Particles is a class that manages simple MovieClip motion. Obviously by it’s name it can be used to create particles such as falling snow, leaves or anything else. And the MovieClips don’t have to fall. They can also float up. ( Download class files ) ( API documentation )
At it’s heart the Particles class is based on the physics from Kieth Peter’s book Making Things Move
Here’s the essentials of how it works:
- You create MovieClip(s) that are linked in the Flash Library – I’ll refer to them as dust.
- Then you create other MovieClip(s) on the Stage that act as the source of where the linked MovieClips will be emmited from – I’ll refer to them as sources.
- Lastly you create a MovieClip where the dust can be created dynamically – I’ll refer to that MovieClip as the holder.
- And if you want you can specify an array of obstacles for the dust to bounce off of.
It’s important to know that there can be any number of dust and source MovieClips but only one holder is needed.
The way the Particles class works is that it attaches dust MovieClips at random intervals depending on the rate you can set.
The dust is always created within the area occupied by one of the source MovieClips that you specify. So if you have a very small source MovieClip then the dust will appear to come out of a point. Or if you have a horizontal line that spans across the Stage as a source then the dust will appear to fall out of that line. That’s something you might do if you wanted to create a snowing effect.
The holder is the MovieClip where the dust is attached at runtime. Each dust particle is always monitoring to see if it still fits within the area that the holder occupies. If it goes outside of that area then it removes itself from the holder so that it doesn’t waste CPU cycles. Therefore you can’t have an emptyMovieClip as the holder because the dust would be removed as soon as it was created. In most cases you would want the holder to be the size of your Stage. But depending on your design it could be smaller or larger as needed.
Lets make some Particles
Here is how you would create an instance of Particles that would look like what you see above. ( download source )
var simpleDust:Particles = new Particles( holder, [sourceMC], ["mote"] );
simpleDust.setDustPerSec( 33 );
The first line creates an instance of the Particles class and the second line sets the rate of dust production at, roughly, 33 per second. I say roughly because it’s all random. You can slow down the rate or speed it up but it will always fluctuate randomly around the value you set.
What’z with the brackets? [ ]
You’ll notice something a little different in the signature. There are brackets around the sourceMC and “mote”. The reason for that is that those are actually arrays that are created inline. That’s because you can have multiple sources and multiple dusts linked in the library. When you have multiples of either then one is randomly selected each time a dust is created.
Here’s an examle.
Here’s the code for that: ( download source )
// balls is an array of strings that is the collection
// of Linkage IDs that you want to use as dust
var balls:Array = ["ball_yellow", "ball_red", "ball_green", "ball_blue"];
// sources is an array of MovieClips - no quotes - that
// are being used as the origins for the dust particles
var sources:Array = [sourceYellow, sourceGreen, sourceRed];
// create an instance of Particles
var multiples:Particles = new Particles( holder, sources, balls );
// set the dust rate so that it will start
multiples.setDustPerSec( 33 );
You’ll notice that these animations don’t slow down no matter how long they run. That’s because once a piece of dust has an _alpha of less than 10 it will automatically delete itself so as to not waste resources. And remember if a dust passes outside the boundaries of the holder it will also delete itself.
There is a lot more that Particles can do that will be covered in later tutorials. And in the near future I’d like to rewrite it in AS3, if I have the time.
Particles originally appeared on polyGeek.com on January 19, 2008.