仅将五个注释加载到 MKMapVIew

Question

我有一个 MKMapView，我想知道如何找到距离用户最近的 5 个注释，并只将它们显示在 MKMapView 上。

我的代码目前是：

- (void)loadFiveAnnotations {
    NSString *string = [[NSString alloc] initWithContentsOfURL:url];
    string = [string stringByReplacingOccurrencesOfString:@"\n" withString:@""];
    NSArray *chunks = [string componentsSeparatedByString:@";"];
    NSArray *keys = [NSArray arrayWithObjects:@"type", @"name", @"street", @"address1", @"address2", @"town", @"county", @"postcode", @"number", @"coffeeclub", @"latlong", nil];   
    // max should be a multiple of 12 (number of elements in keys array)
    NSUInteger max = [chunks count] - ([chunks count] % [keys count]);
    NSUInteger i = 0;

    while (i < max)
    {
        NSArray *subarray = [chunks subarrayWithRange:NSMakeRange(i, [keys count])];
        NSDictionary *dict = [[NSDictionary alloc] initWithObjects:subarray forKeys:keys];
        // do something with dict
        NSArray *latlong = [[dict objectForKey:@"latlong"] componentsSeparatedByString:@","];
        NSString *latitude = [[latlong objectAtIndex:0] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
        NSString *longitude = [[latlong objectAtIndex:1] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
        CLLocationDegrees lat = [latitude floatValue];
        CLLocationDegrees longi = [longitude floatValue];
        Annotation *annotation = [[Annotation alloc] initWithCoordinate:CLLocationCoordinate2DMake(lat, longi)];
        annotation.title = [dict objectForKey:@"name"];
        annotation.subtitle = [NSString stringWithFormat:@"%@, %@, %@",[dict objectForKey:@"street"],[dict objectForKey:@"county"], [dict objectForKey:@"postcode"]];
        [mapView addAnnotation:annotation];
        [dict release];

        i += [keys count];
    }
}

Answer 1

一个很长的答案，大部分是在 Stephen Poletto 发布时写的，并包含有关如何使用内置方法对数组进行排序的示例代码，所以我认为它仍然值得发布，尽管基本答案是相同的（即“选择五个最接近您自己的，仅传递那些”）：

您将需要为自己按距离对注释进行排序，并仅将最接近的五个提交到 MKMapView。如果您有两个 CLLocation，则可以使用 distanceFromLocation: 方法（在 iOS 3.2 之前为 getDistanceFrom:；该名称现已弃用）。

因此，例如，假设您的 Annotation 类有一个方法“setReferenceLocation：”，您可以向该方法传递一个 CLLocation 和一个 getter“distanceFromReferenceLocation”，该方法返回两者之间的距离，您可以这样做：

// create and populate an array containing all potential annotations
NSMutableArray *allPotentialAnnotations = [NSMutableArray array];

for(all potential annotations)
{
    Annotation *annotation = [[Annotation alloc]
                                            initWithCoordinate:...whatever...];
    [allPotentialAnnotations addObject:annotation];
    [annotation release];
}

// set the user's current location as the reference location
[allPotentialAnnotations
      makeObjectsPerformSelector:@selector(setReferenceLocation:) 
      withObject:mapView.userLocation.location];

// sort the array based on distance from the reference location, by
// utilising the getter for 'distanceFromReferenceLocation' defined
// on each annotation (note that the factory method on NSSortDescriptor
// was introduced in iOS 4.0; use an explicit alloc, init, autorelease
// if you're aiming earlier)
NSSortDescriptor *sortDescriptor = 
              [NSSortDescriptor
                  sortDescriptorWithKey:@"distanceFromReferenceLocation" 
                  ascending:YES];

[allPotentialAnnotations sortUsingDescriptors:
                          [NSArray arrayWithObject:sortDescriptor]];

// remove extra annotations if there are more than five
if([allPotentialAnnotations count] > 5)
{
    [allPotentialAnnotations
               removeObjectsInRange:NSMakeRange(5, 
                           [allPotentialAnnotations count] - 5)];
}

// and, finally, pass on to the MKMapView
[mapView addAnnotations:allPotentialAnnotations];

根据您加载的位置，您需要为注释创建本地存储（在内存中或磁盘上），并在用户移动时选择五个最近的存储。将自己注册为 CLLocationManager 委托或地图视图的 userLocation 属性上的键值观察。如果您有相当多的潜在注释，那么对所有注释进行排序有点浪费，最好建议您使用四叉树或 kd 树。

Answer 2

首先，您需要获取用户的当前位置。您可以构建一个 CLLocationManager 并将自己注册为位置更新的委托，如下所示：

locationManager = [[[CLLocationManager alloc] init] autorelease];
[locationManager setDelegate:self];
[locationManager startUpdatingLocation];

将自己设置为委托后，您将收到以下回调：

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation

现在您拥有了用户的位置（newLocation），您可以找到五个最接近的注释。 CoreLocation 中有一个方便的方法：

- (CLLocationDistance)distanceFromLocation:(const CLLocation *)location

当您迭代注释时，只需存储五个最近的位置即可。您可以使用您使用的“lat”和“longi”变量构建 CLLocation：

- (id)initWithLatitude:(CLLocationDegrees)latitude longitude:(CLLocationDegrees)longitude

希望这会有所帮助！