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Adobe InDesign Programming Guide 10.4. Drawing the Layout 335 The use of foreground and background draws means that a page item could be called twice during a redraw. The first call to IShape::Draw() in the order of the document hierarchy for the background draw, and the second call to IHandleShape::Draw() in the order of the selection list for the foreground draw. If the window is not the front view, then a more abbreviated draw takes place. Both the document items and the selection are drawn to a foreground offscreen context, and it is copied to the screen. 10.4.3.2. Introduction to the Draw Manager Until now, document drawing has implied the kLayoutWidgetBoss is directly calling each spread to draw. In fact, kLayoutWidgetBoss calls through InDesign’s Draw Manager facility. The Draw Manager is a session level interface through which all drawing for the screen, PDF, and print passes. As described above, drawing in InDesign occurs on a spread-by-spread basis, and is hierarchical. Without a service like the draw manager, it would be difficult to change the behavior of drawing a sub hierarchy. Funneling all drawing activity through the Draw Manager provides three important features for changing the behavior of draws: • Clipping of areas to be drawn • Filtering of items to be drawn • Interrupting a draw sequence Clipping and filtering provide a means to select items for the draw based on regions. The draw manager maintains the current values for the clip and filter regions. Interruption of the drawing sequence relies on a broader mechanism called draw events. As items draw they broadcast standard messages that announce the beginning and end of discrete drawing operations. Draw events are received by special handlers that register interest in particular types of messages, and each draw event handler has the opportunity to abort the draw at that point. Clients of the Draw Manager (such as the LayoutWidget) install a default draw event handler. The Draw Manager provides services for hit testing and iterating the draw order as well as drawing. More information about all of its functions is available in the Draw Events chapter.
