While helping out on the forums at FlashKit, I came across this post:

Hello,
We have a project that requires free falling snow flakes. When the cursor enters the stage we need the flakes to ‘rush’ toward the pointer and follow it. When the cursor leaves the stage the flakes drift away and down.

Can anyone point me toward an FLA or tutorial?

Many thanks

Well, for some reason I got all obsessed with this question and I wrote the following tutorial:
I did a rough version that emulates this effect. You can probably fine-tune it to be nicer, but this will get you started. Here’s what you need to do:

1. You need some sort of snowflake effect. Rather than start from scratch, you can download one. I downloaded the SnowFlakes class from Pixelfumes: http://pixelfumes.blogspot.com/2006/11/actionscript-3-snow-storm-class.html and I’m using the snowFlake.fla for the example (which you can also download from that link).

2. This class now needs to be modified to do what you were talking about. So first step is to make it so that the movie can register clicks. I created an invisible button called btnClick that was 450 x 450 (covering the whole movie).

3. We want to make sure that the snowflakes are underneath the button, or the user’s click won’t be registered. So in the actions frame of snowFlake.fla, I changed the createFlakes function to:

[[code]]ZnVuY3Rpb24gY3JlYXRlRmxha2VzKCk6dm9pZHsNCiAgICAgdmFyIHNGbGFrZTpTbm93Rmxha2UgPSBuZXcgU25vd0ZsYWtlKCk7DQogICAgIGFkZENoaWxkQXQoc0ZsYWtlLDApOw0KfQ==[[/code]]

This adds the new snowflake underneath everything else.

4. Now we want to have something happen when the user clicks on the movie. So we add the following code:

[[code]]YnRuQ2xpY2suYWRkRXZlbnRMaXN0ZW5lcihNb3VzZUV2ZW50LkNMSUNLLCB0b2dnbGVGbGFrZXMpOw==[[/code]]

5. The toggleFlakes function needs to do a couple of things. First, check to see if it’s snowing. Then, if it IS snowing, it needs to stop snowing and have all the snowflakes go to the mouse. If it’s NOT snowing, it needs to have all the snowflakes go away from the mouse and start moving again. To do this, I added the following code:

[[code]]dmFyIGmµç¥S#WfC&ÇU§§6##—5¥tgT”CvD„£¥G4ä6Õ£&Ôãs—T”…'e£&G5¥U§5•wFÆ7–†Äöµc%¥sSµG&#&Æ´”‡4ä6”t”4vu–t´vÇ¥S#WfC&ÇU§–¶vWs´”4t”4t”4t”„ã#4$v$tg%¥„ÖôµG4ä6”t”4t”4t”4'3çV#6G&Ö6u4&Õ•w‡¥¥G4ä6”t”4ve4&Æ$„æÄ”‡4ä6”t”4t”4t”4'¦Dtg–DU§5•wFÆ7–w÷s´”4t”4t”4t”vÇ¥S#WfC&ÇU§”””…'–EuStEöt”4t”ƒä6ãä6sµ¦ågU“5'##Fv35'f4U§5•wFÆ7–wöå§fuvWs´”4t”4#6Ôf¥¥6v–35'f4U§5•wFÆ7”&¦$vƦ%f´––³tE”Eôä6Õ£&Ôãs—T”„ã•„£&׆†%g¤´6³fFÓ—¤4#tEöt”4t”…'••tæÄ´4§¦Dtg–DU§5•wFÆ7”&¦$vƦ%f´––³tE•µ²ö6öFUÕÐÐ

For now, the stop flakes and start flakes functions are not defined — we’ll get to that in a second.

6. So now we know we need to stop the flakes somehow… enter the stopFlakes function. Without identifying what that function IS, just yet, let’s assume that it will be a function that we’ll add to the SnowFlake class that will stop an instance of the snowflake from animating and cause it to move to the mouse. If we’re going to do it that way, we should create an array which will store references to all the snowflakes so we can trigger them at will. We do this by adding the following code:

[[code]]dmFyIGZsYWtlQXJyYXk6QXJyYXkgPSBuZXcgQXJyYXkoKTs=[[/code]]

And by modifying the createFlakes function to be:

[[code]]ZnVuY3Rpb24gY3JlYXRlRmxha2VzKCk6dm9pZHsNCiAgICAgdmFyIHNGbGFrZTpTbm93Rmxha2UgPSBuZXcgU25vd0ZsYWtlKCk7DQogICAgIGFkZENoaWxkQXQoc0ZsYWtlLDApOw0KICAgICBmbGFrZUFycmF5LnB1c2goc0ZsYWtlKTsNCn0=[[/code]]

