看一组circleparticle类的继续关系：CircleParticle--> AbstractParticle--> AbstractItem
AbstractItem是所有contraints和particles的基类。(The base class for all constraints and particles。)
顺便看一下AbstractParticle和AbstractContraint这两个类。因为依然是定义为抽象类，所以看一看神奇的大自然是怎么构造它的。
if (getQualifiedClassName(this) == "org.cove.ape::AbstractParticle") {
throw new ArgumentError("AbstractParticle can't be instantiated directly");
}
if (getQualifiedClassName(this) == "org.cove.ape::AbstractConstraint") {
throw new ArgumentError("AbstractConstraint can't be instantiated directly");
}
大自然用这个方法来防止是类被实例化。大自然真的好神奇呀~~ so sa la yi ya ~~
好鸟，下面看一下这个类的公有属性:
alwaysRepaint:Boolean
For performance, fixed Particles and SpringConstraints don't have their paint() method called in order to avoid unnecessary redrawing. A SpringConstraint is considered fixed if its two connecting Particles are fixed. Setting this property totrue forcespaint() to be called if this Particle or SpringConstraintfixed property is true. If you are rotating a fixed Particle or SpringConstraint then you would set it's repaintFixed property to true. This property has no effect if a Particle or SpringConstraint is not fixed.
出于性能的考虑，fixed particles和springconstraints没有paint()方法是为了避免不必要的(资源)开销。如果springconstraint的两个连接的粒子是固定的那么这个springconstraint 也被认为是固定的。设置这个属性为true则forces的paint()方法将被调用 如果这个particle和springconstraint的fixed属性设为true。如果你需要旋转一个fixed particle或spriteconstraint那么你需要把它的repaintFixed属性设为true。这个属性没有影响如果一个particle或springconstraint不是固定的。
sprite
Provides a Sprite to use as a container for drawing or adding children. When the sprite is requested for the first time it is automatically added to the global container in the APEngine class.
提供一个可以drawing或添加子children的一个容器。
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