Programming Flash Communication Server

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Chapter 15: Application Design Patterns and Best Practices

• Wait for the shared object to report the change was successful before showing

the change.

• Show the change immediately and again when the shared object synchronizes.

The PersonalCursor component implements the second option. The SharedBall class

listed in Example 8-4 uses the first option. When the mouse is dragging the ball, the

ball’s onMouseMove( ) method checks the x and y values to make sure they aren’t off

the Stage and then updates both the position of the ball clip and the shared object:

this.so.data[this.xSlot] = this._x = x;

this.so.data[this.ySlot] = this._y = y;

When the position updates are accepted by the server, the onSync( ) method is called

with a list containing an information object with

code set to “success”. The Shared-

Ball class simply ignores “success” messages. It updates its position only when some-

one else moves the ball and onSync( ) is passed a list containing an information

object with a

code value of “change” or “reject”. A “reject” code indicates the user’s

attempt to move the ball was rejected by the server in favor of another client’s posi-

tion update.

You might think that there is never a reason to update the visible state of an applica-

tion before and after the changes are synchronized in the shared object. Samuel Wan

posted a nice sample application that makes good use of the third option. The

ScratchPad component, made available as part of his Flash Forward 2003 NY sam-

ple files, updates the screen while the user draws on the scratch pad. When the draw-

ing is complete, the graphic is removed from the Stage and the graphic information is

sent to the server where it is stored in a shared object. When the shared object syn-

chronizes, the original drawing is redrawn on the Stage. From the user’s perspective,

the two-step update sequence is a good choice. The user can draw and see immedi-

ately what the drawing looks like but is also made aware of the delay between releas-

ing the mouse and everyone else seeing the drawing. The source files are available at:

http://www.samuelwan.com/information/archives/000157.html

Example 17-6 demonstrates a fourth option, which is attempting to anticipate an

object’s future position based on its current speed and direction of movement, but

we save that discussion for Chapter 17.

Interval Update Checks

The ScratchPad component waits until the user has completely drawn a shape before

sending the drawing information to the server, whereas the PersonalCursor compo-

nent updates the shared object whenever the cursor moves. Sometimes, something in

between immediate updates and waiting for the user to finish is required. A good

example is a shared Input Text field. The field may be part of a form that multiple

users can each contribute to filling in. Each shared Input Text field cannot be