Now we can reference all the created snowflakes, let’s make it so that by running stopFlakes, you run “gotoMouse” on all the flake instances:

[[code]]ZnVuY3Rpb24gc3RvcEZsYWtlcygpOnZvaWQgew0KICAgICB0cmFjZSgic3RvcEZsYWtlcyBjbGlja2VkIik7DQogICAgIGZvciAodmFyIGk6aW50OyBpPG51bUZsYWtlczsgaSsrKSB7DQogICAgICAgICAgZmxha2VBcnJheVtpXS5nb3RvTW91c2UoKTsNCiAgICAgfQ0KfQ==[[/code]]

7. Now we’ll need to start the flakes once again… so this is the startFlakes function. Let’s assume the function we’ll build in the SnowFlake class is called “restoreFlakes”. So our startFlakes function becomes:

[[code]]ZnVuY3Rpb24gc3RhcnRGbGFrZXMoKTp2b2lkIHsNCiAgICAgdHJhY2UoInN0YXJ0Rmxha2VzIGNsaWNrZWQiKTsNCiAgICAgZm9yICh2YXIgaTppbnQ7IGk8bnVtRmxha2VzOyBpKyspIHsNCiAgICAgICAgICBmbGFrZUFycmF5W2ldLnJlc3RvcmVGbGFrZXMoKTsNCiAgICAgfQ0KfQ==[[/code]]

8. OK — now we’re ready to start editing the SnowFlakes class and add the real functions. First let’s do “gotoMouse”. Open up the SnowFlake class. Let’s think about what the gotoMouse function needs to be and needs to do:
* It needs to be referenced by things OUTSIDE of the class
* It needs to stop the snowflake from moving
* It needs to move the snowflake to the mouse position

9. The first task: It needs to be referenced by things OUTSIDE of the class. This means it needs to be a public function, so add this to the class:

[[code]]cHVibGljIGZ1bmN0aW9uIGdvdG9Nb3VzZSgpOnZvaWR7DQp9[[/code]]

10. The second task: It needs to stop the snowflake from moving. OK, well if we look at the class, we can see that the following code is causing the snowflake to move:

[[code]]cHJpdmF0ZSBmdW5jdGmµçf&”'F#5¦ÅEuVôµG&#&ƶWs´”4t”4#vÛ^zLmFkZEV2ZW50TGlzdGVuZXIoRXZlbnQuRU5URVJfRlJBTUUsZXZlcnlGcmFtZSk7DQp9[[/code]]

Easy enough to counter:

[[code]]cHVibGljIGZ1bmN0aW9uIGdvdG9Nb3VzZSgpOnZvaWR7DQogICAgIHRoaXMucmVtb3ZlRXZlbnRMaXN0ZW5lcihFdmVudC5FTlRFUl9GUkFNRSwgZXZlcnlGcmFtZSk7DQp9[[/code]]

11. The third task: it needs to move the snowflake to the mouse position. We’re dealing with movement, so I personally always use Tweener to handle movement (http://code.google.com/p/tweener/). Download Tweener, put it in the same folder as the FLA, and add the following import command to the top of the class with all the other imports:

[[code]]aW1wb3J0IGNhdXJpbmEudHJhbnNpdGmµçf&ä×Ud†FÅ¥sVÆ6§3Õµ²ö6öFUÕÐÐ

We also need to move it to the mouse position, so let’s pass the mouse position from the original call to this function. So the call in the snowFlake.fla becomes:

[[code]]ZnVuY3Rpb24gc3RvcEZsYWtlcygpOnZvaWQgew0KICAgICB0cmFjZSgic3RvcEZsYWtlcyBjbGlja2VkIik7DQogICAgIGZvciAodmFyIGk6aW50OyBpPG51bUZsYWtlczsgaSsrKSB7DQogICAgICAgICAgZmxha2VBcnJheVtpXS5nb3RvTW91c2UobW91c2VYLG1vdXNlWSk7DQogICAgIH0NCn0=[[/code]]

and our gotoMouse function becomes:

[[code]]cHVibGljIGZ1bmN0aW9uIGdvdG9Nb3VzZSh4UG9zOk51bWJlciwgeVBvczpOdW1iZXIpOnZvaWR7DQogICAgIHRoaXMucmVtb3ZlRXZlbnRMaXN0ZW5lcihFdmVudC5FTlRFUl9GUkFNRSwgZXZlcnlGcmFtZSk7DQp9[[/code]]

Now we need to add the tween, so add the Tweener code and the function becomes:

