Grantlee: Generic type and template support

Grantlee 5.3.0

Generic type and template support

# Generic type support

Grantlee offers powerful support for using any type or container in a QVariant as part of the Context. Qt introspection based on Q_PROPERTY is the most convenient way to access properties on QObject derived type or types decorated with the `Q_GADGET` macro. However, sometimes it is necessary to use classes which do can't have Q_PROPERTY macros (perhaps because they are defined in third-party headers) and where it would not be practical to create QObject/`Q_GADGET` wrappers around all related classes.

In such cases the metatype can be registered with Grantlee and an introspection method can be written.

// Non-QObject
class Person
{
public:
Person() :age(0) {}
Person(const QString &name, int age)
: m_name(name), m_age(age)
{
}
QString name() const
{
return m_name;
}
int age() const
{
return m_age;
}
private:
QString m_name;
int m_age;
};
// Make it possible to put Person in a QVariant.
Q_DECLARE_METATYPE(Person)
// Read-only introspection of Person object.
GRANTLEE_BEGIN_LOOKUP(Person)
if ( property == "name" )
return object.name();
else if ( property == "age" )
return object.age();
GRANTLEE_END_LOOKUP
void someInitializer()
{
// Register the Person type with the %Grantlee introspection system.
Grantlee::registerMetaType<Person>();
}
Grantlee::Context getContext()
{
Grantlee::Context c;
Person leader("The Leader", 19);
QList<Person> followers;
for (int i = 0; i < 3; ++i)
{
Person follower("Follower" + QString::number(i), 18);
people.append(follower);
}
c.insert("leader", QVariant::fromValue(leader));
c.insert("followers", QVariant::fromValue(followers));
return c;
}
QString createOutput()
{
Template t = m_engine->newTemplate(
"<h1>{{ leader.name }}</h1>"
"<ul>"
"{% for person in followers %}"
"<li>{{ person.name }}, {{ person.age }}</li>"
"{% endfor %}"
"</ul>"
);
Context c = getContext();
return t->render(&c);
}
Grantlee::Context
The Context class holds the context to render a Template with.
Definition: context.h:118
Grantlee::Context::insert
void insert(const QString &name, QObject *object)
GRANTLEE_BEGIN_LOOKUP
#define GRANTLEE_BEGIN_LOOKUP(Type)
Definition: metatype.h:213
GRANTLEE_END_LOOKUP
#define GRANTLEE_END_LOOKUP
Definition: metatype.h:239

There are several necessary steps and consequences.

- The type must be registered as a QMetaType with `Q_DECLARE_METATYPE`. Note that this is not needed for QObject derived types.
- All containers are supported. (See Generic container support)
- The `GRANTLEE_BEGIN_LOOKUP` and `GRANTLEE_END_LOOKUP` macros help to define the introspection of the type. Between them is the definition of a method with the signature `QVariant getProperty(const Type &object, const QString &property)`.
- Grantlee::registerMetaType must be called at some point in the program before attempting to use the type in a Context.
- The Context is created and used as normal.

# Generic container support

Grantlee supports most Qt and STL containers by default if they are registered with the QMetaType system as shown in Generic type support. Where a container does not have built in support it can easily be added (See Third party containers).

The following containers have built in support:

- QList<T>
- QVector<T>
- QSet<T>
- QLinkedList<T>
- QStack<T>
- QQueue<T>
- std::vector<T>
- std::deque<T>
- std::list<T>
- QHash<QString, T>
- QHash<qint16, T>
- QHash<qint32, T>
- QHash<qint64, T>
- QHash<quint16, T>
- QHash<quint32, T>
- QHash<quint64, T>
- QMap<QString, T>
- QMap<qint16, T>
- QMap<qint32, T>
- QMap<qint64, T>
- QMap<quint16, T>
- QMap<quint32, T>
- QMap<quint64, T>
- std::map<QString, T>
- std::map<qint16, T>
- std::map<qint32, T>
- std::map<qint64, T>
- std::map<quint16, T>
- std::map<quint32, T>
- std::map<quint64, T>

where T is one of

- bool
- qint16
- qint32
- qint64
- quint16
- quint32
- quint64
- float
- double
- QVariant
- QString
- QDateTime
- A pointer to a type which inherits QObject
- Any type registered with Grantlee::registerMetaType
- Any supported container

Note that QSet<T> is an exception and will only work with types for which `qHash(T)` is defined. This means that QSet<QVariant> is not possible for example.

Note also that containers of pointers to QObject derived types can be stored in containers, and they do not need to be explicitly registered with Grantlee. Where the class has sufficient `Q_PROPERTY`s defined, the introspection method described above with `GRANTLEE_BEGIN_LOOKUP` and `GRANTLEE_END_LOOKUP` is also not necessary. Note also that any type or container can be used through a `Q_PROPERTY`.

