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| 55 | <H2><A NAME="SECTION0018130000000000000000"> |
| 56 | 16.1.3 A (Very) Quick Look at Tcl/Tk</A> |
| 57 | </H2> |
| 58 | <P> |
| 59 | The class hierarchy looks complicated, but in actual practice, |
| 60 | application programmers almost always refer to the classes at the very |
| 61 | bottom of the hierarchy. |
| 62 | |
| 63 | <P> |
| 64 | Notes: |
| 65 | |
| 66 | <UL> |
| 67 | <LI>These classes are provided for the purposes of |
| 68 | organizing certain functions under one namespace. They aren't meant to |
| 69 | be instantiated independently. |
| 70 | |
| 71 | <P> |
| 72 | </LI> |
| 73 | <LI>The <tt class="class">Tk</tt> class is meant to be instantiated only once in |
| 74 | an application. Application programmers need not instantiate one |
| 75 | explicitly, the system creates one whenever any of the other classes |
| 76 | are instantiated. |
| 77 | |
| 78 | <P> |
| 79 | </LI> |
| 80 | <LI>The <tt class="class">Widget</tt> class is not meant to be instantiated, it |
| 81 | is meant only for subclassing to make ``real'' widgets (in C++, this |
| 82 | is called an `abstract class'). |
| 83 | </LI> |
| 84 | </UL> |
| 85 | |
| 86 | <P> |
| 87 | To make use of this reference material, there will be times when you |
| 88 | will need to know how to read short passages of Tk and how to identify |
| 89 | the various parts of a Tk command. |
| 90 | (See section <A href="tkinter-basic-mapping.html#tkinter-basic-mapping">16.1.4</A> for the |
| 91 | <tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> equivalents of what's below.) |
| 92 | |
| 93 | <P> |
| 94 | Tk scripts are Tcl programs. Like all Tcl programs, Tk scripts are |
| 95 | just lists of tokens separated by spaces. A Tk widget is just its |
| 96 | <em>class</em>, the <em>options</em> that help configure it, and the |
| 97 | <em>actions</em> that make it do useful things. |
| 98 | |
| 99 | <P> |
| 100 | To make a widget in Tk, the command is always of the form: |
| 101 | |
| 102 | <P> |
| 103 | <div class="verbatim"><pre> |
| 104 | classCommand newPathname options |
| 105 | </pre></div> |
| 106 | |
| 107 | <P> |
| 108 | <DL> |
| 109 | <DT><STRONG><var>classCommand</var></STRONG></DT> |
| 110 | <DD>denotes which kind of widget to make (a button, a label, a menu...) |
| 111 | |
| 112 | <P> |
| 113 | </DD> |
| 114 | <DT><STRONG><var>newPathname</var></STRONG></DT> |
| 115 | <DD>is the new name for this widget. All names in Tk must be unique. To |
| 116 | help enforce this, widgets in Tk are named with <em>pathnames</em>, just |
| 117 | like files in a file system. The top level widget, the <em>root</em>, |
| 118 | is called <code>.</code> (period) and children are delimited by more |
| 119 | periods. For example, <code>.myApp.controlPanel.okButton</code> might be |
| 120 | the name of a widget. |
| 121 | |
| 122 | <P> |
| 123 | </DD> |
| 124 | <DT><STRONG><var>options</var> </STRONG></DT> |
| 125 | <DD>configure the widget's appearance and in some cases, its |
| 126 | behavior. The options come in the form of a list of flags and values. |
| 127 | Flags are proceeded by a `-', like unix shell command flags, and |
| 128 | values are put in quotes if they are more than one word. |
| 129 | </DD> |
| 130 | </DL> |
| 131 | |
| 132 | <P> |
| 133 | For example: |
| 134 | |
| 135 | <P> |
| 136 | <div class="verbatim"><pre> |
| 137 | button .fred -fg red -text "hi there" |
| 138 | ^ ^ \_____________________/ |
| 139 | | | | |
| 140 | class new options |
| 141 | command widget (-opt val -opt val ...) |
| 142 | </pre></div> |
| 143 | |
| 144 | <P> |
| 145 | Once created, the pathname to the widget becomes a new command. This |
| 146 | new <var>widget command</var> is the programmer's handle for getting the new |
| 147 | widget to perform some <var>action</var>. In C, you'd express this as |
| 148 | someAction(fred, someOptions), in C++, you would express this as |
| 149 | fred.someAction(someOptions), and in Tk, you say: |
| 150 | |
| 151 | <P> |
| 152 | <div class="verbatim"><pre> |
| 153 | .fred someAction someOptions |
| 154 | </pre></div> |
| 155 | |
| 156 | <P> |
| 157 | Note that the object name, <code>.fred</code>, starts with a dot. |
| 158 | |
| 159 | <P> |
| 160 | As you'd expect, the legal values for <var>someAction</var> will depend on |
| 161 | the widget's class: <code>.fred disable</code> works if fred is a |
| 162 | button (fred gets greyed out), but does not work if fred is a label |
| 163 | (disabling of labels is not supported in Tk). |
| 164 | |
| 165 | <P> |
| 166 | The legal values of <var>someOptions</var> is action dependent. Some |
| 167 | actions, like <code>disable</code>, require no arguments, others, like |
| 168 | a text-entry box's <code>delete</code> command, would need arguments |
| 169 | to specify what range of text to delete. |
| 170 | |
| 171 | <P> |
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