COS3711 Summary - An Introduction to Design Patterns in C++ with Qt PDF

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COS3711 Summary For: An Introduction to Design Patterns in with Qt : Wilco Breedt 1. Reflection 2 2. Validation 5 3. XML 7 4. Model View Architecture 8 5. Momento 13 6. Concurrency 15 7. Singleton 17 8. UML 18 9. Factory Method 19 10. Notes 20 COS3711 Wilco Breedt 1 G const QMetaProperty metaProp CO...

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       

COS3711 Summary For: An Introduction to Design Patterns in C++ with Qt By: Wilco Breedt 





1. Reflection

2

2. Validation

5

3. XML

7

4. MV/MVC - Model View Architecture

8

5. Momento

13

6. Concurrency

15

7. Singleton

17

8. UML

18

9. Factory Method

19

10. Notes

20

                                       

COS3711

Wilco Breedt 1

1. Reflection 1. 2. 3. 4. 5. 6.

Inherit QObject Q_OBJECT Macro Q_PROPERTY(int size READ getSize WRITE setSize) Q_PROPERTY(type name READ getter WRITE setter) MOC (Meta Object Compiler) Looks for QOBJECT Macro and compiles a meta object file. Before RUNTIME. Dynamic Properties (When you use setProperty and the property you are trying to set was NOT in the class Q_PROPERTY declared by yourself. This means you cannot use the QMetaProperty to get these values because Dynamic properties are created at RUNTIME where static properties are created BEFORE RUNTIME when the MOC generates the MetaObject for the class 7. QMetaObject - Object that carries meta data about another object (QObject) Used for reflection 8. Static Properties - Defined in the class - Information about static properties are created at compile time and exist within QMetaObject 9. Dynamic Properties- Defined outside the class - created at runtime and you cannot obtain information about them using the QMetaObject

  ===================================image.h=================================== #ifndef IMAGE_H #define IMAGE_H #include   class Image: public  Q  Object / / Inherit   from   QObject { Q_OBJECT // QOBJECTMacro (MOC) Q_PROPERTY(i nt size READ getSize WRITE setSize) // Q_PROPERTY (MOC) Q_PROPERTY(QString   name READ getName WRITE setName) // Q_PROPERTY (MOC)  public: Image(); int getSize(); void setSize(int s); QString getName(); void setName(QString s);  private: int length; // Q_PROPERTY(int size ...) This does not have to be the same as the Q_PROPERTY that gets its value QString name; }; #endif // IMAGE_H      ===================================image.cpp=================================== #include "image.h"  Image::Image() {} void Image::setSize(int s ) {  length = s; } int Image::getSize() {  r eturn length; } void Image::setName(QString s) {   ame = s; n } QString Image  ::getName() {  r eturn name; } 

COS3711

Wilco Breedt 2

====================================main.cpp===================================  #include  #include  #include  #include "image.h"  void printObject(Q  Object *obj)   { / / Method   that uses reflective techniques c onst QMetaObject *meta = obj->metaObject( ); QString className = meta->className(); QString result = "";  // Static Properties ( Was declared by yourself in the Q_PROPERTY of the class ) (Declared at compile time) // for (int i = meta->propertyOffset(); i propertyCount(); i++) Get rid of properties in base class for (int i = 0;  i propertyCount(); i++) { // First method to get value of meta property const QMetaProperty metaProp = meta->property(i); const char *name = metaProp.name(); // Get the name of the meta property QVariant value = obj->property(name); / / Get the value of the meta property  // Convert meta property name to QString if you have to // QString name = QString(metaProp.name());  // Second method to get value of meta property // const QMetaProperty metaProp = meta->property(i); // QVariant value = metaProp.read(obj); result += QString(" %1 %2 ").arg(name).arg(value.toString()); } // Dynamic Properties (Wasn’t declared in the Q_PROPERTY/ Declared at RUNTIME) foreach(QByteArray dynamicPropName, obj->dynamicPropertyNames()) { QVariant value = obj->property(dynamicPropName); result += QString(" %1 %2 ") .arg(Q  String(dynamicPropName)) .arg(value.toString()); } qDebug() dynamicPropertyNames()) { QVariant value = i->property(dynamicPropName); merge.append(QString("DName %1, Value %2").arg(QString(dynamicPropName)).arg(value.toString())); } } // If   the Object is not   a pointer (Image *i)   -> Image i // f oreach(Image i, list)   { // const QMetaObject metaObj   = i.metaObject(); // for (int in   = 0;   in   setProperty("name2", "Wilco2.png"); // Dynamic Property // Getting Values int imageSize = img->property(" size").toInt(); // ->property returns QVariant // OR QVariant name = img->property("name"); // ->property returns QVariant QString imageName = name.toString(); // Convert to QString qDebug() getName()); name.appendChild(nameText); } QString xml = doc.toString(); } 

