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29 \example itemviews/editabletreemodel
30 \title Editable Tree Model Example
32 This example shows how to implement a simple item-based tree model that can
33 be used with other classes the model/view framework.
35 \image itemviews-editabletreemodel.png
37 The model supports editable items, custom headers, and the ability to
38 insert and remove rows and columns. With these features, it is also
39 possible to insert new child items, and this is shown in the supporting
42 \note The model only shows the basic principles used when creating an
43 editable, hierarchical model. You may wish to use the \l{ModelTest}
44 project to test production models.
48 As described in the \l{Model Subclassing Reference}, models must
49 provide implementations for the standard set of model functions:
50 \l{QAbstractItemModel::}{flags()}, \l{QAbstractItemModel::}{data()},
51 \l{QAbstractItemModel::}{headerData()},
52 \l{QAbstractItemModel::}{columnCount()}, and
53 \l{QAbstractItemModel::}{rowCount()}. In addition, hierarchical models,
54 such as this one, need to provide implementations of
55 \l{QAbstractItemModel::}{index()} and \l{QAbstractItemModel::}{parent()}.
57 An editable model needs to provide implementations of
58 \l{QAbstractItemModel::}{setData()} and
59 \l{QAbstractItemModel::}{setHeaderData()}, and must return a suitable
60 combination of flags from its \l{QAbstractItemModel::}{flags()} function.
62 Since this example allows the dimensions of the model to be changed,
63 we must also implement \l{QAbstractItemModel::}{insertRows()},
64 \l{QAbstractItemModel::}{insertColumns()},
65 \l{QAbstractItemModel::}{removeRows()}, and
66 \l{QAbstractItemModel::}{removeColumns()}.
70 As with the \l{itemviews/simpletreemodel}{Simple Tree Model} example,
71 the model simply acts as a wrapper around a collection
72 of instances of a \c TreeItem class. Each \c TreeItem is designed to
73 hold data for a row of items in a tree view, so it contains a list of
74 values corresponding to the data shown in each column.
76 Since QTreeView provides a row-oriented view onto a model, it is
77 natural to choose a row-oriented design for data structures that
78 will supply data via a model to this kind of view. Although this makes
79 the tree model less flexible, and possibly less useful for use with
80 more sophisticated views, it makes it less complex to design and easier
83 \target Relations-between-internal-items
85 \row \li \inlineimage itemviews-editabletreemodel-items.png
86 \li \b{Relations between internal items}
88 When designing a data structure for use with a custom model, it is useful
89 to expose each item's parent via a function like
90 \l{TreeItem::parent}{TreeItem::parent()} because it will make
91 writing the model's own \l{QAbstractItemModel::}{parent()} function easier.
92 Similarly, a function like \l{TreeItem::child}{TreeItem::child()} is
93 helpful when implementing the model's \l{QAbstractItemModel::}{index()}
94 function. As a result, each \c TreeItem maintains information about
95 its parent and children, making it possible for us to traverse the tree
98 The diagram shows how \c TreeItem instances are connected via their
99 \l{TreeItem::parent}{parent()} and \l{TreeItem::child}{child()}
102 In the example shown, two top-level items, \b{A} and
103 \b{B}, can be obtained from the root item by calling its child()
104 function, and each of these items return the root node from their
105 parent() functions, though this is only shown for item \b{A}.
108 Each \c TreeItem stores data for each column in the row it represents
109 in its \c itemData private member (a list of QVariant objects).
110 Since there is a one-to-one mapping between each column in the view
111 and each entry in the list, we provide a simple
112 \l{TreeItem::data}{data()} function to read entries in the \c itemData
113 list and a \l{TreeItem::setData}{setData()} function to allow them to
115 As with other functions in the item, this simplifies the implemention
116 of the model's \l{QAbstractItemModel::}{data()} and
117 \l{QAbstractItemModel::}{setData()} functions.
119 We place an item at the root of the tree of items. This root item
120 corresponds to the null model index, \l{QModelIndex::}{QModelIndex()},
121 that is used to represent the parent of a top-level item when handling
123 Although the root item does not have a visible representation in any of
124 the standard views, we use its internal list of QVariant objects to
125 store a list of strings that will be passed to views for use as
126 horizontal header titles.
