In this post we will go over some of the basic and common constructs that you come across while building GUI programs almost in any language or library. The following are some of the common items that I can think of
- Loop
- Widgets
- Events
- Layout
Loop
Loop or Mainloop as it is called in some GUI libraries is one of the most common element of all GUI programs. In a GUI program this is usually called at the at end of constructing the all the items (widgets) in program. When you call the mainloop, it takes over the control and keeps running until you terminate the GUI.
The main purpose of loop is to poll for user input (mouse and keyboard actions) and fire events which your program can handle in asynchronous way. Following diagram explains loop in simple terms.
GUI main loop
As you can see from the above diagram the GUI loop keeps polling for user input. If there is any event via mouse or keyboard or other input devices (joysticks, game pads ) when input is available loop gives control to callback functions that are bound to input events. Loop keeps running until the main window (also called parent window) exits. Once the main window is destroyed the loop exits and gives control back to your code. In most cases end of the mainloop would be end of many GUI programs.
Widgets
GUIs are collection of widgets. Example of widgets are button, scroll bar, check box etc. Widget is an element of user interaction. Their main purpose is to display or collect information from user. A label displays text to user. A text input widget collects text from user. One way to decide the richness of GUI tool kits is to look at how many widgets they offer. The more widgets a library offers you the better it is for you. So, that you don’t have to write extra code to implement custom widgets. Widgets usually are sub classes of a Widget parent class. So, most of the widgets inherit properties of widget class in respective GUI libraries. In some GUI libraries main window also has properties of a widget. 
Events
When ever user presses a button or enters text using keyboard those actions generate events in GUI libraries. GUI libraries provide a way to hook into these events so that your program can respond to user actions. The event hooking/binding style differs from library to library. But, the aim is same across all libs, that is to provide programmer a way to respond to user input.
Layout
Layout facilities provided by GUI libs helps to make programmer’s life easier when placing widgets in main window. Layouts are simple rules you use to convey to GUI library on how you would like your widgets to be laid out on screen. Instead of manually placing widgets on screen by specifying distance units and placement pixel positions, once you construct your widgets you hand them over to certain layout scheme to let GUI library handle the placement. The benefits are, layouts will take care of expansions, size proportions and other nitty-gritty detail of placing your widgets in orderly way.
The layout features provided by libraries vary greatly. You often have to pick the one that suits your needs. These layouts can be mixed in order to achieve the placement and look you want.
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