1. A method for a reusable selection component of a computing system to facilitate custom actions resulting from the selection of one or more selectable user interface items on a blade on an extendable canvas, the method comprising: an extendable canvas that extends in an extendable direction on a user interface; a reusable selection component including at least a first communication interface defining at least one data model compliance rule, a second communication interface defining at least one user interface compliance rule, and a third communication interface defining at least one action module compliance rule, wherein the interfaces are configured to transmit data between the reusable selection component according to at least one compliance rule associated with a corresponding communication interface; displaying, on the extendable canvas, a first blade that includes one or more selectable user interface items and one or more custom actions associated with one or more of the selectable user interface items, wherein the first blade is a user interface component that occupies a position along the extendable direction of the canvas and substantially all of the orthogonal range of the canvas at that particular position; maintaining, at the reusable selection component, a selection state representing whether or not each of one or more selectable user interface items of the first blade is selected; determining, at the reusable selection component, a selection state of a particular selectable user interface item of the first blade using the maintained selection state, wherein the particular selectable user interface item is associated one of the one more of the custom actions; the reusable selection component notifying, over the second communication interface, the user interface item component of the selection state of the particular selectable user interface item such that the user interface item may visualize its selection state, wherein the selection state notification is compliant with the at least one interface compliance rule; and the reusable selection component notifying, over the third communication interface, an action module of the selection state of the particular selectable user interface item, wherein the custom action module is configured to take further action external to the first blade of the extendable canvas according to the custom action associated with the particular selectable user interface item, wherein the custom action includes at least causing a second blade to appear on the extendable canvas, wherein the second blade is a user interface component that occupies a position along the extendable direction of the canvas, does not overlap with the first blade, and that occupies substantially all of the orthogonal range of the canvas at that particular position, and further wherein the second blade includes one or more new selectable user interface items capable of interacting with the reusable selection component, the data model component, and the action module.