User Interface Events Example

This example illustrates a common use of the alt statement. In the event model of the Limbo/Tk graphics environment, channels are used to send commands from graphic elements (widgets). Using the alt statement enables a program to respond to commands from multiple elements. (For more information about graphics programming in Limbo, see Graphics Programming starting on page 3-45.)

Although Program Listing 3-13 is longer than most examples, the use of the alt statement starts at Line 50.

Program Listing 3-13 wmaltx.b

1. implement WmAltX;
3. include "sys.m";
4. include "draw.m";
5. include "tk.m";
6. include "wmlib.m";
8. sys: Sys;
9. draw: Draw;
10. Display, Image: import draw;
11. tk: Tk;
12. wmlib: Wmlib;
14. WmAltX: module {
15. init: fn(ctxt: ref Draw->Context, argv: list of string);
16. };
18. win_cfg := array[] of {
19. "frame .ft",
20. "text .ft.t -yscrollcommand {.ft.s set} -width 40w
-height 15h",
21. "scrollbar .ft.s -command {.ft.t yview}",
22. "focus .ft.t",
23. "pack .ft.s -side right -fill y",
24. "frame .fb.a -bd 2 -relief sunken",
25. "pack .ft.t -side left -fill y",
26. "button .fb.a.b -height 2h -text {Button 1}
-command {send cmd 1}",
27. "pack .fb.a.b -padx 1 -pady 1",
28. "button .fb.c -height 2h -text {Button 2}
-command {send cmd 2}",
29. "button .fb.q -height 2h -text {Quit}
-command {send cmd Q}",
30. "frame .fb",
31. "pack .fb.a .fb.c .fb.q -padx 5 -pady 5 -side left",
32. "pack .ft .fb",
33. "pack propagate . 0",
34. "update",
35. };
36. init(ctxt: ref Draw->Context, argv: list of string) {
37. sys = load Sys Sys->PATH;
38. draw = load Draw Draw->PATH;
39. tk = load Tk Tk->PATH;
40. wmlib = load Wmlib Wmlib->PATH;
42. wmlib->init();
44. (win, menubut) := wmlib->titlebar(ctxt.screen,
"-x 5 -y 5", "WmAltX", 0);
46. cmd := chan of string;
47. tk->namechan(win, cmd, "cmd");
48. wmlib->tkcmds(win, win_cfg);
50. for(;;) alt {
51. menu := <-menubut =>
52. if(menu[0] == 'e') {
53. tk->cmd(win, ".ft.t insert end '"+
"Close button pressed.\n");
54. tk->cmd(win, "update");
55. wmlib->dialog(win, "warning -fg red",
"Close Button", "Close Button pressed!", 0,
"Close"::nil);
56. exit;
57. }
58. wmlib->titlectl(win, menu);
59. s := <-cmd =>
60. case s[0] {
61. '1' =>
62. tk->cmd(win, ".ft.t insert end '"+
"Button 1 pressed.\n");
63. tk->cmd(win, "update");
64. break;
65. '2' =>
66. tk->cmd(win, ".ft.t insert end '"+
"Button 2 pressed.\n");
67. tk->cmd(win, "update");
68. break;
69. 'Q' =>
70. tk->cmd(win, ".ft.t insert end '"+
"Quit button pressed.\n");
71. tk->cmd(win, "update");
72. wmlib->dialog(win, "info", "Quit Button",
"Quit button pressed!", 0,
"Quit"::nil);
73. exit;
74. }
75. }
76. }