Rendering Templates

Creating Templates

Rendering templates is also very easy in application code. A single Template may be rendered multiple times with different Context objects.

Grantlee::Engine *engine = new Grantlee::Engine( this );
Grantlee::Template t = engine->newTemplate("My name is {{ name }}.", "my_template_name");
QVariantHash mapping;
mapping.insert("name", "Grainne");
Grantlee::Context c(mapping);
t->render(&c); // Returns "My name is Grainne."
mapping.insert("name", "Henry");
c = Grantlee::Context(mapping);
t->render(&c); // Returns "My name is Henry."

Ususally, applications will not create a Template directly, but instead use a Grantlee::Engine to load external files. This allows artists to define the template without needing to recompile.

Grantlee::Template t = engine->loadByName("template.html");
t->render(&c)

Variables

A Context object maps a string to another object for reference in the template. String keys in the Context object are available as variables in the Template, and can be used with the {{ variable }} syntax or inside {% control tags %}. In the above example, we mapped the string "name" to the string "Grainne" and then to the string "Henry". We can create more than just string mappings though.

mapping.insert("myint", 6); // Can insert ints
mapping.insert("mydouble", 6.5); // Can insert doubles
mapping.insert("mybool", false); // Can insert bools
QVariantList mylist;
mylist << "Ingrid" << 3;
mapping.insert("mylist", mylist); // Can insert QVariantList
QVariantHash myhash;
myhash.insert("James", "Artist");
myhash.insert("Kiera", "Dreamer");
mapping.insert("myhash", myhash); // Can insert QVariantHash
QObject *obj;
QVariant objVar = QVariant::fromValue(obj);
mapping.insert("myobject", objVar); // Can insert QObject*

It is additionally possible to insert any type of object or any container (not just QVariantHash and QVariantList) into the Context.

See also: Generic type and template support.

Extending Grantlee

Grantlee has 5 extension points.

Custom object variables
Generic types and containers
Filters
Tags
Loaders

Custom objects

As already noted, QObject* can be inserted into templates. The recommended way to insert custom objects into templates is to create QObject wrappers for your objects. As QObject is introspectable, this will allow lookups to work in a way you define.

Note: If you are already familiar with Django you will know that creating wrappers is not necessary in Django. That is because python objects are already fully introspectable.

#include "myperson.h"
class PersonWrapper : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QString name READ name)
public:
  PersonWrapper(const QString &name, int age);
  QString name() const;
  int age() const;
  QString clear();
};
...
PersonWrapper *linda = new PersonWrapper("Linda", 21);
QVariant lindaVar = QVariant::fromValue<QObject*>(linda);
mapping.insert("person", lindaVar);
...
The name is {{ person.name }} and age is {{ person.age }}.
// Will be rendered as
// The name is Linda and age is .

Note that the 'name' of person is accessible in the template, but the 'age' is not. Note also that rendering fails silently if the method can not be found. Only methods which have a corresponding Q_PROPERTY declaration are accessible from templates. To make age accessible in the template, we would need to add

  Q_PROPERTY(int age READ age)

to the class. Note also that those methods are const. Rendering a template should never change an object it is rendering. Always make sure the READ properties of your wrapper objects are const. It is also possible to lookup dynamic properties of QObjects. In the case of dynamic properties, the property name must be UTF-8 encoded.

Finally, note that the person object (Linda) is inserted into the QVariant as a QObject*, not as a PersonWrapper*. Otherwise, the compiler would generate an error message about an unregistered datatype PersonWrapper*. While using Q_DECLARE_METATYPE(PersonWrapper*) would make the compiler error go away, it would still not be possible to use the Linda object in Grantlee templates. Using Grantlee::registerMetaType<PersonWrapper*>() would work, but it is recommended to use QObject* instead. See also Custom Object Properties.

If you already have QObjects in your application which would be suitable for use in templates, you only need to add some Q_PROPERTY entries to the classes to make them accessible to Grantlee.

Note: If you are familiar with Django you may be aware of the alters_data attribute for methods. This method of using wrappers and const is the equivalent to the alters_data attribute. You the wrapper writer choose the properties which will be accessible from the templates and makes them const, so there's no need to mark other methods as alters_data.

For most uses of Grantlee, this is enough to get make an application themable easily and quickly. For more advanced usage, see Extending the template system. For some example uses of Grantlee, see Examples of Grantlee use.

Custom Object Properties

In Qt4 there are some limitations regarding the types that may be used in Q_PROPERTY entries which should be read by Grantlee.

We might define a Home object like this:

class Home : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int houseNumber READ houseNumber)
    Q_PROPERTY(QString streetName READ streetName)
    Q_PROPERTY(QString city READ city)
public:
    // ...
};
Q_DECLARE_METATYPE(Home*)

And we may wish to use it in our PersonWrapper object:

class PersonWrapper : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QString name READ name)
  Q_PROPERTY(Home* home READ home)
public:
  PersonWrapper(const QString &name, int age, Home *home);
  Home* home() const;
  // ...
}

And then use it in a template:

<h1>{{ person.name }}</h1>
House number: {{ person.home.houseNumber }}
Street: {{ person.home.streetName }}

This will not work with Qt4. The home property will not be readable and there will be a message about an unregistered datatype 'Home*'.

The workaround is to specify the type of the home property to be a QObject* instead of a Home*:

class PersonWrapper : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QString name READ name)
  Q_PROPERTY(QObject* home READ scriptableHome)
public:
  PersonWrapper(const QString &name, int age, Home *home);
  QObject* scriptableHome() const { return home(); }
  Home* home() const;
  // ...
}

In this example, we leave the home() method intact because we may want to use the Home* type from C++, but we add a scriptableHome() method which returns a QObject* instead of a Home*. This method is then used in the READ component of the Q_PROPERTY.

