MKOverlayView Class Reference

Discussion

Overlay views can override this method in situations where they may depend on the availability of other information to draw their contents. For example, an overlay view showing traffic information might want to delay drawing until it has all of the traffic data it needs. In such a case, it can return NO from this method to indicate that it is not ready.

If you return NO from this method, your application is responsible for calling the setNeedsDisplayInMapRect:zoomScale: method when the overlay view subsequently becomes ready to draw its contents.

The default implementation of this method returns YES.

Discussion

The default implementation of this method does nothing. Subclasses are expected to override this method (instead of the drawRect: method) and use it to draw the contents of the view.

In your drawing code, you should specify the position of any rendered content relative to the map itself and not relative to the view’s bounds or frame. In other words, compute the position and size of any overlay content using map points and map rectangles, convert those values to CGPoint and CGRect types (using the methods of this class), and then use the converted points to build paths or specify the rendering location for items.

You should also not make assumptions that the view’s frame matches the bounding rectangle of the overlay. The view’s frame is actually bigger than the bounding rectangle to allow you to draw lines for things like roads that might be located directly on the border of that rectangle. For some types of content, such as gradients, this also means that you might need to apply a clipping rectangle to context to ensure drawing is contained to the correct area.

It is recommended that you use Core Graphics to draw any content for your overlays. If you choose to use UIKit classes and methods for drawing instead, you must push the specified graphics context onto the context stack (using the UIGraphicsPushContext function) before making any drawing calls. When you are done drawing, you must similarly pop the graphics context off the stack using the UIGraphicsPopContext. During drawing, you may draw content normally but should avoid manipulating views and other classes that are safe to use only from the application’s main thread.

To improve drawing performance, the map view may tile overlays that become large enough and render the tiles from separate threads. Your implementation of this method must therefore be capable of safely running from multiple threads simultaneously. In addition, you should avoid drawing the entire contents of the overlay each time this method is called. Instead, your implementation should always take the mapRect parameter into consideration and avoid drawing content outside that rectangle. Failure to do so could lead to performance problems.

Discussion

Upon initialization, the frame of the overlay view is set to CGRectZero. The map view sets the size and position of the view at display time, and you should not change those values yourself.

Discussion

Marking a rectangle as invalid causes that portion of the view to be redrawn during the next update cycle. This method invalidates the overlay regardless of the current zoom scale associated with the map.

Discussion

Marking a rectangle as invalid causes that portion of the view to be redrawn during the next update cycle. This method invalidates the overlay only when it is drawn at the specified zoom scale.