Grantlee: Grantlee for application developers

Grantlee 5.3.0

Grantlee for application developers

Integrating Grantlee into applications is very simple. This page describes

- How to render a Template with a Context
- How to load Templates
- How to extend the syntax of Grantlee
- Patterns of use of Grantlee

If you are not already familiar with Django template syntax and structure, start with Grantlee for theme artists. If you are already familiar with Django, you might find Differences between Django and Grantlee informative.

# Rendering Templates

## Creating Templates

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

auto engine = new Grantlee::Engine( this );
auto 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."
Grantlee::Context
The Context class holds the context to render a Template with.
Definition: context.h:118
Grantlee::Engine
Grantlee::Engine is the main entry point for creating Grantlee Templates.
Definition: engine.h:121

Usually, applications do 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.

auto 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{"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 = getObj();
auto 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

Instances of `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();
};
...
auto linda = new PersonWrapper("Linda", 21);
mapping.insert("person", linda);
...
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 `PersonWrapper` 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 `QObject`s. In the case of dynamic properties, the property name must be UTF-8 encoded.

If you already have `QObject`s 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

The broad introspection features of Qt5 are relied upon by Grantlee via the `Q_PROPERTY` macro. Most types used with that macro may be accessed in Grantlee templates without further code.

We can define a `Home` type 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:
// ...
};

And we can use it in a `Q_PROPERTY` macro in our `PersonWrapper` type:

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 }}

Using containers such as QList and QSharedPointer with the `Q_PROPERTY` macro is also possible.

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

Classes which do not derive from QObject, but which are decorated with the `Q_GADGET` and `<Q_PROPERTY/tt> macros may also be used as expected.`

``

## 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.
...