[[code]]cHVibGljIGZ1bmN0aW9uIGdvdG9Nb3VzZSh4UG9zOk51bWJlciwgeVBvczpOdW1iZXIpOnZvaWR7DQogICAgIHRoaXMucmVtb3ZlRXZlbnRMaXN0ZW5lcihFdmVudC5FTlRFUl9GUkFNRSwgZXZlcnlGcmFtZSk7DQogICAgIFR3ZWVuZXIuYWRkVHdlZW4odGhpcywge3g6eFBvcywgeTp5UG9zLCB0aW1lOjAuNSwgdHJhbnNpdGmµçf&¦ö•¥tg¥¥S“Ddç&ÕV–e6³tE•µ²ö6öFUÕÐÐ

12. OK, now we need to create the function that will start the flakes once again. We’re calling this restoreFlakes. What does it need to do?
* It needs to be referenced by things OUTSIDE of the class
* It needs to move the snowflake back to its original position
* It needs to start the snowflakes moving again.

13. The first task is easy — it needs to be a public function:

[[code]]cHVibGljIGZ1bmN0aW9uIHJlc3RvcmVGbGFrZXMoKTp2b2lkew0KfQ==[[/code]]

14. The second task — move back to the original position. This means we need to track where the snowflake was when it got paused, so we need to modify the gotoMouse function to record where the snowflakes were. To do this we add the following variables to the class:

[[code]]cHJpdmF0ZSB2YXIgdGhpc1g6TnVtYmVyID0gMDsNCnByaXZhdGUgdmFyIHRoaXNZOk51bWJlciA9IDA7[[/code]]

We then modify the gotoMouse function:

[[code]]cHVibGljIGZ1bmN0aW9uIGdvdG9Nb3VzZSh4UG9zOk51bWJlciwgeVBvczpOdW1iZXIpOnZvaWR7DQogICAgIHRoaXMucmVtb3ZlRXZlbnRMaXN0ZW5lcihFdmVudC5FTlRFUl9GUkFNRSwgZXZlcnlGcmFtZSk7DQogICAgIHRoaXNYID0gdGhpcy54Ow0KICAgICB0aGlzWSA9IHRoaXMueTsNCiAgICAgVHdlZW5lci5hZGRUd2Vlbih0aGlzLCB7eDp4UG9zLCB5OnlQb3MsIHRpbWU6MC41LCB0cmFuc2l0aW9uOiJlYXNlT3V0U2luZSJ9KTsNCn0=[[/code]]

Then we modify the restoreFlakes function to move the snowflakes back:

[[code]]cHVibGljIGZ1bmN0aW9uIHJlc3RvcmVGbGFrZXMoKTp2b2lkew0KICAgICBUd2VlbmVyLmFkZFR3ZWVuKHRoaXMsIHt4OnRoaXNYLCB5OnRoaXNZLCB0aW1lOjAuNSwgdHJhbnNpdGmµçf&¦ö•¥tg¥¥S“Ddç&ÕV–e6³tE•µ²ö6öFUÕÐÐ

15. The third task — make them start moving. Well, just add the listener back in and the function becomes

Now we’re done! The revised SnowFlake class becomes this:

