# Fire event when the value of a variable is changed **URL:** \ **Category:** Ruby\ **Created:** [June 14, 2008, 12:24am UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440 "2008-06-14T00:24:58Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 12:24am UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/1 "2008-06-14T00:24:58Z")
hi ! what is the best way to execute a given method each time the value of a given variable is changed. I have told that if the value of the variable is changed via a sitter, it’s possible to use a method chaining. How if there is no setter ? Thanks in advance, Addame ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 4:06am UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/2 "2008-06-14T04:06:25Z")
On Jun 13, 2008, at 6:24 PM, Mehdi A. wrote: > hi ! > hej! > what is the best way to execute a given method each time the value > of a > given variable is changed. sweetsauce. this is a pretty underused trick in ruby: a = 5 a.trace\_var {|new| puts “a was changed to ‘#{new}’” } huzzah! -------------------------------------------------------| ~ Ari seydar: it’s like a crazy love triangle of Kernel commands and C code ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 5:37am UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/3 "2008-06-14T05:37:37Z")
On Fri, Jun 13, 2008 at 10:04 PM, fedzor [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > a = 5 > a.trace\_var {|new| puts “a was changed to ‘#{new}’” } Did you actually try this? And if so, on what version of Ruby? According to RI and the results of my own experimentation, trace\_var is called like so: trace\_var(:$a) {|value| …} And it only works for globals, not local variables. – Avdi Home: [http://avdi.org](http://avdi.org) Developer Blog: [Avdi Grimm, Code Cleric](http://avdi.org/devblog/) Twitter: [http://twitter.com/avdi](http://twitter.com/avdi) Journal: [http://avdi.livejournal.com](http://avdi.livejournal.com) ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 9:15am UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/4 "2008-06-14T09:15:47Z")
In article [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), Mehdi A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I have told that if the value of the variable is changed via a sitter, > it’s possible to use a method chaining. How if there is no setter ? What kind of variable? Global, local, instance? For the latter you could write your own attr\_accessor method that either creates a call to a custom method or uses update() of Observer. ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 4:06pm UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/5 "2008-06-14T16:06:41Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Jun 14, 2008, at 3:07 PM, Mehdi A. wrote: > > write your own attr\_accessor method that either creates a call to a > > Addame Even if you change the setter, you won’t be able to trace direct assignment. Regards, Florian G. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.8 (Darwin) iEYEARECAAYFAkhT0DsACgkQJA/zY0IIRZZtMQCcDXUu3UKxEiOi9ixEW3J5LUKr TaoAoIXU3Dom1cDnzGCoOUmRI0Lt/XDl =SYa5 -----END PGP SIGNATURE----- ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 3:08pm UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/6 "2008-06-14T15:08:38Z")
Tobias W. wrote: > In article [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), > Mehdi A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > I have told that if the value of the variable is changed via a sitter, > > it’s possible to use a method chaining. How if there is no setter ? > > What kind of variable? Global, local, instance? For the latter you could > write your own attr\_accessor method that either creates a call to a > custom method or uses update() of Observer. Thanks for the answer … if fact when it’s an instance variable, it’s possible to (re)define the setter… the problem is when the variable is local. I checked the use of trace\_var, and as Avdi said, it’s only used to trace global variables assignments. Addame ---
**Author:** ![addame](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@addame](https://www.ruby-forum.com/u/addame)\ **Post date:** [June 14, 2008, 6:37pm UTC](https://www.ruby-forum.com/t/fire-event-when-the-value-of-a-variable-is-changed/140440/7 "2008-06-14T18:37:06Z")
On Jun 13, 2008, at 11:36 PM, Avdi G. wrote: > On Fri, Jun 13, 2008 at 10:04 PM, fedzor [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > a = 5 > > a.trace\_var {|new| puts “a was changed to ‘#{new}’” } > > Did you actually try this? And if so, on what version of Ruby? > > According to RI and the results of my own experimentation, trace\_var > is called like so: > > trace\_var(:$a) {|value| …} I tried it a while ago… but not _Exactly_ that line of code… sorry! -------------------------------------------------------| ~ Ari seydar: it’s like a crazy love triangle of Kernel commands and C code