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Using Quartz Debug

Quartz Debug is an important tool for determining the efficiency of your drawing code. The tool collects information from your program’s drawing calls to find out where your program is drawing and whether it is redrawing content unnecessarily. Figure 4-1 shows the Quartz Debug options window.

In its primary mode, Quartz Debug shows you visually where your code is drawing. It places a yellow rectangle over an area where a redraw operation is about to occur and then pauses briefly before redrawing the content. This flickering yellow pattern can point out places where you are drawing more than is necessary. For example, if you update only a small portion of a custom view, you probably do not want to be forced to redraw the entire view. Alternatively, if you see a system control being redrawn several times in succession, it might point out the need to hide that control before changing its attributes.

The tool also has a mode that identifies parts of the screen that are being redrawn with the exact same content. If you select the “Flash identical updates” option, the tool displays red over any areas whose resulting bits are identical to the current content.

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