A graphical user interface builder (or GUI builder), also known as GUI designer, is a software development tool that simplifies the creation of GUIs by allowing the designer to arrange graphical control elements (often called widgets) using a drag-and-drop WYSIWYG editor. Without a GUI builder, a GUI must be built by manually specifying each widget's parameters in source-code, with no visual feedback until the program is run.
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User interfaces are commonly programmed using an event-driven architecture, so GUI builders also simplify creating event-driven code. This supporting code connects widgets with the outgoing and incoming events that trigger the functions providing the application logic.
Some graphical user interface builders, such as e.g. Glade Interface Designer, automatically generate all the source code for a graphical control element. Others, like Interface Builder, generate serialized object instances that are then loaded by the application.