ActionScript 3.0 Design Patterns

Example: Car Steering Adapter | 189

ignition key of the car (assume that the car is in gear all the time). The

setSteeringWheelAngle( )public method takes in the angle of the steering wheel and

rotates the car accordingly. The interface is illustrated in Figure 5-4.

The new context

Let’s assume that the new context requires the car to be driven, not only by a steer-

ing wheel, but by alternative input devices as well. For example, we need to drive the

car using the keyboard or the mouse. Think of the new context as an effort to mod-

ify the car to be driven by someone disabled who can’t use a steering wheel. The

original interface allowed the car to be steered with very fine-grained movements

using thesetSteeringWheelAngle( )method. You could make the steering wheel turn

half a degree or 900 degrees (2.5 complete rotations of the steering wheel). How-

ever, generating such a wide range of values is not possible with key presses. A key

press essentially has two values, key down and key up. It is evident that we’ll have to

turn the steering wheel in discrete steps. For example, we can assume that every

press of the left-arrow key represents turning the steering wheel by 50 degrees coun-

terclockwise. The right-arrow key will, in turn, represent turning the steering wheel

in the clockwise direction. It will also be helpful to have a way to get the steering

wheel back to its original position. The up arrow key can be used to set the steering

wheel angle to zero. We need to convert the interface to fit this new context.

The new interface

Based on the previous analysis, the new interface needs several methods to meet the

requirements of the new context. TheICartarget interface, shown in Example 5-10

and depicted in Figure 5-5, is designed for the new context.

Figure 5-4. The original interface of the LegacyCar class

Ignition key

public function start() :void

Steering wheel

public function setSteeringWheelAngle (nAngle:int) :void