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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()}, and
52 \l{QAbstractItemModel::}{rowCount()}. In addition, hierarchical models,
53 such as this one, need to provide implementations of
54 \l{QAbstractItemModel::}{index()} and \l{QAbstractItemModel::}{parent()}.
56 An editable model needs to provide implementations of
57 \l{QAbstractItemModel::}{setData()} and
58 \l{QAbstractItemModel::}{headerData()}, and must return a suitable
59 combination of flags from its \l{QAbstractItemModel::}{flags()} function.
61 Since this example allows the dimensions of the model to be changed,
62 we must also implement \l{QAbstractItemModel::}{insertRows()},
63 \l{QAbstractItemModel::}{insertColumns()},
64 \l{QAbstractItemModel::}{removeRows()}, and
65 \l{QAbstractItemModel::}{removeColumns()}.
69 As with the \l{itemviews/simpletreemodel}{Simple Tree Model} example,
70 the model simply acts as a wrapper around a collection
71 of instances of a \c TreeItem class. Each \c TreeItem is designed to
72 hold data for a row of items in a tree view, so it contains a list of
73 values corresponding to the data shown in each column.
75 Since QTreeView provides a row-oriented view onto a model, it is
76 natural to choose a row-oriented design for data structures that
77 will supply data via a model to this kind of view. Although this makes
78 the tree model less flexible, and possibly less useful for use with
79 more sophisticated views, it makes it less complex to design and easier
82 \target Relations-between-internal-items
84 \row \o \inlineimage itemviews-editabletreemodel-items.png
85 \o \bold{Relations between internal items}
87 When designing a data structure for use with a custom model, it is useful
88 to expose each item's parent via a function like
89 \l{TreeItem::parent}{TreeItem::parent()} because it will make
90 writing the model's own \l{QAbstractItemModel::}{parent()} function easier.
91 Similarly, a function like \l{TreeItem::child}{TreeItem::child()} is
92 helpful when implementing the model's \l{QAbstractItemModel::}{index()}
93 function. As a result, each \c TreeItem maintains information about
94 its parent and children, making it possible for us to traverse the tree
97 The diagram shows how \c TreeItem instances are connected via their
98 \l{TreeItem::parent}{parent()} and \l{TreeItem::child}{child()}
101 In the example shown, two top-level items, \bold{A} and
102 \bold{B}, can be obtained from the root item by calling its child()
103 function, and each of these items return the root node from their
104 parent() functions, though this is only shown for item \bold{A}.
107 Each \c TreeItem stores data for each column in the row it represents
108 in its \c itemData private member (a list of QVariant objects).
109 Since there is a one-to-one mapping between each column in the view
110 and each entry in the list, we provide a simple
111 \l{TreeItem::data}{data()} function to read entries in the \c itemData
112 list and a \l{TreeItem::setData}{setData()} function to allow them to
114 As with other functions in the item, this simplifies the implemention
115 of the model's \l{QAbstractItemModel::}{data()} and
116 \l{QAbstractItemModel::}{setData()} functions.
118 We place an item at the root of the tree of items. This root item
119 corresponds to the null model index, \l{QModelIndex::}{QModelIndex()},
120 that is used to represent the parent of a top-level item when handling
122 Although the root item does not have a visible representation in any of
123 the standard views, we use its internal list of QVariant objects to
124 store a list of strings that will be passed to views for use as
125 horizontal header titles.
128 \row \o \inlineimage itemviews-editabletreemodel-model.png
129 \o \bold{Accessing data via the model}
131 In the case shown in the diagram, the piece of information represented
132 by \bold{a} can be obtained using the standard model/view API:
134 \snippet doc/src/snippets/code/doc_src_examples_editabletreemodel.cpp 0
136 Since each items holds pieces of data for each column in a given row,
137 there can be many model indexes that map to the same \c TreeItem object.
138 For example, the information represented by \bold{b} can be obtained
139 using the following code:
141 \snippet doc/src/snippets/code/doc_src_examples_editabletreemodel.cpp 1
143 The same underlying \c TreeItem would be accessed to obtain information
144 for the other model indexes in the same row as \bold{b}.
147 In the model class, \c TreeModel, we relate \c TreeItem objects to
148 model indexes by passing a pointer for each item when we create its
149 corresponding model index with QAbstractItemModel::createIndex() in
150 our \l{TreeModel::index}{index()} and \l{TreeModel::parent}{parent()}
152 We can retrieve pointers stored in this way by calling the
153 \l{QModelIndex::}{internalPointer()} function on the relevant model
154 index - we create our own \l{TreeModel::getItem}{getItem()} function to
155 do this work for us, and call it from our \l{TreeModel::data}{data()}
156 and \l{TreeModel::parent}{parent()} implementations.
158 Storing pointers to items is convenient when we control how they are
159 created and destroyed since we can assume that an address obtained from
160 \l{QModelIndex::}{internalPointer()} is a valid pointer.
