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• Qt Signals And Slots Observer Pattern Chart
• Qt Signal Slot Thread

QT and Signals/Slots. On the whole, QT is an excellent framework. It is well-designed and documented, portable, performant, versatile and it offers a wide range of features. Not to mention, it introduced the innovative signals and slots paradigm as an implementation of the Observer pattern. To accomplish this task, we use Signal and Slot concept. This concept had been introduced in Trolltech Qt library and Boost C library. Using Signal and Slots. To demonstrate how signals and slots work, we create a model class containing CppSignal member and a view class containing CppSlot. @lukasm said in Signal-slot-mechanism vs. Observer pattern - Good practice. Years ago I learnt about the observer pattern, where we usually have some class (the subject) which is observed by some observer. The observers register to the subject, and if the subject does some action, e.g., sets some new value, it notifies the observers. Observer pattern is a beautiful way to make a loosely coupled system and I absolutely do not want to shoot that down. What I wanted to say is that signals and slots come with all the advantages of Observer pattern but less of its disadvantages. If Observer pattern is great, then signal-and-slots is great Let me give you an example. QT’s specific implementation of the observer pattern. The QObject class represents the application of a simplified observer pattern in its use of signals and slots. QObject represents the Subject, Observer, ConcreteSubject and ConcreteObserver classes all rolled into one. QObject has a static member called connect which is used to connect a subject to an observer. It also allows you to specify which change of state (the signal) should cause which update (the slot).

Qt 4 uses the MVC pattern for its item view classes(QListView, QTable, etc.). But MVC is more than just a pattern foritem views: it can be used generally as a means of keeping differentwidgets synchronized. In this article, we show how to apply it, takingfull advantage of Qt's signal--slot mechanism.

MVC and Subject-Observer pattern in C & QT. Wheel of fortune. Ask Question Asked 3 years, 8 months ago. Viewed 512 times 5 $begingroup$ I am trying to implement the MVC pattern in C & QT, similar to the question here. Would it be better to connect the buttons signals to the Controller slots directly (like in the portion commented out in View.

In Qt Quarterly 7's article, A Model/ViewTable for Large Datasets, we saw how to create a model/view QTable subclass. Here we take a more genericapproach that can be applied to any of Qt's widget classes (and to ourown widget classes).

A model is a set of data, and a view is a GUI component thatcan present a visual representation of the model to the user. If themodel (data) cannot be changed by the user, having a model and a viewis sufficient; but if the model can be modified, then we also need acontroller, which is a means by which the user can modify thedata shown in a view, and have their changes reflected back into thesource data.

Qt Signals And Slots Observer Pattern Chart

Let's imagine that we want to provide color management for anapplication. We want to provide a palette of colors that the user canmake use of throughout the application, for example to specify thecolor of text, or the color of shapes that they have drawn. We mightwant to present the palette in different ways in different parts ofthe application, but we want all the colors displayed to be taken froma single palette.

In this example, the model is a palette of colors, and the view is awidget that can display the colors. The controller could be a separate'editor' widget, or could be built-in to the view widget. Wheneverthe model's data changes (for example, because a user has edited thedata in one of the views), all active views must be informed so thatthey can update themselves. For the purpose of illustration, we willdesign just one view widget, but we could design any number of viewwidgets, each presenting the data in their own way.

Our model uses the Singleton design pattern, since we only want onepalette to be used throughout the entire application. Let's look atthe definition of our palette model class.

The PaletteModelManager class is unusual in some respects. Firstlyit provides a static getInstance() function that returns a pointerto the one and only PaletteModelManager object that can exist inthe application. Secondly, it has a private constructor; this isto ensure that users cannot instantiate instances of the class itself.These two features are used to make a Singleton class in C++.

The palette itself is a simple map of string IDs X colors. TheaddColor() slot is uncommon in that it has anon-void return value to support its use as both a slot andas a function.

Conceptually, the class provides several key interfaces. An accessinterface which gives us a handle through which we can interact withthe model---getInstance(); a read interface through which wecan read the current state of the model---getPalette() andgetColor(); a change interface through which the model's datacan be modified---the slots provide this; and an inform interfacethat notifies views of changes to the model's state---the signalsprovide this.

The global PaletteModelManager pointer is statically initializedto 0. (It might be tempting to use a static object rather than apointer, but some compilers fail to call the constructor for staticobjects, especially in libraries, so our approach is more robust.) IngetInstance(), we create the single instance if it doesn't exist.

We have omitted the implementation of all the slots except forchangeColor():

Here we've emitted the colorChanged() signal before performingthe change. This is to ensure that in any slot connected to thecolorChanged() signal, the palette is still in its original state,with the ID and color that are going to be changed passed asparameters. This is especially useful if you want to track thechanges, for example to support an undo stack or a history. Sometimesit may be more appropriate to emit the change signal after thechange (as we do in PaletteModelManager::addColor()), but in suchcases the previous state cannot be accessed directly from the palettemanager, so if it is needed it must be passed in the parameters of thesignal that notifies the change of state. Another strategy would be toemit notifying signals before and after a change in state.

Now that we've seen how to implement our model, let's see how to makeuse of it. We'll create a custom icon view that shows the colors andtheir ID strings; here's the definition:

Our custom icon view holds a pointer to the PaletteModelManager.Since we only have a single PaletteModelManager, we could simplyhave used its static getInstance() function instead, but we'vechosen a more general approach, since most models are not implementedas Singletons. The private slots are used internally to update theicon view and the palette manager. Our controller is built-in to ourview, in this case as a context menu.

Now we'll review the PaletteIconView's main functions.

The constructor is straightforward; we justset the PaletteModelManager, and connect thecontext menu.

When a new palette manager is set, the connections to the old one (ifany) are broken, and new ones established. We haven't shown theclearOld() function; it clears the item X color IDmaps, and clears the icon view itself.

Qt Signal Slot Thread

The fillNew() function populates the maps with the IDs andcolors from the palette, and adds each color into the icon view.

When the user adds a new color via the context menu we create a newicon view item and update the item X ID maps.

The context menu is straightforward. First we check that we have apalette manager, since we can't do anything without one. Then wecreate the menu items, but disable those that only apply to an item ifthe user didn't invoke the menu on an item (i.e. if item0). If the user chose Add, we pop up a color dialog, and if theychoose a color, we pop up an input dialog to get the color's ID. Ifthey chose Change, we pop up a color dialog so that they can choose anew color, and if they chose Remove we remove the color.

Note that the adding, changing, and removing are all applied to thepalette manager, not to the icon view; this is because the palettemanager is responsible for the color data, and it will emit signalsconcerning its change of state to all associated views, includingthis one, so that they can update themselves. This is a much cleanerand safer approach than updating the view directly, since it ensuresthat all views are updated only via the model, and using the samecode.

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Here's a simple main() function that creates two views of apalette.

Once we create our views, we add a few colors. The user can add,change, and remove colors using the context menu that each viewprovides, and whatever the user does to one view is applied to both.

The classes presented here could be enhanced in several ways, forexample, by creating a PaletteIconView plugin for use with Qt Designer,or by providing signals and slots for updating and notifying changinga color's ID. The PaletteIconView could be enhanced by providingdrag and drop support, while the PaletteModelManager could provideloading and saving of palettes. Additional views that operate on apalette could also be created, for example, combobox and listboxsubclasses. A more ambitious extension would be the implementation ofan undo/redo mechanism. The full source code for this article is here:qq10-mvc.zip(8K).