129 \row \li \inlineimage itemviews-editabletreemodel-model.png
130 \li \b{Accessing data via the model}
132 In the case shown in the diagram, the piece of information represented
133 by \b{a} can be obtained using the standard model/view API:
136 QVariant a = model->index(0, 0, QModelIndex()).data();
139 Since each items holds pieces of data for each column in a given row,
140 there can be many model indexes that map to the same \c TreeItem object.
141 For example, the information represented by \b{b} can be obtained
142 using the following code:
145 QVariant b = model->index(1, 0, QModelIndex()).data();
148 The same underlying \c TreeItem would be accessed to obtain information
149 for the other model indexes in the same row as \b{b}.
152 In the model class, \c TreeModel, we relate \c TreeItem objects to
153 model indexes by passing a pointer for each item when we create its
154 corresponding model index with QAbstractItemModel::createIndex() in
155 our \l{TreeModel::index}{index()} and \l{TreeModel::parent}{parent()}
157 We can retrieve pointers stored in this way by calling the
158 \l{QModelIndex::}{internalPointer()} function on the relevant model
159 index - we create our own \l{TreeModel::getItem}{getItem()} function to
160 do this work for us, and call it from our \l{TreeModel::data}{data()}
161 and \l{TreeModel::parent}{parent()} implementations.
163 Storing pointers to items is convenient when we control how they are
164 created and destroyed since we can assume that an address obtained from
165 \l{QModelIndex::}{internalPointer()} is a valid pointer.
166 However, some models need to handle items that are obtained from other
167 components in a system, and in many cases it is not possible to fully
168 control how items are created or destroyed. In such situations, a pure
169 pointer-based approach needs to be supplemented by safeguards to ensure
170 that the model does not attempt to access items that have been deleted.
173 \row \li \b{Storing information in the underlying data structure}
175 Several pieces of data are stored as QVariant objects in the \c itemData
176 member of each \c TreeItem instance
178 The diagram shows how pieces of information,
179 represented by the labels \b{a}, \b{b} and \b{c} in the
180 previous two diagrams, are stored in items \b{A}, \b{B} and
181 \b{C} in the underlying data structure. Note that pieces of
182 information from the same row in the model are all obtained from the
183 same item. Each element in a list corresponds to a piece of information
184 exposed by each column in a given row in the model.
186 \li \inlineimage itemviews-editabletreemodel-values.png
189 Since the \c TreeModel implementation has been designed for use with
190 QTreeView, we have added a restriction on the way it uses \c TreeItem
191 instances: each item must expose the same number of columns of data.
192 This makes viewing the model consistent, allowing us to use the root
193 item to determine the number of columns for any given row, and only
194 adds the requirement that we create items containing enough data for
195 the total number of columns. As a result, inserting and removing
196 columns are time-consuming operations because we need to traverse the
197 entire tree to modify every item.
199 An alternative approach would be to design the \c TreeModel class so
200 that it truncates or expands the list of data in individual \c TreeItem
201 instances as items of data are modified. However, this "lazy" resizing
202 approach would only allow us to insert and remove columns at the end of
203 each row and would not allow columns to be inserted or removed at
204 arbitrary positions in each row.
206 \target Relating-items-using-model-indexes
209 \li \inlineimage itemviews-editabletreemodel-indexes.png
210 \li \b{Relating items using model indexes}
212 As with the \l{itemviews/simpletreemodel}{Simple Tree Model} example,
213 the \c TreeModel needs to be able to take a model index, find the
214 corresponding \c TreeItem, and return model indexes that correspond to
215 its parents and children.
217 In the diagram, we show how the model's \l{TreeModel::parent}{parent()}
218 implementation obtains the model index corresponding to the parent of
219 an item supplied by the caller, using the items shown in a
220 \l{Relations-between-internal-items}{previous diagram}.
