{"id":73,"date":"2020-01-22T13:00:20","date_gmt":"2020-01-22T13:00:20","guid":{"rendered":"https:\/\/www.vcreatelogic.com\/?p=73"},"modified":"2020-01-23T05:05:16","modified_gmt":"2020-01-23T05:05:16","slug":"qobjectfactory","status":"publish","type":"post","link":"https:\/\/www.vcreatelogic.com\/index.php\/2020\/01\/22\/qobjectfactory\/","title":{"rendered":"QObjectFactory"},"content":{"rendered":"\n

Factory Design Pattern<\/a> is a well known pattern. Its a way to organise code so that class-instances can be created at run time against a developer defined key. Factories specialise in create objects of a specific type. For example (yes, this example is inspired from the example on the Wikipedia page about Factory Design Pattern<\/a>s)<\/p>\n\n\n\n

#define interface class\n#define implements : public\n\ninterface IPerson\n{\npublic:\n    virtual std::string getType() = 0;\n};\n\nclass VillagePerson implements IPerson\n{\npublic:\n    virtual std::string getType() { return \"VillagePerson\"; }\n};\n\nclass TownPerson implements IPerson\n{\npublic:\n    virtual std::string getType() { return \"TownPerson\"; }\n};\n\nclass CityPerson implements IPerson\n{\npublic:\n    virtual std::string getType() { return \"CityPerson\"; }\n};\n\nclass PersonFactory\n{\npublic:\n    IPerson *createPerson(const std::string &type) {\n        if(type == \"VillagePerson\")\n            return new VillagePerson;\n        if(type == \"TownPerson\")\n            return new TownPerson;\n        if(type == \"CityPerson\")\n            return new CityPerson;\n        return nullptr;\n    }\n};<\/code><\/pre>\n\n\n\n
So, we can now create instances of IPerson using the PersonFactory class. For example:<\/p>\n\n\n\n
PersonFactory factory;\nIPerson *person = factory.createPerson(\"CityPerson\");<\/code><\/pre>\n\n\n\n
Now person will point to an instance of CityPerson class. Thats the whole point of a factory, to create object instances based on a key. In this case the key was type name as string.<\/p>\n\n\n\n
Qt uses Factory Design Pattern in several places. For example the QItemEditorFactory<\/a> class helps create editor widgets for items in a QAbstractItemView. Over there the type is integer. There is also a QStyleFactory<\/a> that creates instances of QStyle based on style-name as key. <\/p>\n\n\n\n
Almost every software project that I have been a part of makes use of the factory design pattern. Almost all of the factory classes I have written create QObject subclasses based on a QString or QByteArray key. Typically the key is name of the QObject subclass, but its not always the case. After one writes enough number of factories, it becomes boring to write the same thing over and over again. So I came up with a simple solution. <\/p>\n\n\n\n
QObjectFactory class<\/h3>\n\n\n\n
Lets take the same example as before. Suppose we wanted to create a Person factory class. Lets also suppose that we can make peace with the fact that objects created by our factory will be QObject subclasses. So the person interface and subclasses would look like this<\/p>\n\n\n\n
class AbstractPerson : public QObject\n{\n    Q_OBJECT\n\npublic:\n    AbstractPerson(QObject *parent=nullptr)\n        : QObject(parent) { }\n    ~AbstractPerson() { }\n\n    virtual QByteArray getType() const = 0;\n};\n\nclass VillagePerson : public AbstractPerson\n{\n    Q_OBJECT\n\npublic:\n    Q_INVOKABLE VillagePerson(QObject *parent=nullptr)\n        : AbstractPerson(parent) { }\n    ~VillagePerson() { }\n\n    virtual QByteArray getType() const {\n        return \"VillagePerson\";\n    }\n};\n\nclass TownPerson : public AbstractPerson\n{\n    Q_OBJECT\n\npublic:\n    Q_INVOKABLE TownPerson(QObject *parent=nullptr)\n        : AbstractPerson(parent) { }\n    ~TownPerson() { }\n\n    virtual QByteArray getType() const {\n        return \"VillagePerson\";\n    }\n};<\/code><\/pre>\n\n\n\n
Notice the following<\/p>\n\n\n\n