The above is the recommended method of using Q_PROPERTY with QObject derived types, however, if the class containing the Q_PROPERTY (PersonWrapper in the example above) can not be modified, it is still possible to make the property accessible to Grantlee by using the Grantlee::registerMetaType() method near the start of the program.

// Somewhere among other initializers
Grantlee::registerMetaType<Home*>();
// ...
// Creating the context.
PersonWrapper *person = new PersonWrapper;
context.insert("person", person);
t->render(&c);
// ...
// Template can now use home property as normal. Eg
 Street: {{ person.home.streetName }}

Note that prior to Grantlee version 0.1.9, Grantlee::registerMetaType<Home*, QObject*>(); must be used instead.

Using containers such as QList with Q_PROPERTY macro is also possible.

class PersonWrapper : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QString name READ name)
  Q_PROPERTY(QVariant friends READ friends)
public:
  PersonWrapper(const QString &name, int age, Home *home);
  QVariant friends() const { return QVariant::fromValue(m_friends); }
  void setFriends(const QList<QObject*> friends) { m_friends = friends; }
  // ...
private:
  QList<QObject*> m_friends;
};
Q_DECLARE_METATYPE(QList<QObject*>)

It is also necessary to register QList<QObject*> with the Qt metatype system and with Grantlee

Grantlee::registerSequentialContainer<QList<QObject*> >();

generic_variables

An alternative to writing wrapper QObject classes with Q_PROPERTY macros is to use the Grantlee::MetaType system to provide introspection for any type or any container. This subject is treated in detail in Generic type and template support

Enumerators

Grantlee has built-in support for enumerators which use the Q_ENUMS macro.

class MyClass : public QObject
{
  Q_OBJECT
  Q_ENUMS(PersonName)
  Q_PROPERTY(PersonName personName READ personName)
public:
  enum PersonName
  {
    Mike,
    Natalie,
    Oliver,
    Patrica = 9
    Quentin
  };
  MyClass(QObject *parent = 0 );
  PersonName personName() const { return Quentin; }
};
...
{
  QObject *object = new MyClass(this);
  context.insert("myObj", object);
  t->render(context);
}

The enums can be used and accessed in various ways. See QMetaEnum for details.

Oliver is value {{ myObj.Oliver }}.
// Output: "Oliver is value 2".
...
Oliver key is {{ myObj.Oliver.key }}.
// Output: "Oliver key is Oliver".
...
Oliver value is {{ myObj.Oliver.value }}.
// Output: "Oliver value is 2".
...
Oliver scope is {{ myObj.Oliver.scope }}.
// Output: "Oliver scope is MyClass".
...
Oliver scope is {{ myObj.Oliver.name }}.
// Output: "Oliver scope is PersonName".
...
Patrica is value {{ myObj.Patrica }}.
// Output: "Patrica is value 9".
...
PersonName has {{ myObj.PersonName.keyCount }} items.
// Output: "PersonName has 5 items".
...
Second item is {{ myObj.PersonName.1 }}.
// Output: "Second item is 1".
...
Second item is {{ myObj.PersonName.1.key }}.
// Output: "Second item is Natalie".
...
Fourth item is {{ myObj.PersonName.3 }}.
// Output: "Fourth item is 9".
...
Fourth item is {{ myObj.PersonName.3.key }}.
// Output: "Fourth item is Patrica".
...
The personName property is {{ myObj.personName }}.
// Output: "The personName property is 10".
...
The personName property is {{ myObj.personName.key }}.
// Output: "The personName property is Quentin".
...
The personName type is {{ myObj.personName.scope }}.
// Output: "The personName type is PersonName".
...
The personName is {% with myObj.personName as var %}{{ var }}, {{ var.key }}{% endwith %}.
// Output: "The personName is 10, Quentin".
...
The enum is {% with myObj.PersonName as enum %}{{ enum.3 }}, {{ enum.4 }}, {{ enum.4.key }}{% endwith %}.
// Output: "The enum is 9, 10, Quentin".
...
The enum is {% for enum in myObj.PersonName %}{{ enum }} : {{ enum.key }}, {% endfor %}.
// Output: "The enum 0 : Mike, 1 : Natalie, 2 : Oliver, 9 : Patrica, 10 : Quentin, ".
...
The enum is {% for enum in myObj.PersonName %}
                {% ifequal enum Oliver %}**{{ enum }} : {{ enum.key }}**
                {% else %}{{ enum }} : {{ enum.key }},
                {% endifequal %}
            {% endfor %}.
// Output: "The enum 0 : Mike, 1 : Natalie, **2 : Oliver**, 9 : Patrica, 10 : Quentin, ".
...
The enum is {% for enum in myObj.PersonName %}
                {% ifequal enum myObj.personName %}**{{ enum }} : {{ enum.key }}**
                {% else %}{{ enum }} : {{ enum.key }},
                {% endifequal %}
            {% endfor %}.
// Output: "The enum 0 : Mike, 1 : Natalie, 2 : Oliver, 9 : Patrica, **10 : Quentin**, ".

Enumerators in the Qt namespaces are also supported.

AlignRight value is {{ Qt.AlignRight }}.
// Output: "AlignRight value is 2".
...
AlignRight key is {{ Qt.AlignRight.key }}.
// Output: "AlignRight value is AlignRight".
...
AlignRight scope is {{ Qt.AlignRight.scope }}.
// Output: "AlignRight scope is Qt".
...
AlignRight name is {{ Qt.AlignRight.name }}.
// Output: "AlignRight scope is Alignment".
...
{% ifequal myObj.alignment Qt.AlignRight %}RightAligned{% else %}Not RightAligned{% endifequal %}.
// Output: "RightAligned". or "Not RightAligned", depending on the value of myObj.alignment.
...