despite the brevity of the code, this code is fully functional,
and fits on less than a page.
Running Example of a Presentation Layer
The following is an implementation of a GUI display framework that uses the simple
Transformation Interface approach. A customer object is created and a form is displayed in
which the user can edit the attributes of that customer business object.
The following code is written in the clear, syntactically minimalist language of JPython -
which is Python running on a Java VM, with seamless access to all Java classes, including
swing! (See Appendix for easy tips on reading Python code).
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class GuiTransformationInterface:
def displayObjectInGui(self, obj):
self.currentObj = obj
def rePopulateObjectFromGui(self):
pass
def getCurrentObjectBeingDisplayed(self):
return self.currentObj
class Customer:
def __init__(self, name=''): # Pass customer name to Constructor
if name:
self.Firstname, self.Surname = string.split(name)
from pawt import swing
import string, java
class CustomerAsSwingForm(GuiTransformationInterface): # Model-Gui Mediator
def __init__(self, customer):
self.frame = swing.JDialog ( swing.JFrame(), 1)
self.frame.contentPane.layout = java.awt.FlowLayout()
addwidget = lambda obj, topane : topane.add(obj)
pane = self.frame.contentPane
self.edFirstname = swing.JTextField("",10)
self.edSurname = swing.JTextField("",10)
buttonOk = swing.JButton('Ok', actionPerformed=self.ok)
buttonCancel = swing.JButton('Cancel', actionPerformed=self.cancel)
addwidget(swing.JLabel("First Name: "),pane)
addwidget(self.edFirstname,pane)
addwidget(swing.JLabel("Surname: "),pane)
addwidget(self.edSurname,pane)
addwidget(buttonOk,pane)
addwidget(buttonCancel,pane)
self.displayObjectInGui(customer)
self.frame.setSize(200,120)
def show(self): self.frame.show()
def close(self):
self.frame.setVisible(0)
self.frame.dispose()
def ok(self,e): self.rePopulateObjectFromGui() ; self.close()
def cancel(self,e): self.close()
def displayObjectInGui(self, obj):
GuiTransformationInterface.displayObjectInGui
(self, obj) # call inherited
self.edFirstname.text = obj.Firstname
self.edSurname.text = obj.Surname
def rePopulateObjectFromGui(self):
self.getCurrentObjectBeingDisplayed().Firstname = self.edFirstname.text
self.getCurrentObjectBeingDisplayed().Surname = self.edSurname.text
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The following test code creates a customer and displays it in a form, allowing user to edit.
cust = Customer('Anastasia Bones')
CustomerAsSwingForm(cust).show() # Display the modal form
The user edits the customer form then clicks
Ok or Cancel.
The form represent the attributes of the
Customer Object passed to it.
print 'Customer is now: ', cust.Firstname, cust.Surname
java.lang.System.exit(0)
Creating more forms
For every GUI form required in your application, simply create a new mediating adapter class
which implements the transformational methods in the required way to move object attribute
data to the widgets and out again. All the widget specific code required to achieve this
mapping is nicely encapsulated in the code of the transformational methods of the adapter
class.
The combined architecture
In this paper, we have mentioned how the Transformation Interface metaphor is applicable to
many ‘transformational situations’. Our specific examples show the implementation of both
persistence and GUI displays - using the same pattern! (there may be an comprehensibility
advantage in this for both designers and implementers). Also, because of this broad
applicability, the Transformation Interface arguably has an architectural aspect about it. For
example, the above working implementations of a persistence layer and GUI display
mechanism can be ‘plugged together’ to form the hub of a complete application architecture.
Persistence transformation Pattern GUI display transformation Pattern
Figure 7. Applying the transformation interface pattern twice, provides a solution to both
business object persistence and GUI display functionality.
Related Patterns
Serializer, Riehle, Siberski, Bäumer, Megert and Züllighoven. PLoPD 3, 1997 page 293.
Serializer deals with the context of converting objects and arbitrarily complex data structures
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into different data representations and back, with an emphasis on streamed and persistent data
formats - making no reference to its applicability in a GUI presentation layer context.