222 A pointer to item \b{C} is obtained from the corresponding model index
223 using the \l{QModelIndex::internalPointer()} function. The pointer was
224 stored internally in the index when it was created. Since the child
225 contains a pointer to its parent, we use its \l{TreeItem::parent}{parent()}
226 function to obtain a pointer to item \b{B}. The parent model index is
227 created using the QAbstractItemModel::createIndex() function, passing
228 the pointer to item \b{B} as the internal pointer.
231 \section1 TreeItem Class Definition
233 The \c TreeItem class provides simple items that contain several
234 pieces of data, and which can provide information about their parent
237 \snippet itemviews/editabletreemodel/treeitem.h 0
239 We have designed the API to be similar to that provided by
240 QAbstractItemModel by giving each item functions to return the number
241 of columns of information, read and write data, and insert and remove
242 columns. However, we make the relationship between items explicit by
243 providing functions to deal with "children" rather than "rows".
245 Each item contains a list of pointers to child items, a pointer to its
246 parent item, and a list of QVariant objects that correspond to
247 information held in columns in a given row in the model.
249 \section1 TreeItem Class Implementation
251 Each \c TreeItem is constructed with a list of data and an optional
254 \snippet itemviews/editabletreemodel/treeitem.cpp 0
256 Initially, each item has no children. These are added to the item's
257 internal \c childItems member using the \c insertChildren() function
260 The destructor ensures that each child added to the item is deleted
261 when the item itself is deleted:
263 \snippet itemviews/editabletreemodel/treeitem.cpp 1
265 \target TreeItem::parent
266 Since each item stores a pointer to its parent, the \c parent() function
269 \snippet itemviews/editabletreemodel/treeitem.cpp 9
271 \target TreeItem::child
272 Three functions provide information about the children of an item.
273 \c child() returns a specific child from the internal list of children:
275 \snippet itemviews/editabletreemodel/treeitem.cpp 2
277 The \c childCount() function returns the total number of children:
279 \snippet itemviews/editabletreemodel/treeitem.cpp 3
281 The \c childNumber() function is used to determine the index of the child
282 in its parent's list of children. It accesses the parent's \c childItems
283 member directly to obtain this information:
285 \snippet itemviews/editabletreemodel/treeitem.cpp 4
287 The root item has no parent item; for this item, we return zero to be
288 consistent with the other items.
290 The \c columnCount() function simply returns the number of elements in
291 the internal \c itemData list of QVariant objects:
293 \snippet itemviews/editabletreemodel/treeitem.cpp 5
295 \target TreeItem::data
296 Data is retrieved using the \c data() function, which accesses the
297 appropriate element in the \c itemData list:
299 \snippet itemviews/editabletreemodel/treeitem.cpp 6
301 \target TreeItem::setData
302 Data is set using the \c setData() function, which only stores values
303 in the \c itemData list for valid list indexes, corresponding to column
306 \snippet itemviews/editabletreemodel/treeitem.cpp 11
308 To make implementation of the model easier, we return true to indicate
309 whether the data was set successfully, or false if an invalid column
311 Editable models often need to be resizable, enabling rows and columns to
312 be inserted and removed. The insertion of rows beneath a given model index
313 in the model leads to the insertion of new child items in the corresponding
314 item, handled by the \c insertChildren() function:
316 \snippet itemviews/editabletreemodel/treeitem.cpp 7
318 This ensures that new items are created with the required number of columns
319 and inserted at a valid position in the internal \c childItems list.
320 Items are removed with the \c removeChildren() function:
322 \snippet itemviews/editabletreemodel/treeitem.cpp 10
324 As discussed above, the functions for inserting and removing columns are
325 used differently to those for inserting and removing child items because
326 they are expected to be called on every item in the tree. We do this by
327 recursively calling this function on each child of the item:
329 \snippet itemviews/editabletreemodel/treeitem.cpp 8
331 \section1 TreeModel Class Definition
333 The \c TreeModel class provides an implementation of the QAbstractItemModel
334 class, exposing the necessary interface for a model that can be edited and
337 \snippet itemviews/editabletreemodel/treemodel.h 0
339 The constructor and destructor are specific to this model.
341 \snippet itemviews/editabletreemodel/treemodel.h 1
343 Read-only tree models only need to provide the above functions. The
344 following public functions provide support for editing and resizing:
346 \snippet itemviews/editabletreemodel/treemodel.h 2
348 To simplify this example, the data exposed by the model is organized into
349 a data structure by the model's \l{TreeModel::setupModelData}{setupModelData()}
350 function. Many real world models will not process the raw data at all, but
351 simply work with an existing data structure or library API.
