Optimizing drawing tasks

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A special manager feature minimizes the cost of drawing tasks to be done after geometric operations have been performed. This is useful in situations where you want to see the results of your work. This feature uses a region of invalidated parts of the display called the update region. The update region stores the appropriate regions before any modifications are carried out on objects, as well as those regions that are relevant after these modifications have been carried out for each view.

To successfully apply an applicable function, you must mark the regions where the objects are located as invalid, apply the function, and then invalidate the regions where the objects involved are now located (applying the function may change the location of the objects). This mechanism is greatly simplified by a set of methods of the IlvManager class. Regions to be updated are only refreshed when the method reDrawViews is called. This means that refreshing the views of a manager is done by marking regions to be redrawn in a cycle of initReDraws and reDrawViews .

These cycles can be nested so that only the last call to the method reDrawViews actually updates the display. The IlvManager methods that help you optimize drawing tasks are:

initReDraws

Marks the beginning of the drawing optimization operation by emptying the region to update for each view being managed. Once this step is taken, direct or indirect calls to a draw instruction are deferred. For every initReDraws , there should be one call to reDrawViews , otherwise, a warning is issued. Calls to initReDraws can be embedded so that the actual refresh only takes place when the last call to reDrawViews is reached.
invalidateRegion

Defines a new region as invalid, that is, this region will be redrawn later. Each call to invalidateRegion adds the region to the update region in every view.
reDrawViews

Sends the drawing commands for the whole update region. All the objects involved in previous calls to invalidateRegion are then updated.
abortReDraws

Aborts the mechanism of deferred redraws (for example, if you need to refresh the whole screen). This function resets the update region to empty. If needed, you should start again with an initReDraws call.
isInvalidating

Returns true when the manager is in an initReDraws / reDrawViews state.

This mechanism is used in the applyToObject method.

In fact the call:

manager.applyToObject(obj, func, userArg, true);

is equivalent to:

manager.initReDraws(); 
manager.invalidateRegion(obj);
manager.applyToObject(obj, func, userArg, false); 
manager.invalidateRegion(obj); 
manager.reDrawViews();

The invalidateRegion method works with the bounding box of the object given as a parameter. When an operation applied to the object modifies its bounding box, invalidateRegion must be called twice: once before and once after the operation. For example, for a move operation, you must invalidate the initial region where the object was before being moved and invalidate the final region so that the object can be redrawn. In other situations, such as changing the background, only the call after the operation is necessary.

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Managing graphic objects