How is it possible to Manipulate depending on what changed?

*To*: mathgroup at smc.vnet.net*Subject*: [mg113467] How is it possible to Manipulate depending on what changed?*From*: Dims <dims12 at hotmail.com>*Date*: Sat, 30 Oct 2010 04:36:19 -0400 (EDT)

I want to draw arc between two locators. When moving "first" locator, the arc should be moved with constant center and length. When moving "second" locator, the arc should change it's length only. It is obvious that these requirements imply detecting of what locator was moved. It looks impossible to write algorith without this knowledge. I wrote the following: Manipulate[ \[Phi]1 = VectorAngle[{1, 0}, beg]; \[Phi]2 = VectorAngle[{1, 0}, end]; r = Norm[beg]; l = (\[Phi]2 - \[Phi]1) r; end = {r Cos[\[Phi]2], r Sin[\[Phi]2]}; Graphics[Circle[{0, 0}, r, {\[Phi]1, \[Phi]2}], PlotRange -> {{-3, 3}, {-3, 3}}], {{beg, {1, 0}}, Locator}, {{end, {Cos[1], Sin[1]}}, Locator}] This has 2 bugs: 1) wrong angle calculation 2) and wrong behavior: when first locator is moved, the arc does not conserve it's length So, how to fix? Thanks.