什么时候在 viewForAnnotation 方法中 link 注解 属性

Question

我在 Apple 的 Location and Maps Programming Guide:

中看到了这段代码

- (MKAnnotationView *)mapView:(MKMapView *)mapView
                      viewForAnnotation:(id <MKAnnotation>)annotation
{
    // If the annotation is the user location, just return nil.
    if ([annotation isKindOfClass:[MKUserLocation class]])
        return nil;

    // Handle any custom annotations.
    if ([annotation isKindOfClass:[MyCustomAnnotation class]])
    {
        // Try to dequeue an existing pin view first.
        MKPinAnnotationView*    pinView = (MKPinAnnotationView*)[mapView
        dequeueReusableAnnotationViewWithIdentifier:@"CustomPinAnnotationView"];

        if (!pinView)
        {
            // If an existing pin view was not available, create one.
            pinView = [[MKPinAnnotationView alloc] initWithAnnotation:annotation
                       reuseIdentifier:@"CustomPinAnnotationView"];
            pinView.pinColor = MKPinAnnotationColorRed;
            pinView.animatesDrop = YES;
            pinView.canShowCallout = YES;

            // If appropriate, customize the callout by adding accessory views (code not shown).
        }
        else
            pinView.annotation = annotation;

        return pinView;
    }

    return nil;
}

我对这部分有疑问：

else
    pinView.annotation = annotation;

我在 Whosebug 中看到了相同的代码 answer:

Unrelated, but you also need to set the view's annotation property when it is being re-used (in the case when it is not nil after the dequeue). So add an else block to the if (pinAnnotation == nil):

else {
    //annotation view being re-used, set annotation to current...
    pinAnnotation.annotation = annotation;
}

为什么在else块中设置注解属性？无论是否重用视图，是否都不应设置注释？

Answer 1

你用注解初始化 MKPinAnnotationView 所以 else 只是为了你不会设置它两次。设置两次完全没问题，当然效率低下。