ios - 使用Geocoder获取经度和纬度值

Question

我写下面的代码从一个地址获取纬度和经度值。我有2个地址字段从和到。

-(void)getLatLong:(NSString *)string
{
CLGeocoder *geocoder = [[CLGeocoder alloc] init];

[geocoder geocodeAddressString:string completionHandler:^(NSArray *placemarks, NSError *error) {
    if([placemarks count]) {
        CLPlacemark *placemark = [placemarks objectAtIndex:0];
        CLLocation *location = placemark.location;
        CLLocationCoordinate2D coordinate = location.coordinate;
        NSLog(@"coordinate = (%f, %f)", coordinate.latitude, coordinate.longitude);
        if([string isEqualToString:self.from.text])
        {
            fromLat = coordinate.latitude;
            fromLng = coordinate.longitude;

        }
        else if([string  isEqualToString:self.to.text])
        {
            toLat = coordinate.latitude;
            toLng = coordinate.longitude;
        }
    }
}];
}  
- (void)display
{
    NSLog(@"%f",fromLat);
    NSLog(@"%f",fromLng);
    NSLog(@"%f",toLat);
    NSLog(@"%f",toLng);
}
- (IBAction)getLatLongs:(id)sender
{
    [self getLatLong:self.from.text];
    [self getLatLong:self.to.text];
    [self display];
}

单击“首次提交”按钮后，输出为:

 2014-03-20 10:15:28.178 Application[746:70b] 0.000000  
 2014-03-20 10:15:28.179 Application[746:70b] 0.000000  
 2014-03-20 10:15:28.180 Application[746:70b] 0.000000  
 2014-03-20 10:15:28.181 Application[746:70b] 0.000000   
 2014-03-20 10:15:29.019 Application[746:70b] coordinate = (17.387337, 78.480835)  
 2014-03-20 10:15:29.134 Application[746:70b] coordinate = (16.516667, 80.616667) 

第二次单击提交按钮后，输出为:

 2014-03-20 10:15:28.178 Application[746:70b] 17.387337  
 2014-03-20 10:15:28.179 Application[746:70b] 78.480835 
 2014-03-20 10:15:28.180 Application[746:70b] 16.516667  
 2014-03-20 10:15:28.181 Application[746:70b] 80.616667  
 2014-03-20 10:15:29.019 Application[746:70b] coordinate = (17.387337, 78.480835)  
 2014-03-20 10:15:29.134 Application[746:70b] coordinate = (16.516667, 80.616667) 

我想知道为什么如果我在获取LaltLong方法之后调用显示方法，这是第一次获得全零的原因。

Answer 1

当您调用[self getLatLong:self.from.text];时

它调用的[geocoder geocodeAddressString:string completionHandler:^{}];是GCD。哪个运行在不同的线程上。

然后，您立即调用[self display];，直到在GCD中未收到值为止。这就是为什么您第一次获得 0 的原因。
下次调用[self display];时，值已经存在，因此按原样打印。

在-(void)display和GCD内使用断点以获得更多信息。

尝试在GCD中调用[self display];。

-(void)getLatLong:(NSString *)string
{
    CLGeocoder *geocoder = [[CLGeocoder alloc] init];

    [geocoder geocodeAddressString:string completionHandler:^(NSArray *placemarks, NSError *error) {
    if([placemarks count]) {
        CLPlacemark *placemark = [placemarks objectAtIndex:0];
        CLLocation *location = placemark.location;
        CLLocationCoordinate2D coordinate = location.coordinate;
        NSLog(@"coordinate = (%f, %f)", coordinate.latitude, coordinate.longitude);
        if([string isEqualToString:self.from.text])
        {
            fromLat = coordinate.latitude;
            fromLng = coordinate.longitude;
            [self display];
        }
        else if([string  isEqualToString:self.to.text])
        {
            toLat = coordinate.latitude;
            toLng = coordinate.longitude;
            [self display];
        }
    }
    }];
} 

- (void)display
{
   NSLog(@"%f",fromLat);
   NSLog(@"%f",fromLng);
   NSLog(@"%f",toLat);
   NSLog(@"%f",toLng);
}

- (IBAction)getLatLongs:(id)sender
{
   [self getLatLong:self.from.text];
   [self getLatLong:self.to.text];
}