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| 56 | 16.1.6.2 The Packer</A> |
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| 58 | |
| 59 | <P> |
| 60 | The packer is one of Tk's geometry-management mechanisms. |
| 61 | |
| 62 | <P> |
| 63 | Geometry managers are used to specify the relative positioning of the |
| 64 | positioning of widgets within their container - their mutual |
| 65 | <em>master</em>. In contrast to the more cumbersome <em>placer</em> |
| 66 | (which is used less commonly, and we do not cover here), the packer |
| 67 | takes qualitative relationship specification - <em>above</em>, <em>to |
| 68 | the left of</em>, <em>filling</em>, etc - and works everything out to |
| 69 | determine the exact placement coordinates for you. |
| 70 | |
| 71 | <P> |
| 72 | The size of any <em>master</em> widget is determined by the size of |
| 73 | the "slave widgets" inside. The packer is used to control where slave |
| 74 | widgets appear inside the master into which they are packed. You can |
| 75 | pack widgets into frames, and frames into other frames, in order to |
| 76 | achieve the kind of layout you desire. Additionally, the arrangement |
| 77 | is dynamically adjusted to accommodate incremental changes to the |
| 78 | configuration, once it is packed. |
| 79 | |
| 80 | <P> |
| 81 | Note that widgets do not appear until they have had their geometry |
| 82 | specified with a geometry manager. It's a common early mistake to |
| 83 | leave out the geometry specification, and then be surprised when the |
| 84 | widget is created but nothing appears. A widget will appear only |
| 85 | after it has had, for example, the packer's <tt class="method">pack()</tt> method |
| 86 | applied to it. |
| 87 | |
| 88 | <P> |
| 89 | The pack() method can be called with keyword-option/value pairs that |
| 90 | control where the widget is to appear within its container, and how it |
| 91 | is to behave when the main application window is resized. Here are |
| 92 | some examples: |
| 93 | |
| 94 | <P> |
| 95 | <div class="verbatim"><pre> |
| 96 | fred.pack() # defaults to side = "top" |
| 97 | fred.pack(side = "left") |
| 98 | fred.pack(expand = 1) |
| 99 | </pre></div> |
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