淡入淡出的效果

MovieClip.prototype.roll = function(alpha) {
this.onEnterFrame = function() {
this._alpha += (alpha-this._alpha)*.1;
};
};
for (var i = 0; i<4; i++) {
this["mc"+i]._alpha = 20;
this["mc"+i].onRollOver = function() {
this.roll(100); //over的时候透明度调整至100
};
this["mc"+i].onRollOut = function() {
this.roll(20); //out的时候透明度降至20
};
}