QModelRoleDataSpan Class Reference

\inmodule QtCore More...

#include <qabstractitemmodel.h>

Collaboration diagram for QModelRoleDataSpan:

Public Member Functions
constexpr	QModelRoleDataSpan () noexcept
	Constructs an empty QModelRoleDataSpan.
constexpr	QModelRoleDataSpan (QModelRoleData &modelRoleData) noexcept
	Constructs an QModelRoleDataSpan spanning over modelRoleData, seen as a 1-element array.
constexpr	QModelRoleDataSpan (QModelRoleData *modelRoleData, qsizetype len)
	Constructs an QModelRoleDataSpan spanning over the array beginning at modelRoleData and with length len.
template<typename Container , if_compatible_container< Container > = true>
constexpr	QModelRoleDataSpan (Container &c) noexcept(noexcept(std::data(c)) &&noexcept(std::size(c)))
	Constructs an QModelRoleDataSpan spanning over the container c, which can be any contiguous container of QModelRoleData objects.
constexpr qsizetype	size () const noexcept
	Returns the length of the span represented by this object.
constexpr qsizetype	length () const noexcept
	Returns the length of the span represented by this object.
constexpr QModelRoleData *	data () const noexcept
	Returns a pointer to the beginning of the span represented by this object.
constexpr QModelRoleData *	begin () const noexcept
	Returns a pointer to the beginning of the span represented by this object.
constexpr QModelRoleData *	end () const noexcept
	Returns a pointer to the imaginary element one past the end of the span represented by this object.
constexpr QModelRoleData &	operator[] (qsizetype index) const
	Returns a modifiable reference to the QModelRoleData at position index in the span.
constexpr QVariant *	dataForRole (int role) const
	Returns the data associated with the first QModelRoleData in the span that has its role equal to role.

Detailed Description

\inmodule QtCore

Since

6.0

The QModelRoleDataSpan class provides a span over QModelRoleData objects.

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}, QAbstractItemModel::multiData()

Definition at line 65 of file qabstractitemmodel.h.

Constructor & Destructor Documentation

QModelRoleDataSpan() [1/4]

QModelRoleDataSpan::QModelRoleDataSpan ( )

inlineconstexprnoexcept

Constructs an empty QModelRoleDataSpan.

Its data() will be set to \nullptr, and its length to zero.

Definition at line 74 of file qabstractitemmodel.h.

QModelRoleDataSpan() [2/4]

QModelRoleDataSpan::QModelRoleDataSpan ( QModelRoleData &  modelRoleData )

inlineconstexprnoexcept

Constructs an QModelRoleDataSpan spanning over modelRoleData, seen as a 1-element array.

Definition at line 76 of file qabstractitemmodel.h.

QModelRoleDataSpan() [3/4]

QModelRoleDataSpan::QModelRoleDataSpan ( QModelRoleData *  modelRoleData, qsizetype  len  )

inlineconstexpr

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.

Definition at line 81 of file qabstractitemmodel.h.

QModelRoleDataSpan() [4/4]

template<typename Container , if_compatible_container< Container > = true>

template< typename Container > QModelRoleDataSpan::QModelRoleDataSpan ( Container &  c )

inlineconstexprnoexcept

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.

Definition at line 87 of file qabstractitemmodel.h.

Member Function Documentation

begin()

QModelRoleData * QModelRoleDataSpan::begin ( ) const

inlineconstexprnoexcept

Returns a pointer to the beginning of the span represented by this object.

Definition at line 95 of file qabstractitemmodel.h.

Referenced by dataForRole().

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data()

QModelRoleData * QModelRoleDataSpan::data ( ) const

inlineconstexprnoexcept

Returns a pointer to the beginning of the span represented by this object.

Definition at line 94 of file qabstractitemmodel.h.

dataForRole()

const QVariant * QModelRoleDataSpan::dataForRole ( int  role ) const

inlineconstexpr

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()

Definition at line 99 of file qabstractitemmodel.h.

References begin(), e, end(), Q_ASSERT, and roleData.

Referenced by QStyledItemDelegate::initStyleOption().

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end()

QModelRoleData * QModelRoleDataSpan::end ( ) const

inlineconstexprnoexcept

Returns a pointer to the imaginary element one past the end of the span represented by this object.

Definition at line 96 of file qabstractitemmodel.h.

Referenced by dataForRole().

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length()

qsizetype QModelRoleDataSpan::length ( ) const

inlineconstexprnoexcept

Returns the length of the span represented by this object.

Definition at line 93 of file qabstractitemmodel.h.

operator[]()

QModelRoleData & QModelRoleDataSpan::operator[] ( qsizetype  index ) const

inlineconstexpr

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()).

Definition at line 97 of file qabstractitemmodel.h.

size()

qsizetype QModelRoleDataSpan::size ( ) const

inlineconstexprnoexcept

Returns the length of the span represented by this object.

Definition at line 92 of file qabstractitemmodel.h.

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The documentation for this class was generated from the following files:

• qtbase/src/corelib/itemmodels/qabstractitemmodel.h
• qtbase/src/corelib/itemmodels/qabstractitemmodel.cpp