add a FactoryHelper constraint helper

This constraints helper allows the user to define a function (or any
callable) which will be invoked as-needed to generate the constraints
for the layout.

enaml/layout/api.py

@@ -12,6 +12,6 @@
     FloatItem, FloatArea, RemoveItem, ExtendItem, RetractItem
 )
 from .layout_helpers import (
-    align, hbox, vbox, horizontal, vertical, grid, spacer,
+    align, hbox, vbox, horizontal, vertical, factory, grid, spacer,
 )
 from .geometry import Box, BoxF, Pos, PosF, Rect, RectF, Size, SizeF

enaml/layout/factory_helper.py

@@ -0,0 +1,63 @@
+#------------------------------------------------------------------------------
+# Copyright (c) 2014, Nucleic Development Team.
+#
+# Distributed under the terms of the Modified BSD License.
+#
+# The full license is in the file COPYING.txt, distributed with this software.
+#------------------------------------------------------------------------------
+from atom.api import Callable, Tuple, Dict
+
+from .constraint_helper import ConstraintHelper
+
+
+class FactoryHelper(ConstraintHelper):
+    """ A constraint helper which delegates to a factory callable.
+
+    """
+    #: The callable object which will generate the list of constraints.
+    #: It will be passed the constraints widget owner as the first
+    #: argument, and should return the list of generated constraints.
+    factory = Callable()
+
+    #: Additional positional arguments to pass to the callable.
+    args = Tuple()
+
+    #: Additional keyword arguments to pass to the callable.
+    kwargs = Dict()
+
+    def __init__(self, factory, *args, **kwargs):
+        """ Initialize a FactoryHelper.
+
+        Parameters
+        ----------
+        factory : callable
+            The callable object which generates the constraints.
+
+        *args
+            Additional positional arguments to pass to the factory.
+
+        **kwargs
+            Additional keyword arguments to pass to the factory.
+
+        """
+        self.factory = factory
+        self.args = args
+        self.kwargs = kwargs
+
+    def constraints(self, component):
+        """ Generate the constraints using the underlying factory.
+
+        This method will automatically expand ConstraintHelper objects
+        which are generated by the factory.
+
+        """
+        factory = self.factory
+        if factory is None:
+            return []
+        cns = []  # inline `expand_constraints`
+        for cn in factory(component, *self.args, **self.kwargs):
+            if isinstance(cn, ConstraintHelper):
+                cns.extend(cn.create_constraints(component))
+            elif cn is not None:
+                cns.append(cn)
+        return cns

enaml/layout/layout_helpers.py

@@ -6,6 +6,7 @@
 # The full license is in the file COPYING.txt, distributed with this software.
 #------------------------------------------------------------------------------
 from .constraint_helper import ConstraintHelper
+from .factory_helper import FactoryHelper
 from .grid_helper import GridHelper
 from .linear_box_helper import LinearBoxHelper
 from .sequence_helper import SequenceHelper
@@ -94,6 +95,25 @@ def align(anchor, *items, **config):
     return SequenceHelper(anchor, anchor, items, **config)
 
 
+def factory(func, *args, **kwargs):
+    """ Create a FactoryHelper with the given factory function.
+
+    Parameters
+    ----------
+    func : callable
+        The callable which will generate the list of constraints.
+        The owner widget will be passed as the first argument.
+
+    *args
+        Additional positional arguments to pass to the factory.
+
+    **kwargs
+        Additional keyword arguments to pass to the factory.
+
+    """
+    return FactoryHelper(func, *args, **kwargs)
+
+
 def grid(*rows, **config):
     """ Create a GridHelper object with the given rows.