353 \section1 TreeModel Class Implementation
355 The constructor creates a root item and initializes it with the header
358 \snippet itemviews/editabletreemodel/treemodel.cpp 0
360 We call the internal \l{TreeModel::setupModelData}{setupModelData()}
361 function to convert the textual data supplied to a data structure we can
362 use with the model. Other models may be initialized with a ready-made
363 data structure, or use an API to a library that maintains its own data.
365 The destructor only has to delete the root item; all child items will
366 be recursively deleted by the \c TreeItem destructor.
368 \snippet itemviews/editabletreemodel/treemodel.cpp 1
370 \target TreeModel::getItem
371 Since the model's interface to the other model/view components is based
372 on model indexes, and the internal data structure is item-based, many of
373 the functions implemented by the model need to be able to convert any
374 given model index to its corresponding item. For convenience and
375 consistency, we have defined a \c getItem() function to perform this
378 \snippet itemviews/editabletreemodel/treemodel.cpp 4
380 This function assumes that each model index it is passed corresponds to
381 a valid item in memory. If the index is invalid, or its internal pointer
382 does not refer to a valid item, the root item is returned instead.
384 The model's \c rowCount() implementation is simple: it first uses the
385 \c getItem() function to obtain the relevant item, then returns the
386 number of children it contains:
388 \snippet itemviews/editabletreemodel/treemodel.cpp 8
390 By contrast, the \c columnCount() implementation does not need to look
391 for a particular item because all items are defined to have the same
392 number of columns associated with them.
394 \snippet itemviews/editabletreemodel/treemodel.cpp 2
396 As a result, the number of columns can be obtained directly from the root
399 To enable items to be edited and selected, the \c flags() function needs
400 to be implemented so that it returns a combination of flags that includes
401 the Qt::ItemIsEditable and Qt::ItemIsSelectable flags as well as
404 \snippet itemviews/editabletreemodel/treemodel.cpp 3
406 \target TreeModel::index
407 The model needs to be able to generate model indexes to allow other
408 components to request data and information about its structure. This task
409 is performed by the \c index() function, which is used to obtain model
410 indexes corresponding to children of a given parent item:
412 \snippet itemviews/editabletreemodel/treemodel.cpp 5
414 In this model, we only return model indexes for child items if the parent
415 index is invalid (corresponding to the root item) or if it has a zero
418 We use the custom \l{TreeModel::getItem}{getItem()} function to obtain
419 a \c TreeItem instance that corresponds to the model index supplied, and
420 request its child item that corresponds to the specified row.
422 \snippet itemviews/editabletreemodel/treemodel.cpp 6
424 Since each item contains information for an entire row of data, we create
425 a model index to uniquely identify it by calling
426 \l{QAbstractItemModel::}{createIndex()} it with the row and column numbers
427 and a pointer to the item. In the \l{TreeModel::data}{data()} function,
428 we will use the item pointer and column number to access the data
429 associated with the model index; in this model, the row number is not
430 needed to identify data.
432 \target TreeModel::parent
433 The \c parent() function supplies model indexes for parents of items
434 by finding the corresponding item for a given model index, using its
435 \l{TreeItem::parent}{parent()} function to obtain its parent item,
436 then creating a model index to represent the parent. (See
437 \l{Relating-items-using-model-indexes}{the above diagram}).
439 \snippet itemviews/editabletreemodel/treemodel.cpp 7
441 Items without parents, including the root item, are handled by returning
442 a null model index. Otherwise, a model index is created and returned as
443 in the \l{TreeModel::index}{index()} function, with a suitable row number,
444 but with a zero column number to be consistent with the scheme used in
445 the \l{TreeModel::index}{index()} implementation.
447 \target TreeModel::data
448 \target TreeModel::setupModelData