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Figure 6-27. Create a circle object, and move it so that it sits on top of the cylinder. With the drum skin selected, choose Object Duplicate from Blender s lower toolbar. The duplicated skin is already selected; drag it downward on the Z axis (the opposite direction from the blue arrowhead) until it sits below the drum. Then, hold down the middle mouse button, and drag the mouse around to flip your view to the underside of the drum. You can then position the lower drum skin so that it sits neatly on the shell. After that, create the metal drum hoops by making another open-ended cylinder, but slightly larger in diameter and much thinner. A diameter of 1.02 and depth of 0.1 are about right. In real life, the drum hoop must be seated on the outside of the shell and raised slightly above the skin; to make your model as realistic as possible, you have to do the same. When you re happy with the look of the hoop, duplicate it, and move it along the Z axis until it s roughly in position. Then, flip the view, and finalize the position of the hoop copy over the lower drum skin (see Figure 6-28).

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A common established way to manage asynchronous communication is to espouse the Asynchronous Completion Token (ACT) pattern, which originated at the Department of Computer Science and Engineering at Washington University in St. Louis. This pattern s universal applicability in the area of asynchronous communication caused it to become popular rather quickly. You can find a description and an explanation of the pattern online at http://www.cs.wustl.edu/~schmidt/PDF/ACT.pdf. The Flex framework adopts ACT ideas natively in its framework constructs. Almost all asynchronous communication in Flex follows this pattern:

) Update operators are thus only conditionally polymorphic The Principle of (Update) Operator Polymorphism If Op is an update operator and P is a parameter to Op that is subject to update and S is a subtype of the declared type T of P for which Op is inherited, then by definition it is possible to invoke Op with an argument variable corresponding to parameter P that is of declared type S The Principle of (Variable) Substitutability I ll illustrate these ideas by repeating some remarks I made in Is a Circle an Ellipse ( 23 in the present book) Let types T and S be ELLIPSE and CIRCLE, respectively (of course, I do assume here that CIRCLE is a proper subtype of ELLIPSE).

A call is made and an asynchronous token is returned. Every response dispatches events, indicating either success or failure. Event listeners are registered with the object that is capable of receiving the response. Sometimes the implementation of a marker interface makes a class eligible to receive responses. When a response arrives, events are dispatched. The response comes back with the asynchronous token. Call and response are correlated using this token. The appropriate handler gets the response. On receipt, it processes the response.

Then: All read-only operations that apply to ellipse values get the area, for example apply to circle values too (because circle values are ellipse values); that is, read-only operations associated with type ELLIPSE are inherited unconditionally by type CIRCLE However, there are some read-only operations associated with type CIRCLE get the radius, for example that don t apply to type ELLIPSE In other words, the set of read-only operations that apply to circle values is a proper superset of the set of read-only operations that apply to ellipse values Some update operations that apply to ellipse variables change the center, for example apply to circle variables too.

Figure 6-28. With duplicated skins and hoops, your drum model is starting to take shape. Next, you need to model the bolts that hold the metal hoop onto the wooden shell, tensioning the drum skin. If you aren t sure what these look like, refer to a close-up picture of a drum kit on the

By default, a call is dispatched as soon as it s made, and there is never any certainty about when the response will come. Let s take a simple example of an editable collection in a data grid, where this behavior could make our life difficult. Say we have a few rows of data. Now we select one of the rows of data and edit the values, and then we do the same with another row. When we are editing the second row, we realize our first row modification wasn t accurate, and so we need to revisit it. Under normal circumstances, the first row modification call has already been dispatched by now, and rolling back the modification is cumbersome. This situation can get more complex if our operation deletes a row and we need to recover it, or if we make two modifications to the same data element and the first modification completes after the second one. In general, to take care of these types of complexities, we need to allow the following: