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POMKPinAnnotationView and custom classes conforming to the MKAnnotation protocol
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  1. POMKPinAnnotationView and custom classes conforming to the MKAnnotation protocol
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    copied!<p>I have created the following method for determining the view for my annotations in a map view. </p> <pre><code>- (MKAnnotationView *)mapView:(MKMapView *)mv viewForAnnotation:(id &lt;MKAnnotation&gt;)annotation { if ([annotation isKindOfClass:[MKUserLocation class]]) { return nil; } MKPinAnnotationView *pin; if ([annotation isKindOfClass:[AnnotationsWithIndices class]]) { int i = [(AnnotationsWithIndices *)annotation index]; if (i &gt; currentCheckpointIndex ) { pin = (MKPinAnnotationView *)[mv dequeueReusableAnnotationViewWithIdentifier:@"unvisited"]; if (!pin) { pin = [[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"unvisited"]; } [pin setPinColor:MKPinAnnotationColorRed]; } if (i == currentCheckpointIndex) { pin = (MKPinAnnotationView *)[mv dequeueReusableAnnotationViewWithIdentifier:@"current"]; if (!pin) { pin = [[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"current"]; } [pin setPinColor:MKPinAnnotationColorGreen]; } if (i &lt; currentCheckpointIndex) { pin = (MKPinAnnotationView *)[mv dequeueReusableAnnotationViewWithIdentifier:@"visited"]; if (!pin) { pin = [[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"visited"]; } [pin setPinColor:MKPinAnnotationColorPurple]; } [pin setAnimatesDrop:YES]; return [pin autorelease]; } else return nil; } </code></pre> <p>The idea is that I want the different annotation views (pins) be of different colors depending on whether the user has visited them to signal which annotation to visit next. </p> <p>This code works fine, but I have a few questions about it which I hope someone can answer. </p> <p>Firstly, the MKPinAnnotations are dequeued from a map view and reused whenever possible. In the line doing so (which I have found in multiple blogs and forums) </p> <pre><code>pin = (MKPinAnnotationView *)[mv dequeueReusableAnnotationViewWithIdentifier:@"unvisited"]; </code></pre> <p>I understand that the return value of the <code>dequeueReusableAnnotationViewWithIdentifier:</code> is an instance of MKAnnotationView, whereas <code>pin</code> is a pointer to an instance of MKPinAnnotationView (which is a subclass o MKAnnotationView). This, I guess, is why there is a 'cast' seemingly going on with the prefix <code>(MKPinAnnotationView *)</code> in front of the method call. Is this really a cast and in this case, is it not dangerous since the MKPinAnnotationView contains more instance variables (than MKAnnotationView) and hence more space in memory?</p> <p>I have tried to find some information about this, but nowhere I have not found anyone explaining this in particular.</p> <p>Furthermore, the pointer annotation is either of class MKUserLocation or of my own custom class <code>AnnotationsWithIndices</code> conforming to the MKAnnotation protocol. Now, in order to determine which color the annotation view should have I have added an instance variable called index into the <code>AnnotationsWithIndices</code> class. Now, when I call the getter for the index I write </p> <pre><code>int i = [(AnnotationsWithIndices *)annotation index]; </code></pre> <p>Now, I basically have the same question about this. Is there a cast going on here or is it just to let the compiler know that it is ok to send the message index to annotation? I guess that the compiler expects annotation to be an id whereas it really is a pointer to an instance of AnnotationsWithIndices. Of course I know that this is the case since the annotation will be of my custom class, and I also check this explicitly to be sure. Is the <code>(AnnotationsWithIndices)</code> there only to signal to the compiler that this is OK? </p> <p>I have also tried to find information about this without luck. </p> <p>I am very thankful for any answers on this. </p>
 

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