f[x_] := Module[{},
  If[x > 1, Print["print: ", x]; Message[f::msg, x]];
  x];

f::msg = "`1` is greater than 1";

f[1.2]
(* print: 1.2 *)
(* f::msg f:1.2` is greater than 1 *)
(* 1.2 *)

StreamPlot[{f[x], y}, {x, 0, 1.2}, {y, 0, 1}]
(* txt: 1.22403 *)
(* txt: 1.22403 *)
(* ... *)