COS3711

Wilco Breedt 7

4. MV/MVC - Model View Architecture 1.

Views - ONLY displays the data, declare one instance and set the Model (that is basically the only thing you do with it) The rest of the things comes from the Model (use setModel to set the view) a.

QTableView (Model Based)

b. QListView (Model Based) c.

QTreeView (Model Based)

d. QTableWidget (Item Based/ Convenience class) e.

QListWidget (Item Based/ Convenience class)

f.

QTreeWidget (Item Based/ Convenience class)

2. Models - Serve the DATA to the Views (Remember Abstract means you cannot use it, you can not make an instance of it, you cannot instantiate it ! you need to inherit it and then you can use it !!! QAbstractItemModel, QAbstractTableModel, QAbstractListModel) Concrete models are models of which you can make a instance of



Objectives when using a QAbstractTableModel (Nothing is done on the View, everything is done on the MODEL!) : 1.

Headers

2. Edit table data (Double click on row and edit it)

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Wilco Breedt 8

3.

Add table data (Add rows)

4.

Remove / Delete data (Delete rows)

 How to implement a QAbstractTableModel (subclassing it): ●

Functions YOU MUST IMPLEMENT !!!!!!! ○

rowCount() (pure virtual)→ How many rows are in your data

○

columnCount() (pure virtual) → How many columns are there in your data

○

data() (pure virtual) → To return the actual value / data (provides the data for each and every CELL in

○

headerData() (This is just to make it a WELL behaved model)

your table) ●

Functions to implement if you want your model to be EDITABLE ! ○

setData() (If you want your table to be editable)

○

flags() that returns a value containing Qt::ItemIsEditable / Qt::ItemIsSelectable / Qt::ItemIsEnabled

 int MyModel::rowCount(const Q  QModelIndex &parent) const  { // Q_UNUSED(parent); // Sodat jy nie warnings kry vir goed wat nie gebruik word nie. return list.count(); // list.length - 1 ?? }  int MyModel::columnCount(const Q  QModelIndex &parent)   const  { // Q_UNUSED(parent); // Sodat jy nie warnings kry vir goed wat nie gebruik word nie. return headers.count(); // headers.length - 1 ?? }  // Get   the   header   data QVariant MyModel::headerData(int section,   Qt::Orientation orientation, i nt role)   const // MUST   IMPLEMENT  { if (role != Qt::DisplayRole) // If it is not for display purposes don't return QVariant {

// Readonly return QVariant();

} if (orientation == Qt::Vertical) // orientation != Qt::Horizontal { return QVariant(); } return headers.at(section); // Headers are stored in QList || QStringList; // return headers[section]; }    // Get the data for each   cell QVariant MyModel::data(const QModelIndex  &  index, int role)   const / / MUST IMPLEMENT {

