It’s helpful to think of a rich client application as having two distinct categories of threads: UI threads and worker threads. UI threads instantiate (and subsequently “own”) UI elements; worker threads do not. Worker threads typically execute long-running tasks such as fetching data.
So, when would an application have multiple UI threads? The main scenario is when you have an application with multiple top-level windows, often called a Single Document Interface (SDI) application, such as Microsoft Word. Each SDI window typically shows itself as a separate “application” on the taskbar and is mostly isolated, functionally, from other SDI windows. By giving each such window its own UI thread, the application can be made more responsive.
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