The Serializer pattern is a much more complex solution than Transformation Interface, and
resolves different forces. The Serializable interface consists of a transformational method pair
writeTo(Writer)/readFrom(Reader) but these do not encapsulate any format specific
mapping code, rather they are coded specifically to persist themselves, but are coded
generically in terms of a reader or writer class which reads and writes object attributes via
generic operations like
writer->writeString( customername)
writer->writeInteger( age )
Neither the readers, writers, nor the object being serialized know anything about any
particular data format – specific concrete readers and writer classes override writeString
and writeInteger etc. in order to implement these generic operations for a particular data
format. Thus reader and writer objects know nothing about the actual application classes they
are serializing. The benefit of this complex design pattern is that new concrete reader and
writer objects for different representational formats can be seamlessly plugged in as needed.
The Transformation Interface is a simpler pattern, with less classes required to implement it –
though the pattern does require that the transformation methods be re-implemented for each
new format. The Transformation Interface pattern encourages the transformational method
code to be customising to the transformational task at hand, which may involve things like
taking object attributes and storing them in GUI widget attributes or even may involve special
calculations and drawings onto a graphic canvas area. The serializer pattern’s transformation
are limited to reading and writing value types like string and integer etc. to an abstract stream.
Model Gui Mediator (MGM), Bulka, A. KoalaPlop2000, RMIT Australia departmental
technical report, http://www.cs.rmit.edu.au/reports/2000/00-7.html. MGM implements a GUI
display transformation interface but also implements Observer to gain the benefits of more
granular control of screen updates.
Known Uses
Reason! Software for critical thinking (1998-2000), Melbourne University, Australia.
RIPS 2001 payroll system uses load/save method pairs for persistence and similar GUI
methods pairs for all its modal forms. Used by retiree investment company.
The Delphi class library (VCL) offer loadFromFile/SaveToFile method pairs for many
component classes e.g. TBitmap, TPicture, and TStrings classes. They are also available for
some data-aware components (TBlobField, TMemoField,
and TGraphicField), for other graphic formats (TGraphic, TIcon, and TMetaFile), for
OLE (Object Linking and Embedding) containers, and for the TreeView and other
Windows common controls.
APPENDIX
Easy Tips on Reading Python Code
Python is a simple, straightforward and elegant language. It uses standard conventions of
accessing methods and properties and is fully OO. Types are associated with objects not
variables, so you don’t need to declare variables. Functions are called like
afunction(param1, param2) and objects are created from classes the same way e.g. o =
Transformation Interface Page 16 21/08/01
MyClass(). Python is case sensitive.
There are no begin end reserved words or { } symbols in Python to indicate code blocks –
this is done through indentation. The colon in if lzt: simply means ‘then’. The idiom of
testing objects (rather than expressions) in an if statement makes sense as python treats empty
lists, None and 0 as false.
Python understands named parameters e.g. In a method call, afunction(From=None) means
you are passing None (null / nil) as the ‘From’ parameter, whilst in a method definition
From=None means that if the caller does not supply this parameter, then it will default to
None.
The first argument of all methods defined inside classes must be ‘self’. This argument fills the
role of the reserved word this in C++ or Java or self in Delphi. Most languages supply this
parameter (a reference to the current instance) implicitly whereas Python makes this concept
explicit. At runtime this parameter is supplied by the system, not the caller of the method,
thus the def AddOrder(self, order) method in reality takes one parameter when calling
it: AddOrder( order ).
The statement pass means do nothing.
You can return multiple items at once e.g. return (2, 50) and also assign multiple items at
once e.g. x, y, z = 0 or even expressions like result, status, errmsg =
myfunction(1, 90).
Other class files/modules are imported using import somefile. __init__ methods are
simply constructors. Finally, a lambda is just a one line function that reads functionname =
lambda paramlist : returnedexpression.
Both Python and JPython (Java Python, now called Jython) are open source, free and
available from www.python.org
-Andy Bulka
abulka@netspace.net.au
www.andypatterns.com
Copyright (c) 2001, Andy Bulka, All Rights Reserved.
Permission granted to reproduce this work verbatim in its entirety
for non-commercial use provided this copyright notice appears.
