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UDBarTrackballItem is a button view for iOS which can position the text cursor (caret) and define the text selection of a text view in parallel to its touch drag movement. The user interaction pattern is similar to a Thinkpad TrackPoint.
The intention was to create a text navigation component for text editor apps that makes the task of frequent text positioning and selection more convenient. The default way of positioning and selection requires a touch down delay to distinguish view scrolling and editing mode as well as the magnifying glass which distract the precise positioning more that it helps. The tradeoff is a decrease in view real-estate for the input accessory view bar (which is quite common in text editors nowadays).