[[code]]cGFja2FnZSB7DQoNCiAgICAgaW1wb3J0IGZsYXNoLmRpc3BsYXkuTW92aWVDbGlwOw0KICAgICBpbXBvcnQgZmxhc2guZXZlbnRzLio7DQogICAgIGmµçF4s—–D4&Ö$tg¦3VÖwƒ¥„§¤Æ´§6E„¤vwƒ¥„“tEöt”4t”vÇF4s—–D4&¥•…g–sV„Æå'••sW¦…'##W¤ÆÅ#5¥ugU¥„“tEôä6”t”4v4…f–$vƤ”tç5•„礔dçV#6Dv$tg%¥4&ÆT…&Æ&Õ'¤”SfFÖÆÅ'‡4‡4ä6s´”4t”4t”4t”„'–…¦†DuVvFÔg””„çu¥ufµE…g6DvÇv$vÆÆ6§&åu4÷s´”4t”4t”4t”„'–…¦†DuVvFÔg””„çu¥ufµfÔg–S$…'4w‡¥„“fsS”CtÕDtEöt”4t”´”4t”4t”4t”„'–…¦†DuVvFÔg””„çu¥uf´öåg&åu4$å•…&ôÆ䦆&Õ'f%6w¶äçu¥ufµE…g6DvÇv$vÆÆ6§4ä6”t”4t”4t”4'v6ÖÃ%•…&Ä”…¦†6”'¦4ufŤe¦†6ÖƆDvÇf&”””S†DvwV6ÔgU¤s—D´6·34&Å¥u%u•„§E…g6DvÛ^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µç¤—4Ö–³tEöt”4t”4t”4t”4t”4#vǤÆÕ§$…&Æ6äÖu4&%–Õ¦D÷s´Eöt”4t”4t”4t”4t”4'¦–‡¦4ufŤ3‡””5§6DG6v34&Å¥u%u•„§•…'##GWs´”4t”4t”4t”4t”4t”4t”4#vǤÆä楕w†Åt4””…&ö„×V3$æ†$ue¤”CtÆ¥StEöt”4t”4t”4t”4t”4#”Eôä6”t”4t”4t”4#”Eôä6”t”4t”4t”4'v6ÖÃ%•…&Ä”u£&Ôãs—T”vFÆDU'–u£´6³fFÓ—¤‡4ä6”t”4t”4t”4t”4t”u'–u£”CuEtc3W••sV¶##ôµ6÷¤÷s´”4t”4t”4t”4t”4vFÔg””u'–u£$vÇ•¥tãs—TöÖÇVD4””S†DvwV6ÔgU¤s—D´6·ÕDtEöt”4t”4t”4t”4t”4'¦–†¶6ÖÆÖDU'6Õf¦DvÇf&”Ö$…t”EWWs´”4t”4t”4t”4t”4t”4t”4&¶6ÖÆÖD44DÕG4ä6”t”4t”4t”4t”4t”ƒä6s´”4t”4t”4t”ƒä6s´”4t”4t”4t”„'–…¦†DuVu¦ågU“5'##Fv%s“%¥SÄ´6³fFÓ—¤‡4ä6”t”4t”4t”4t”4t”…&ö„×U•u&µ%…¦Æ&å$Ö„ã¥sVÆ6–„fFÕgVD3TeFÅ$eVÔuV´då%7†ÆFÕg–UU§••sĵG4ä6”t”4t”4t”4#”Eôä6”t”4t”4t”4'v6ÖÃ%•…&Ä”u£&Ôãs—T”uc%¥„£U&䦆%uVõ¥GfFÕgVD6³fFÓ—¤‡4ä6”t”4t”4t”4t”4t”…&ö„×VU4%4'¦4ufŤG4ä6”t”4t”4t”4t”4t”…&ö„×VT4%4&¶6ÖÆÖDG4ä6”t”4t”4t”4t”4t”vÆÔ´…&ö„×VU4Õ£5t”…§¥†D•¥vÆæ…vd‡vvDv‡7“SD”5§6DG6tÔ4#†d4#vÛ^zLnggJmd0OyB2aWV3V2lkdGgpew0KICAgICAgICAgICAgICAgICAgICByZWFkeUZsYWtlKCk7DQogICAgICAgICAgICAgICB9DQoNCiAgICAgICAgICB9DQoNCiAgICAgICAgICBwdWJsaWMgZnVuY3Rpb24gZ290b01vdXNlKHhQb3M6TnVtYmVyLCB5UG9zOk51bWJlcik6dm9pZHsNCiAgICAgICAgICAgICAgIHRoaXMucmVtb3ZlRXZlbnRMaXN0ZW5lcihFdmVudC5FTlRFUl9GUkFNRSwgZXZlcnlGcmFtZSk7DQogICAgICAgICAgICAgICB0aGmµç¥t4””…&ö„×VTG4ä6”t”4t”4t”4t”4t”…&ö„夔CvDv‡7“ST÷s´”4t”4t”4t”4t”4ud†FÅ¥sVÆ6“V…¤u%VC%fÆ&–ƒvÛ^zLCB7eDp4UG9zLCB5OnlQb3MsIHRpbWU6MC41LCB0cmFuc2l0aW9uOiJlYXNlT3V0U2luZSJ9KTsNCiAgICAgICAgICB9DQoNCiAgICAgICAgICBwdWJsaWMgZnVuY3Rpb24gcmVzdG9yZUZsYWtlcygpOnZvaWR7DQogICAgICAgICAgICAgICBUd2VlbmVyLmFkZFR3ZWVuKHRoaXMsIHt4OnRoaXNYLCB5OnRoaXNZLCB0aW1lOjAuNSwgdHJhbnNpdGmµçf&¦ö•¥tg¥¥S“Ddç&ÕV–e6³tEöt”4t”4t”4t”4t”4#vǤÆÔfµ¤Uc%¥sSDvǦDugU¥„–õ%…¦Æ&åU%SUU%d¦e&Ĥ%EUW4”uc%¥„£U&䦆%uW÷s´”4t”4t”4t”ƒä6s´”4t”4#”Eôä6ãä6sÓÕµ²ö6öFUÕÐÐ

The code on the main timeline becomes this:

[[code]]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[[/code]]

And you’re good to go! Here’s what it should look like (click on it in order to contract/expand the snowflakes):

And you can download the source files here:
snowflake.zip