QModelRoleDataSpan Class
The QModelRoleDataSpan class provides a span over QModelRoleData objects. More...
Header: | #include <QModelRoleDataSpan> |
CMake: | find_package(Qt6 COMPONENTS Core REQUIRED) target_link_libraries(mytarget PRIVATE Qt6::Core) |
qmake: | QT += core |
Since: | Qt 6.0 |
Public Functions
QModelRoleDataSpan(Container &c) | |
QModelRoleDataSpan(QModelRoleData *modelRoleData, qsizetype len) | |
QModelRoleDataSpan(QModelRoleData &modelRoleData) | |
QModelRoleDataSpan() | |
QModelRoleData * | begin() const |
QModelRoleData * | data() const |
QVariant * | dataForRole(int role) const |
QModelRoleData * | end() const |
qsizetype | length() const |
qsizetype | size() const |
QModelRoleData & | operator[](qsizetype index) const |
Detailed Description
A QModelRoleDataSpan is used as an abstraction over an array of QModelRoleData objects.
Like a view, QModelRoleDataSpan provides a small object (pointer and size) that can be passed to functions that need to examine the contents of the array. A QModelRoleDataSpan can be constructed from any array-like sequence (plain arrays, QVector, std::vector, QVarLengthArray, and so on). Moreover, it does not own the sequence, which must therefore be kept alive longer than any QModelRoleDataSpan objects referencing it.
Unlike a view, QModelRoleDataSpan is a span, so it allows for modifications to the underlying elements.
QModelRoleDataSpan's main use case is making it possible for a model to return the data corresponding to different roles in one call.
In order to draw one element from a model, a view (through its delegates) will generally request multiple roles for the same index by calling data()
as many times as needed:
QVariant text = model->data(index, Qt::DisplayRole); QVariant decoration = model->data(index, Qt::DecorationRole); QVariant checkState = model->data(index, Qt::CheckStateRole); // etc.
QModelRoleDataSpan allows a view to request the same data using just one function call.
This is achieved by having the view prepare a suitable array of QModelRoleData objects, each initialized with the role that should be fetched. The array is then wrapped in a QModelRoleDataSpan object, which is then passed to a model's multiData()
function.
std::array<QModelRoleData, 3> roleData = { { QModelRoleData(Qt::DisplayRole), QModelRoleData(Qt::DecorationRole), QModelRoleData(Qt::CheckStateRole) } }; // Usually, this is not necessary: A QModelRoleDataSpan // will be built automatically for you when passing an array-like // container to multiData(). QModelRoleDataSpan span(roleData); model->multiData(index, span); // Use roleData[0].data(), roleData[1].data(), etc.
Views are encouraged to store the array of QModelRoleData objects (and, possibly, the corresponding span) and re-use it in subsequent calls to the model. This allows to reduce the memory allocations related with creating and returning QVariant objects.
Finally, given a QModelRoleDataSpan object, the model's responsibility is to fill in the data corresponding to each role in the span. How this is done depends on the concrete model class. Here's a sketch of a possible implementation that iterates over the span and uses setData()
on each element:
void MyModel::multiData(const QModelIndex &index, QModelRoleDataSpan roleDataSpan) const { for (QModelRoleData &roleData : roleDataSpan) { int role = roleData.role(); // ... obtain the data for index and role ... roleData.setData(result); } }
See also Model/View Programming and QAbstractItemModel::multiData().
Member Function Documentation
template <typename Container, if_compatible_container<Container>> QModelRoleDataSpan::QModelRoleDataSpan(Container &c)
Constructs an QModelRoleDataSpan spanning over the container c, which can be any contiguous container of QModelRoleData objects. For instance, it can be a QVector<QModelRoleData>
, a std::array<QModelRoleData, 10>
and so on.
Note: The container must be kept alive as long as this object has not been destructed.
QModelRoleDataSpan::QModelRoleDataSpan(QModelRoleData *modelRoleData, qsizetype len)
Constructs an QModelRoleDataSpan spanning over the array beginning at modelRoleData and with length len.
Note: The array must be kept alive as long as this object has not been destructed.
QModelRoleDataSpan::QModelRoleDataSpan(QModelRoleData &modelRoleData)
Constructs an QModelRoleDataSpan spanning over modelRoleData, seen as a 1-element array.
QModelRoleDataSpan::QModelRoleDataSpan()
Constructs an empty QModelRoleDataSpan. Its data() will be set to nullptr
, and its length to zero.
QModelRoleData *QModelRoleDataSpan::begin() const
Returns a pointer to the beginning of the span represented by this object.
QModelRoleData *QModelRoleDataSpan::data() const
Returns a pointer to the beginning of the span represented by this object.
QVariant *QModelRoleDataSpan::dataForRole(int role) const
Returns the data associated with the first QModelRoleData in the span that has its role equal to role. If such a QModelRoleData object does not exist, the behavior is undefined.
Note: Avoid calling this function from the model's side, as a model cannot possibly know in advance which roles are in a given QModelRoleDataSpan. This function is instead suitable for views and delegates, which have control over the roles in the span.
See also QModelRoleData::data().
QModelRoleData *QModelRoleDataSpan::end() const
Returns a pointer to the imaginary element one past the end of the span represented by this object.
qsizetype QModelRoleDataSpan::length() const
Returns the length of the span represented by this object.
qsizetype QModelRoleDataSpan::size() const
Returns the length of the span represented by this object.
QModelRoleData &QModelRoleDataSpan::operator[](qsizetype index) const
Returns a modifiable reference to the QModelRoleData at position index in the span.
Note: index must be a valid index for this span (0 <= index < size()).