class PersonObject : public QObject
{
Q_OBJECT
Q_PROPERTY(QString name READ name)
Q_PROPERTY(int age READ age)
public:
PersonObject(const QString &name, int age, QObject *parent = 0);
QString name() const;
int age() const;
};
class SportsClub : public QObject
{
Q_OBJECT
Q_PROPERTY(QString name READ name)
Q_PROPERTY(QString sport READ sport)
Q_PROPERTY(std::vector<QObject*> members READ members)
public:
SportsClub(const QString &name, const QString &sport, QObject *parent = 0);
QString name() const;
QString sport() const;
std::vector<QObject*> members() const;
void setMembers(std::vector<QObject*> members);
};
void someInitializer()
{
// QObject* already has built in support. No need to register the types
// with Grantlee::registerMetaType
}
Grantlee::Context SomeClass::getContext()
{
Grantlee::Context c;
QSet<QObject*> clubs;
{
auto club = new SportsClub("Smithfield Tennis Club", "Tennis", this);
std::vector<QObject*> members;
{
auto member = new PersonObject("Alice", 21, this);
members.push_back(member);
}
{
auto member = new PersonObject("Bob", 22, this);
members.push_back(member);
}
club.setMembers(members);
}
// ... specify other clubs and members
c.insert("sportsClubs", QVariant::fromValue(clubs));
return c;
}
QString createOutput()
{
auto t = m_engine->newTemplate(
"{% regroup sportsClubs by sport as groupedSports %}"
"{% for groupedClub in groupedSports %}"
"<h1>{{ groupedClub.grouper }}</h1>"
"{% for club in groupedClub.list %}"
"<h2>{{ club.name }}</h2>"
"<ul>"
"{% for member in club.members %}"
"<li>{{ member.name, }}, {{ member.age }}"
"{% endfor %}"
"</ul>"
"{% endfor %}"
"{% endfor %}"
);
auto c = getContext();
return t->render(&c);
}

See also
  The regroup tag

The output would be something like

      <h1>Tennis</h1>
        <h2>Smithfield Tennis Club</h2>
        <ul>
          <li>Alice, 21</li>
          <li>Bob, 22</li>
        </ul>
        <h2>Greenore Lawn Tennis and Fitness Club</h2>
        <ul>
          <li>Charlie, 23</li>
          <li>David, 24</li>
          <li>Elaine, 25</li>
          <li>Frank, 26</li>
        </ul>
      <h1>Basketball</h1>
        <h2>Sandyford Basketball Club</h2>
        <ul>
          <li>Gilian, 27</li>
          <li>Henry, 28</li>
        </ul>

Of course, it is possible to use containers of pointers to concrete QObject subclasses, such as QSet<PersonObject*> and std::vector<SportsClub*> too.

Because any supported container can also be used as a contained type, nested containers such as `QVector<QList<PersonObject*>>` are also supported.

Note that if a type is registered with Grantlee::registerMetaType, built in containers of that type do not also need to be registered. Third party containers do need to be registered however (See Third party containers)

Q_DECLARE_METATYPE(Person)
void someInitializer()
{
Grantlee::registerMetaType<Person>();
// Now any of the nested containers can be put
// in a Context and used in a Template.
}

# Third party containers

To support another, non-built in container it is necessary to use some macros to register it with Grantlee.

For a sequential container, `Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE`, and `Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE` are needed.

#include <boost/circular_buffer>
// Enable looping over the contents of the container
Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(boost::circular_buffer)
Q_DECLARE_METATYPE(boost::circular_buffer<Person>)
void someInitializer()
{
Grantlee::registerMetaType<Person>();
}
Grantlee::Context getContext()
{
Grantlee::Context c;
boost::circular_buffer<Person> buffer(5);
// loop
{
Person p("Grant", 21);
b.push_back(p);
}
c.insert("people", QVariant::fromValue(buffer));
}

See also
  Variable lookups

For associative containers `Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE` is needed.

# Smart Pointers

Shared pointer types containing a custom type should be introspected as normal using `GRANTLEE_BEGIN_LOOKUP` and `GRANTLEE_END_LOOKUP`

Q_DECLARE_METATYPE(QSharedPointer<Person>)
void someInitializer()
{
Grantlee::registerMetaType<QSharedPointer<Person> >();
}
GRANTLEE_BEGIN_LOOKUP(QSharedPointer<Person>)
if (property == "name")
return object->name();
GRANTLEE_END_LOOKUP

Note that if only shared pointers to the type is in your introspectable API you only need to define the property access for the shared pointer. In the case above, there is no need to use `Q_DECLARE_METATYPE` or `GRANTLEE_BEGIN_LOOKUP` with `Person` or `Person*`.

This is of course true of any smart pointer:

Q_DECLARE_METATYPE(boost::shared_ptr<Person>)
GRANTLEE_BEGIN_LOOKUP(boost::shared_ptr<Person>)
if (property == "name")
return object->name();
GRANTLEE_END_LOOKUP

QSharedPointers containing QObject derived types get special treatment.

QObjects are automatically introspected for their `Q_PROPERTY`s (See Custom objects).

If you have QSharedPointer<PersonObject> where `PersonObject` is derived from QObject it will be automatically introspected just like a QObject* is without requiring the `GRANTLEE_BEGIN_LOOKUP` and `GRANTLEE_END_LOOKUP` macros, the `Q_DECLARE_METATYPE` macro, or registration with the Grantlee metatype system. All of the access registration is handled by Qt.

void getContext()
{
Grantlee::Context c;
auto p = QSharedPointer<PersonObject>::create();
c.insert("person", QVariant::fromValue(p));
return c;
}
QString createOutput()
{
// Uses Q_PROPERTYs defined on PersonObject for name and age
auto t = m_engine->newTemplate( "{{ person.name }}, {{ person.age }}" );
auto c = getContext();
return t->render(&c);
}

The same technique can be used to support QObject derived types in third party shared pointers, but that requires an extra macro, `Q_DECLARE_SMART_POINTER_METATYPE`.

Q_DECLARE_SMART_POINTER_METATYPE(boost::shared_ptr)
void getContext()
{
Grantlee::Context c;
boost::shared_ptr<PersonObject> p( new PersonObject );
c.insert("person", QVariant::fromValue(p));
return c;
}