161 However, some models need to handle items that are obtained from other
162 components in a system, and in many cases it is not possible to fully
163 control how items are created or destroyed. In such situations, a pure
164 pointer-based approach needs to be supplemented by safeguards to ensure
165 that the model does not attempt to access items that have been deleted.
168 \row \o \bold{Storing information in the underlying data structure}
170 Several pieces of data are stored as QVariant objects in the \c itemData
171 member of each \c TreeItem instance
173 The diagram shows how pieces of information,
174 represented by the labels \bold{a}, \bold{b} and \bold{c} in the
175 previous two diagrams, are stored in items \bold{A}, \bold{B} and
176 \bold{C} in the underlying data structure. Note that pieces of
177 information from the same row in the model are all obtained from the
178 same item. Each element in a list corresponds to a piece of information
179 exposed by each column in a given row in the model.
181 \o \inlineimage itemviews-editabletreemodel-values.png
184 Since the \c TreeModel implementation has been designed for use with
185 QTreeView, we have added a restriction on the way it uses \c TreeItem
186 instances: each item must expose the same number of columns of data.
187 This makes viewing the model consistent, allowing us to use the root
188 item to determine the number of columns for any given row, and only
189 adds the requirement that we create items containing enough data for
190 the total number of columns. As a result, inserting and removing
191 columns are time-consuming operations because we need to traverse the
192 entire tree to modify every item.
194 An alternative approach would be to design the \c TreeModel class so
195 that it truncates or expands the list of data in individual \c TreeItem
196 instances as items of data are modified. However, this "lazy" resizing
197 approach would only allow us to insert and remove columns at the end of
198 each row and would not allow columns to be inserted or removed at
199 arbitrary positions in each row.
201 \target Relating-items-using-model-indexes
204 \o \inlineimage itemviews-editabletreemodel-indexes.png
205 \o \bold{Relating items using model indexes}
207 As with the \l{itemviews/simpletreemodel}{Simple Tree Model} example,
208 the \c TreeModel needs to be able to take a model index, find the
209 corresponding \c TreeItem, and return model indexes that correspond to
210 its parents and children.
212 In the diagram, we show how the model's \l{TreeModel::parent}{parent()}
213 implementation obtains the model index corresponding to the parent of
214 an item supplied by the caller, using the items shown in a
215 \l{Relations-between-internal-items}{previous diagram}.
217 A pointer to item \bold{C} is obtained from the corresponding model index
218 using the \l{QModelIndex::internalPointer()} function. The pointer was
219 stored internally in the index when it was created. Since the child
220 contains a pointer to its parent, we use its \l{TreeItem::parent}{parent()}
221 function to obtain a pointer to item \bold{B}. The parent model index is
222 created using the QAbstractItemModel::createIndex() function, passing
223 the pointer to item \bold{B} as the internal pointer.
226 \section1 TreeItem Class Definition
228 The \c TreeItem class provides simple items that contain several
229 pieces of data, and which can provide information about their parent
232 \snippet examples/itemviews/editabletreemodel/treeitem.h 0
234 We have designed the API to be similar to that provided by
235 QAbstractItemModel by giving each item functions to return the number
236 of columns of information, read and write data, and insert and remove
237 columns. However, we make the relationship between items explicit by
238 providing functions to deal with "children" rather than "rows".
240 Each item contains a list of pointers to child items, a pointer to its
241 parent item, and a list of QVariant objects that correspond to
242 information held in columns in a given row in the model.
244 \section1 TreeItem Class Implementation
246 Each \c TreeItem is constructed with a list of data and an optional
249 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 0
251 Initially, each item has no children. These are added to the item's
252 internal \c childItems member using the \c insertChildren() function
255 The destructor ensures that each child added to the item is deleted
256 when the item itself is deleted:
258 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 1
260 \target TreeItem::parent
261 Since each item stores a pointer to its parent, the \c parent() function
264 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 9
266 \target TreeItem::child
267 Three functions provide information about the children of an item.
268 \c child() returns a specific child from the internal list of children:
270 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 2
272 The \c childCount() function returns the total number of children:
274 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 3
276 The \c childNumber() function is used to determine the index of the child
277 in its parent's list of children. It accesses the parent's \c childItems
278 member directly to obtain this information:
280 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 4
282 The root item has no parent item; for this item, we return zero to be
283 consistent with the other items.