COS3711

Wilco Breedt 9

if (!index.isValid()) // Check if the index is VALID ! { return QVariant(); } if (role == Qt::DisplayRole || role == Qt::EditRole) // Important { // int row = index.row(); // int col = index.column(); // QObject *object = list[row]; // QString columnName = headers[col]; / / QVariant value = object->property(columnName.toStdString().c_str()); // Return const char // return value; r eturn list[index.row()]->property(headers[index.column()].toStdString().c_str()); // Return the data requested. } return QVariant(); }  void MyModel::insert() / / Do   not   have   to   implement, just   for   fun. { QObject *object = new QObject(); foreach(QString header, headers) { object->setProperty(header.toStdString().c_str(), "MyTest"); } list.append(object); }                    a cell // Must   be   here to   edit the   MODEL   !! // Set the data for bool MyModel::setData(const Q  ModelIndex &  index, const Q  Variant &  value, int   r ole) / / MUST   I MPLEMENT EDITABLE {

COS3711

Wilco Breedt 10

if (role == Qt::EditRole && index.isValid()) { int row = index.row(); int col = index.col(); // list[row]->setProperty(headers[col].toStdString().c_str(), value); l ist.at(row)->setProperty(headers.at(col).toStdString().c_str(), value); emit dataChanged(index, index); // Signal the view that the data changed. return true; } return false; }   // Must   be here   to edit   the MODEL   !! Qt::ItemFlags M  yModel::flags(const QModelIndex &  index)  // MUST   IMPLEMENT EDITABLE { // Do some check here to see if you are allowed to edit it ? // If everything is editable then return everything ! return (Qt::ItemIsEditable | Qt::ItemIsSelectable | Qt::ItemIsEnabled); }  MVC M: Model → Application object / data V: View → Presentation C: Controller → Controls data flow from the model to the view or from the view to the model. Notifies everyone of what is happening.  MV Delegate: Renders items for editing (QSpinBox, QLineEdit etc.) Sits between the Model and View  Transition from MVC to MV (Why MV is a good example of MVC): ●

Do not see the C (Controller)

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View and Controller are merged together

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If something changes in the model, the view is notified, if something changes on the view the model is notified

 The controller is like an observer (it observes the changes on the model & view).  What is the relationship between MVC and observer pattern ●

In the observer pattern the observer automatically respond to events occurring in the subject/ its subjects

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In the same way the model, view and controller automatically respond to data changes within each other.

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This can hence be seen as they are observing each other

   MV vs MVC

COS3711

Wilco Breedt 11

In the MV pattern there is no controller, there is however a delegate, which is used to render items for editing on the view and update the model with values from the view when editing is done from the view. A delegate can therefore be seen as playing the role of a controller in the MV vs MVC architecture.  Difference between QTableView/QAbstractItemModel and QTableWidget (What is the difference between item based and model based views) ●

QTableWidget (Item based) handles the data for you/data is stored in the widget (This is a convenience class)

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QTableView (Model based) requires a model to handle the data for you

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Item based Less flexible ○

In Model based the view and the model are separated, this makes it more flexible because you can have multiple views for the same model, or multiple models for the same view.

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Model and View are tightly coupled together in the item based.

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Item based is not reusable because the data is stored within the widget (the view moves with the data)

                   

       COS3711

Wilco Breedt 12

5. Momento ●

When you want to serve the state of an object (state = data within the class) in a previous time frame (basically it is used to restore data)

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3 classes involved ○

Originator → Class that has the state that you want to serve (The class you want to backup) The originator must handle the back-up logic (So all the logic basically comes here)

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Memento → Copy the state of the Originator (This is where the backup is stored)

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Caretaker → Stores the memonto’s

Momento class (Backup class) → Only has the state + a getter and a setter to set the data

 // Originator  class ImageList { public: ImageList(); Backup getBackup(); void restore(Backup b); private: QList list; };  // Implementation of the getters and setters within the ImageList Backup ImageList::getBackup() { Backup b; b.setList(l ist); return b; }  void ImageList::restore(Backup b) { list = b.getList(); }   // Memento  class Backup { public: Backup(); void setList(Q  List l); QList getList(); private:

COS3711

Wilco Breedt 13

QList list; }; // Caretaker class BackupList: public Q  List { public: BackupList(); }; Basic steps of the client: 1.

