Class: RGen::Transformer

Inherits:
Object
  • Object
show all
Defined in:
lib/rgen/transformer.rb

Overview

The Transformer class can be used to specify model transformations.

Model transformations take place between a source model (located in the source environment being an instance of the source metamodel) and a target model (located in the target environment being an instance of the target metamodel). Normally a “model” consists of several model elements associated with each other.

Transformation Rules

The transformation is specified within a subclass of Transformer. Within the subclass, the Transformer.transform class method can be used to specify transformation blocks for specific metamodel classes of the source metamodel.

If there is no transformation rule for the current object’s class, a rule for the superclass is used instead. If there’s no rule for the superclass, the class hierarchy is searched this way until Object.

Here is an example:

class MyTransformer < RGen::Transformer

transform InputClass, :to => OutputClass do { :name => name, :otherClass => trans(otherClass) } end

transform OtherInputClass, :to => OtherOutputClass do { :name => name } end end

In this example a transformation rule is specified for model elements of class InputClass as well as for elements of class OtherInputClass. The former is to be transformed into an instance of OutputClass, the latter into an instance of OtherOutputClass. Note that the Ruby class objects are used to specifiy the classes.

Transforming Attributes

Besides the target class of a transformation, the attributes of the result object are specified in the above example. This is done by providing a Ruby block with the call of transform. Within this block arbitrary Ruby code may be placed, however the block must return a hash. This hash object specifies the attribute assignment of the result object using key/value pairs: The key must be a Symbol specifying the attribute which is to be assigned by name, the value is the value that will be assigned.

For convenience, the transformation block will be evaluated in the context of the source model element which is currently being converted. This way it is possible to just write :name => name in the example in order to assign the name of the source object to the name attribute of the target object.

Transforming References

When attributes of elements are references to other elements, those referenced elements have to be transformed as well. As shown above, this can be done by calling the Transformer#trans method. This method initiates a transformation of the element or array of elements passed as parameter according to transformation rules specified using transform. In fact the trans method is the only way to start the transformation at all.

For convenience and performance reasons, the result of trans is cached with respect to the parameter object. I.e. calling trans on the same source object a second time will return the same result object without a second evaluation of the corresponding transformation rules.

This way the trans method can be used to lookup the target element for some source element without the need to locally store a reference to the target element. In addition this can be useful if it is not clear if certain element has already been transformed when it is required within some other transformation block. See example below.

Special care has been taken to allow the transformation of elements which reference each other cyclically. The key issue here is that the target element of some transformation is created before the transformation’s block is evaluated, i.e before the elements attributes are set. Otherwise a call to trans within the transformation’s block could lead to a trans of the element itself.

Here is an example:

transform ModelAIn, :to => ModelAOut do { :name => name, :modelB => trans(modelB) } end transform ModelBIn, :to => ModelBOut do { :name => name, :modelA => trans(modelA) } end

Note that in this case it does not matter if the transformation is initiated by calling trans with a ModelAIn element or ModelBIn element due to the caching feature described above.

Transformer Methods

When code in transformer blocks becomes more complex it might be useful to refactor it into smaller methods. If regular Ruby methods within the Transformer subclass are used for this purpose, it is necessary to know the source element being transformed. This could be achieved by explicitly passing the @current_object as parameter of the method (see Transformer#trans).

A more convenient way however is to define a special kind of method using the Transformer.method class method. Those methods are evaluated within the context of the current source element being transformed just the same as transformer blocks are.

Here is an example:

transform ModelIn, :to => ModelOut do { :number => doubleNumber } end

method :doubleNumber do number * 2; end

In this example the transformation assigns the ‘number’ attribute of the source element multiplied by 2 to the target element. The multiplication is done in a dedicated method called ‘doubleNumber’. Note that the ‘number’ attribute of the source element is accessed without an explicit reference to the source element as the method’s body evaluates in the source element’s context.

Conditional Transformations

Using the transformations as described above, all elements of the same class are transformed the same way. Conditional transformations allow to transform elements of the same class into elements of different target classes as well as applying different transformations on the attributes.

Conditional transformations are defined by specifying multiple transformer blocks for the same source class and providing a condition with each block. Since it is important to create the target object before evaluation of the transformation block (see above), the conditions must also be evaluated separately before the transformer block.

Conditions are specified using transformer methods as described above. If the return value is true, the corresponding block is used for transformation. If more than one conditions are true, only the first transformer block will be evaluated.

If there is no rule with a condition evaluating to true, rules for superclasses will be checked as described above.

Here is an example:

transform ModelIn, :to => ModelOut, :if => :largeNumber do { :number => number * 2} end

transform ModelIn, :to => ModelOut, :if => :smallNumber do { :number => number / 2 } end method :largeNumber do number > 1000 end method :smallNumber do number < 500 end

In this case the transformation of an element of class ModelIn depends on the value of the element’s ‘number’ attribute. If the value is greater than 1000, the first rule as taken and the number is doubled. If the value is smaller than 500, the second rule is taken and the number is divided by two.

Note that it is up to the user to avoid cycles within the conditions. A cycle could occure if the condition are based on transformation target elements, i.e. if trans is used within the condition to lookup or transform other elements.

Direct Known Subclasses

ECore::ECoreTransformer, UML13ToECore

Defined Under Namespace

Classes: TransformationDescription, TransformerJob

Constant Summary collapse

@@methods = 
{}
@@transformer_blocks = 
{}

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(env_in = nil, env_out = nil) ⇒ Transformer

Creates a new transformer Optionally an input and output Environment can be specified.