285 The \c columnCount() function simply returns the number of elements in
286 the internal \c itemData list of QVariant objects:
288 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 5
290 \target TreeItem::data
291 Data is retrieved using the \c data() function, which accesses the
292 appropriate element in the \c itemData list:
294 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 6
296 \target TreeItem::setData
297 Data is set using the \c setData() function, which only stores values
298 in the \c itemData list for valid list indexes, corresponding to column
301 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 11
303 To make implementation of the model easier, we return true to indicate
304 whether the data was set successfully, or false if an invalid column
306 Editable models often need to be resizable, enabling rows and columns to
307 be inserted and removed. The insertion of rows beneath a given model index
308 in the model leads to the insertion of new child items in the corresponding
309 item, handled by the \c insertChildren() function:
311 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 7
313 This ensures that new items are created with the required number of columns
314 and inserted at a valid position in the internal \c childItems list.
315 Items are removed with the \c removeChildren() function:
317 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 10
319 As discussed above, the functions for inserting and removing columns are
320 used differently to those for inserting and removing child items because
321 they are expected to be called on every item in the tree. We do this by
322 recursively calling this function on each child of the item:
324 \snippet examples/itemviews/editabletreemodel/treeitem.cpp 8
326 \section1 TreeModel Class Definition
328 The \c TreeModel class provides an implementation of the QAbstractItemModel
329 class, exposing the necessary interface for a model that can be edited and
332 \snippet examples/itemviews/editabletreemodel/treemodel.h 0
334 The constructor and destructor are specific to this model.
336 \snippet examples/itemviews/editabletreemodel/treemodel.h 1
338 Read-only tree models only need to provide the above functions. The
339 following public functions provide support for editing and resizing:
341 \snippet examples/itemviews/editabletreemodel/treemodel.h 2
343 To simplify this example, the data exposed by the model is organized into
344 a data structure by the model's \l{TreeModel::setupModelData}{setupModelData()}
345 function. Many real world models will not process the raw data at all, but
346 simply work with an existing data structure or library API.
348 \section1 TreeModel Class Implementation
350 The constructor creates a root item and initializes it with the header
353 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 0
355 We call the internal \l{TreeModel::setupModelData}{setupModelData()}
356 function to convert the textual data supplied to a data structure we can
357 use with the model. Other models may be initialized with a ready-made
358 data structure, or use an API to a library that maintains its own data.
360 The destructor only has to delete the root item; all child items will
361 be recursively deleted by the \c TreeItem destructor.
363 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 1
365 \target TreeModel::getItem
366 Since the model's interface to the other model/view components is based
367 on model indexes, and the internal data structure is item-based, many of
368 the functions implemented by the model need to be able to convert any
369 given model index to its corresponding item. For convenience and
370 consistency, we have defined a \c getItem() function to perform this
373 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 4
375 This function assumes that each model index it is passed corresponds to
376 a valid item in memory. If the index is invalid, or its internal pointer
377 does not refer to a valid item, the root item is returned instead.
379 The model's \c rowCount() implementation is simple: it first uses the
380 \c getItem() function to obtain the relevant item, then returns the
381 number of children it contains:
383 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 8
385 By contrast, the \c columnCount() implementation does not need to look
386 for a particular item because all items are defined to have the same
387 number of columns associated with them.
389 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 2
391 As a result, the number of columns can be obtained directly from the root
394 To enable items to be edited and selected, the \c flags() function needs
395 to be implemented so that it returns a combination of flags that includes
396 the Qt::ItemIsEditable and Qt::ItemIsSelectable flags as well as
399 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 3
401 \target TreeModel::index
402 The model needs to be able to generate model indexes to allow other
403 components to request data and information about its structure. This task
404 is performed by the \c index() function, which is used to obtain model
405 indexes corresponding to children of a given parent item:
407 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 5
409 In this model, we only return model indexes for child items if the parent
410 index is invalid (corresponding to the root item) or if it has a zero
413 We use the custom \l{TreeModel::getItem}{getItem()} function to obtain
414 a \c TreeItem instance that corresponds to the model index supplied, and
415 request its child item that corresponds to the specified row.
417 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 6
419 Since each item contains information for an entire row of data, we create
420 a model index to uniquely identify it by calling
421 \l{QAbstractItemModel::}{createIndex()} it with the row and column numbers
422 and a pointer to the item. In the \l{TreeModel::data}{data()} function,
423 we will use the item pointer and column number to access the data
424 associated with the model index; in this model, the row number is not
425 needed to identify data.
427 \target TreeModel::parent
428 The \c parent() function supplies model indexes for parents of items
429 by finding the corresponding item for a given model index, using its
430 \l{TreeItem::parent}{parent()} function to obtain its parent item,
431 then creating a model index to represent the parent. (See
432 \l{Relating-items-using-model-indexes}{the above diagram}).
434 \snippet examples/itemviews/editabletreemodel/treemodel.cpp 7
436 Items without parents, including the root item, are handled by returning
437 a null model index. Otherwise, a model index is created and returned as
438 in the \l{TreeModel::index}{index()} function, with a suitable row number,
439 but with a zero column number to be consistent with the scheme used in
440 the \l{TreeModel::index}{index()} implementation.
442 \target TreeModel::data
443 \target TreeModel::setupModelData