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| 2 | **
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| 3 | ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
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| 4 | ** All rights reserved.
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| 40 | ****************************************************************************/
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| 41 |
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| 42 | /*!
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| 43 | \example itemviews/simpletreemodel
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| 44 | \title Simple Tree Model Example
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| 45 |
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| 46 | The Simple Tree Model example shows how to create a basic, read-only
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| 47 | hierarchical model to use with Qt's standard view classes. For a
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| 48 | description of simple non-hierarchical list and table models, see the
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| 49 | \l{model-view-programming.html}{Model/View Programming} overview.
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| 50 |
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| 51 | \image simpletreemodel-example.png
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| 52 |
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| 53 | Qt's model/view architecture provides a standard way for views to manipulate
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| 54 | information in a data source, using an abstract model of the data to
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| 55 | simplify and standardize the way it is accessed. Simple models represent
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| 56 | data as a table of items, and allow views to access this data via an
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| 57 | \l{model-view-model.html}{index-based} system. More generally, models can
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| 58 | be used to represent data in the form of a tree structure by allowing each
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| 59 | item to act as a parent to a table of child items.
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| 60 |
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| 61 | Before attempting to implement a tree model, it is worth considering whether
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| 62 | the data is supplied by an external source, or whether it is going to be
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| 63 | maintained within the model itself. In this example, we will implement an
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| 64 | internal structure to hold data rather than discuss how to package data from
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| 65 | an external source.
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| 66 |
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| 67 | \section1 Design and Concepts
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| 68 |
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| 69 | The data structure that we use to represent the structure of the data takes
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| 70 | the form of a tree built from \c TreeItem objects. Each \c TreeItem
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| 71 | represents an item in a tree view, and contains several columns of data.
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| 72 |
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| 73 | \target SimpleTreeModelStructure
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| 74 | \table
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| 75 | \row \i \inlineimage treemodel-structure.png
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| 76 | \i \bold{Simple Tree Model Structure}
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| 77 |
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| 78 | The data is stored internally in the model using \c TreeItem objects that
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| 79 | are linked together in a pointer-based tree structure. Generally, each
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| 80 | \c TreeItem has a parent item, and can have a number of child items.
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| 81 | However, the root item in the tree structure has no parent item and it
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| 82 | is never referenced outside the model.
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| 83 |
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| 84 | Each \c TreeItem contains information about its place in the tree
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| 85 | structure; it can return its parent item and its row number. Having
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| 86 | this information readily available makes implementing the model easier.
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| 87 |
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| 88 | Since each item in a tree view usually contains several columns of data
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| 89 | (a title and a summary in this example), it is natural to store this
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| 90 | information in each item. For simplicity, we will use a list of QVariant
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| 91 | objects to store the data for each column in the item.
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| 92 | \endtable
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| 93 |
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| 94 | The use of a pointer-based tree structure means that, when passing a
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| 95 | model index to a view, we can record the address of the corresponding
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| 96 | item in the index (see QAbstractItemModel::createIndex()) and retrieve
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| 97 | it later with QModelIndex::internalPointer(). This makes writing the
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