Backup a.

Get an instance from the originator using the getter you declared

b. Store that backup instance in the caretaker. 2. Restore a.

Get the instance of the backup in the caretaker

b. Use the setter you declared in the originator to restore the backup // CLIENT  I mageList i; BackupList bl; // Backup Backup b = i.getBackup(); // Get a instance from originator. bl.append(b); // Store the backup in the caretaker  // Restore Backup r = bl.at(0  ); // bl[0] // Get the backup from the caretaker. i.restore(r); // Restore the backup in the originator.  Encapsulation REQUIREMENT Should always satisfy this ! ●

Caretaker must NOT be able to modify the memento

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Make everything private and make the originator a friend class

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See below how to do this !

 // Memento  class Backup { private: friend class ImageList; Backup();   List l); void s etList(Q QList getList(); QList list; };    

COS3711

Wilco Breedt 14

6. Concurrency ●

The ability to run multiple processes at the same time (Threads)

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Example: You want to loop a 900 times over 0 - 900. Then check if the numbers are prime numbers. Imagine if it took 1 second for a check, this means your program will be busy for 900 seconds. But if you split it into threads [0 - 300, 301 - 600, 601 - 900] and execute those loops at the same time, then it will take a ⅓ of the time to complete the 900 loop.

 Have two things involved in threads (Two classes involved) 1.

Worker → Does the actual work (Does the loop) a.

Must be a QObject

b. Use signals to communicate with the Client 2. Client → The one telling the worker to do its stuffies  // Worker, USING THE QOBJECT is the RECOMMENDED APPROACH !! class SearchImages: public  Q  Object { Q_OBJECT public: SearchImages(); // QList search(QList list, int size); void search(QList list, int size); // Changed the above to this to make it use signals and slots signals: void imageFound(Image); // Signal for when a image is found to emit outside the thread. void finished(); };  // Client QList imgList; QThread *thread = new QThread(); SearchImages *s = new SearchImages(); // Move the Q  Object t o t hread, moveToThread e  xists o  n Q  Object c lass s->moveToThread(thread); // connect(senderObject, signalOnSenderObject, receiverObject, slotOnReceiverObject) Connect Basic Params // When the thread starts, start the search function in SearchImage Class connect(thread, SIGNAL(started()), s, SLOT(search(imgList, 300))); // Connect when the SearchImage finds an image and emits it and then sends the image to the handleImage function. connect(s, SIGNAL(imageFound(I mage)), this, SLOT(handleImage(I mage))); // The following 3 steps ARE FOR CLEANUP !! connect(s, SIGNAL(finished()), thread, SLOT(quit()); // When done, quit the thread, CLEANUP connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()); // Delete the thread nicely, CLEANUP connect(thread, SIGNAL(finished()), si, SLOT(deleteLater())); // Delete the worker nicely, CLEANUP thread->start(); // Emits the signal started ... and then the above search(size) SIGNAL SLOT gets handled, 

COS3711

Wilco Breedt 15

   QProcess ●

Run an exe

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readyReadStandardOutput - Signal that gets emitted when something is printed to the CONSOLE !!!

 QProcess *process =  n  ew QProcess  (); connect(process, SIGNAL(readyReadStandardOutput()), this, SLOT(manage()); process->start("image.exe");  void manage() { QByteArray ba = process->readAllStandardOutput(); // Readline QString output = QString(ba); QStringList lines = output.split("/n"); }    

              COS3711

Wilco Breedt 16

7. Singleton ●

ONE instance

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Can be asked with Memento

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Steps ○

1. Hide the constructor → Move the constructor to the private members

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2. Create a method on the class to get an instance. (getInstance)

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3. Make that method that you created (getInstace) a static method, this makes it easier so you can call it like so. BookList bl = BookList::getInstance();

○

This makes the getInstance function INSTANCE INDEPEN...