本质是层次结构的表（table），
If you do not make use of the hierarchy, then the model is a simple table of rows and columns. Each item has a unique index specified by a QModelIndex.
如果不关心层次，model就是简单的表。
Every item of data that can be accessed via a model has an associated model index. You can obtain this model index using the index() function. Each index may have a sibling() index; child items have a parent() index.
Each item has a number of data elements associated with it and they can be retrieved by specifying a role (see Qt::ItemDataRole) to the model's data() function. Data for all available roles can be obtained at the same time using the itemData() function.
Data for each role is set using a particular Qt::ItemDataRole. Data for individual roles are set individually with setData(), or they can be set for all roles with setItemData().
Items can be queried with flags() (see Qt::ItemFlag) to see if they can be selected, dragged, or manipulated in other ways.
If an item has child objects, hasChildren() returns true for the corresponding index.
The model has a rowCount() and a columnCount() for each level of the hierarchy. 每个层次都有rowCount和columnCount。Rows and columns can be inserted and removed with insertRows(), insertColumns(), removeRows(), andremoveColumns().
The model emits signals to indicate changes. For example, dataChanged() is emitted whenever items of data made available by the model are changed. Changes to the headers supplied by the model cause headerDataChanged() to be emitted. If the structure of the underlying data changes, the model can emit layoutChanged() to indicate to any attached views that they should redisplay any items shown, taking the new structure into account.
The items available through the model can be searched for particular data using the match() function.
To sort the model, you can use sort().
Subclassing
Note: Some general guidelines for subclassing models are available in the Model Subclassing Reference.
When subclassing QAbstractItemModel, at the very least you must implement index(), parent(), rowCount(), columnCount(), and data(). These functions are used in all read-only models, and form the basis of editable models.
You can also reimplement hasChildren() to provide special behavior for models where the implementation of rowCount() is expensive. This makes it possible for models to restrict the amount of data requested by views, and can be used as a way to implement lazy population of model data.
当rowCount开销很多时候，可以重定义hasChildren，这样可以使得model可以限制view要求的数据，可以实现lazy population。
To enable editing in your model, you must also implement setData(), and reimplement flags() to ensure that ItemIsEditable is returned. You can also reimplement headerData() andsetHeaderData() to control the way the headers for your model are presented.
The dataChanged() and headerDataChanged() signals must be emitted explicitly when reimplementing the setData() and setHeaderData() functions, respectively.
Custom models need to create model indexes for other components to use. To do this, call createIndex() with suitable row and column numbers for the item, and an identifier for it, either as a pointer or as an integer value. The combination of these values must be unique for each item. Custom models typically use these unique identifiers in other reimplemented functions to retrieve item data and access information about the item's parents and children. See the Simple Tree Model Example for more information about unique identifiers.
自定义model需要创建index。这要调用createIndex，用row，column和identifier（pointer或integer）。这些值的组合必须唯一。自定义model经常用这些唯一值来取得信息，看例子。
It is not necessary to support every role defined in Qt::ItemDataRole. Depending on the type of data contained within a model, it may only be useful to implement the data() function to return valid information for some of the more common roles. Most models provide at least a textual representation of item data for the Qt::DisplayRole, and well-behaved models should also provide valid information for the Qt::ToolTipRole and Qt::WhatsThisRole. Supporting these roles enables models to be used with standard Qt views. However, for some models that handle highly-specialized data, it may be appropriate to provide data only for user-defined roles.
Models that provide interfaces to resizable data structures can provide implementations of insertRows(), removeRows(), insertColumns(),and removeColumns(). When implementing these functions, it is important to notify any connected views about changes to the model's dimensions both before and after they occur:
可变大小的model需要通知view这些改变。
- An insertRows() implementation must call beginInsertRows() before inserting new rows into the data structure, and endInsertRows() immediately afterwards.
- An insertColumns() implementation must call beginInsertColumns() before inserting new columns into the data structure, and endInsertColumns() immediately afterwards.
- A removeRows() implementation must call beginRemoveRows() before the rows are removed from the data structure, and endRemoveRows() immediately afterwards.
- A removeColumns() implementation must call beginRemoveColumns() before the columns are removed from the data structure, and endRemoveColumns() immediately afterwards.
The private signals that these functions emit give attached components the chance to take action before any data becomes unavailable. The encapsulation of the insert and remove operations with these begin and end functions also enables the model to manage persistent model indexes correctly. If you want selections to be handled properly, you must ensure that you call these functions. If you insert or remove an item with children, you do not need to call these functions for the child items. In other words, the parent item will take care of its child items.
To create models that populate incrementally, you can reimplement fetchMore() and canFetchMore(). If the reimplementation of fetchMore() adds rows to the model, beginInsertRows() andendInsertRows() must be called.
See also Model Classes, Model Subclassing Reference, QModelIndex, QAbstractItemView, Using Drag and Drop with Item Views, Simple DOM Model Example, Simple Tree Model Example,Editable Tree Model Example, and Fetch More Example.
int QAbstractItemModel::columnCount ( const QModelIndex & parent = QModelIndex() ) const [pure virtual]
Returns the number of columns for the children of the given parent.
返回制定parent的children的column数
In most subclasses, the number of columns is independent of the parent.
For example:
int DomModel::columnCount(const QModelIndex &/*parent*/) const
{
return 3;
}
Note: When implementing a table based model, columnCount() should return 0 when the parent is valid.
table model 实现的columnCount中单parent合法时候要返回0
Here is an example implementation of insertRows assuming the model holds its data in a QList<QPair<QString, int> > called m_rows:
bool CustomModel::insertRows(int row, int count, const QModelIndex &parent){
if(parent.isValid())
return false; // the model is flat 没有层次结构的子表
if(rowCount()<row)
row = rowCount()+1;
QPair<QString, int> pair(QString::null, 0);
beginInsertRows(QModelIndex(), row, row+count); // new rows get indexes [row ; row+count]
for(int i=0;i<count;i++){
m_rows.insert(row+i, pair);
}
endInsertRows();
return true;
}