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# File 'lib/rgen/transformer.rb', line 231

def initialize(env_in=nil, env_out=nil)
  @env_in = env_in
  @env_out = env_out
  @transformer_results = {}
  @transformer_jobs = []
end

Dynamic Method Handling

This class handles dynamic methods through the method_missing method

#method_missing(m, *args) ⇒ Object

Each call which is not handled by the transformer object is passed to the object currently being transformed. If that object also does not respond to the call, it is treated as a transformer method call (see Transformer.method).



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# File 'lib/rgen/transformer.rb', line 305

def method_missing(m, *args) #:nodoc:

  if @current_object.respond_to?(m)
    @current_object.send(m, *args)
  else
    _invokeMethod(m, *args)
  end
end

Class Method Details

._methods ⇒ Object

:nodoc:



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# File 'lib/rgen/transformer.rb', line 176

def self._methods # :nodoc:

  @@methods[self] ||= {}
end

._transformer_blocks ⇒ Object

:nodoc:



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# File 'lib/rgen/transformer.rb', line 172

def self._transformer_blocks # :nodoc:

  @@transformer_blocks[self] ||= {}
end

.copy(from, to = nil) ⇒ Object

This class method specifies that all objects of class from are to be copied into an object of class to. If to is omitted, from is used as target class. During copy, all attributes and references of the target object are set to their transformed counterparts of the source object.



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# File 'lib/rgen/transformer.rb', line 211

def self.copy(from, to=nil)
  transform(from, :to => to || from) do
    Hash[*@current_object.class.ecore.eAllStructuralFeatures.inject([]) {|l,a|
      l + [a.name.to_sym, trans(@current_object.send(a.name))]
    }]
  end
end

.method(name, &block) ⇒ Object

Define a transformer method for the current transformer class. In contrast to regular Ruby methods, a method defined this way executes in the context of the object currently being transformed.



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# File 'lib/rgen/transformer.rb', line 223

def self.method(name, &block)
  _methods[name.to_s] = block
end

.transform(from, desc = nil, &block) ⇒ Object

This class method is used to specify a transformation rule.

The first argument specifies the class of elements for which this rule applies. The second argument must be a hash including the target class (as value of key ‘:to’) and an optional condition (as value of key ‘:if’).

The target class is specified by passing the actual Ruby class object. The condition is either the name of a transformer method (see Transfomer.method) as a symbol or a proc object. In either case the block is evaluated at transformation time and its result value determines if the rule applies.

Raises:

  • (StandardError) 


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# File 'lib/rgen/transformer.rb', line 191

def self.transform(from, desc=nil, &block)
  to = (desc && desc.is_a?(Hash) && desc[:to])
  condition = (desc && desc.is_a?(Hash) && desc[:if])
  raise StandardError.new("No transformation target specified.") unless to
  block_desc = TransformationDescription.new(block, to)
  if condition
    _transformer_blocks[from] ||= {}
    raise StandardError.new("Multiple (non-conditional) transformations for class #{from.name}.") unless _transformer_blocks[from].is_a?(Hash)
    _transformer_blocks[from][condition] = block_desc
  else
    raise StandardError.new("Multiple (non-conditional) transformations for class #{from.name}.") unless _transformer_blocks[from].nil?
    _transformer_blocks[from] = block_desc
  end
end

Instance Method Details

#_transformProperties(obj, block_desc) ⇒ Object

:nodoc:

Raises:

  • (StandardError) 


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# File 'lib/rgen/transformer.rb', line 283

def _transformProperties(obj, block_desc) #:nodoc:

  old_object, @current_object = @current_object, obj
  block_result = instance_eval(&block_desc.block)
  raise StandardError.new("Transformer must return a hash") unless block_result.is_a? Hash
  @current_object = old_object
  _attributesFromHash(@transformer_results[obj], block_result)
end

#trans(obj) ⇒ Object

Transforms a given model element according to the rules specified by means of the Transformer.transform class method.

The transformation result element is created in the output environment and returned. In addition, the result is cached, i.e. a second invocation with the same parameter object will return the same result object without any further evaluation of the transformation rules. Nil will be transformed into nil. Ruby “singleton” objects true, false, numerics and symbols will be returned without any change. Ruby strings will be duplicated with the result being cached.

The transformation input can be given as:

  • a single object

  • an array each element of which is transformed in turn

  • a hash used as input to Environment#find with the result being transformed

Raises:

  • (StandardError) 


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# File 'lib/rgen/transformer.rb', line 254

def trans(obj)
  if obj.is_a?(Hash)
    raise StandardError.new("No input environment available to find model element.") unless @env_in
    obj = @env_in.find(obj) 
  end
  return nil if obj.nil?
  return obj if obj.is_a?(TrueClass) or obj.is_a?(FalseClass) or obj.is_a?(Numeric) or obj.is_a?(Symbol)
  return @transformer_results[obj] if @transformer_results[obj]
  return @transformer_results[obj] = obj.dup if obj.is_a?(String)
  return obj.collect{|o| trans(o)}.compact if obj.is_a? Array
  raise StandardError.new("No transformer for class #{obj.class.name}") unless _transformerBlock(obj.class)
  block_desc = _evaluateCondition(obj)
  return nil unless block_desc
  @transformer_results[obj] = _instantiateTargetClass(obj, block_desc.target)
  # we will transform the properties later

  @transformer_jobs << TransformerJob.new(self, obj, block_desc)
  # if there have been jobs in the queue before, don't process them in this call

  # this way calls to trans are not nested; a recursive implementation 

  # may cause a "Stack level too deep" exception for large models

  return @transformer_results[obj] if @transformer_jobs.size > 1
  # otherwise this is the first call of trans, process all jobs here

  # more jobs will be added during job execution

  while @transformer_jobs.size > 0
    @transformer_jobs.first.execute
    @transformer_jobs.shift
  end
  @transformer_results[obj]
end