iPhone：MKMapView縮小並返回導致地圖region.center漂移

Question

我有一個簡單的iPhone應用程序，它使用MKMapView。它有一個分段控制，將調整區域以縮放（街道，鄰居，城市，州，世界）。它工作正常，但如果我縮小到州級並回到街道，我注意到中心點已經移動。

以下是之前並得到由分段控制調用的區域變化和我changeZoom方法後得到叫了兩個委託方法：

 
- (void)mapView:(MKMapView *)mView regionDidChangeAnimated:(BOOL)animated 
{ 
MKCoordinateRegion region = [mapView region]; 

NSLog(@"Region DID change. Center is now %f,%f, Deltas=%f,%f", 
    region.center.latitude, region.center.longitude, 
    region.span.latitudeDelta, region.span.longitudeDelta); 
} 

- (void)mapView:(MKMapView *)mView regionWillChangeAnimated:(BOOL)animated 
{ 
MKCoordinateRegion region = [mapView region]; 

NSLog(@"Region WILL change. Center is now %f,%f, Deltas=%f,%f", 
    region.center.latitude, region.center.longitude, 
    region.span.latitudeDelta, region.span.longitudeDelta); 
} 


- (IBAction)changeZoom:(id)sender 
{ 
UISegmentedControl *sc = (UISegmentedControl *)sender; 

float mile_range = 1; 

NSLog(@"changeZoom called. MapView currently is Center=%f,%f Deltas=%f,%f", 
    mapView.region.center.latitude, mapView.region.center.longitude, 
    mapView.region.span.latitudeDelta, mapView.region.span.longitudeDelta); 

switch ([sc selectedSegmentIndex]) 
{ 
    case 0: /* Street */ 
    mile_range=1; 
    break; 
    case 1: /* Hood */ 
    mile_range=2; 
    break; 
    case 2: /* City */ 
    mile_range=30; 
    break; 
    case 3: /* State */ 
    mile_range=500; 
    break; 
    case 4: /* World */ 
    mile_range=4000; 
    break; 
    default: /* Error */ 
    NSLog(@"Unknown segment selected from the UISegmentedControl!!!!!!!!!!!!!!"); 
    return; 
    break; 
} 

originalZoomCenter = mapView.region.center; 
MKCoordinateRegion region; 
    region = MKCoordinateRegionMakeWithDistance(originalZoomCenter, mile_range/MILES_PER_METER, mile_range/MILES_PER_METER); 
    region = [mapView regionThatFits: region]; 

NSLog(@"Calling setRegion Center=%f,%f Deltas=%f,%f", 
    region.center.latitude, region.center.longitude, 
    region.span.latitudeDelta, region.span.longitudeDelta); 

    [mapView setRegion: region animated: YES]; 

NSLog(@"After setRegion Center=%f,%f Deltas=%f,%f", 
    mapView.region.center.latitude, mapView.region.center.longitude, 
    mapView.region.span.latitudeDelta, mapView.region.span.longitudeDelta); 
} 

如果我運行的應用程序，它在街上變焦開出在我的當前位置的水平，然後我點擊縮小以國家級和回街道，我得到了在控制檯下面的輸出：

 
Region WILL change. Center is now 30.145127,-40.078125, Deltas=151.851332,225.000000 
Region DID change. Center is now 37.331676,-122.030725, Deltas=0.024842,0.027466 
changeZoom called. MapView currently is Center=37.331676,-122.030725 Deltas=0.024842,0.027466 
Calling setRegion Center=37.331676,-122.030725 Deltas=12.708100,14.062500 
Region WILL change. Center is now 37.331676,-122.030725, Deltas=0.024842,0.027466 
After setRegion Center=37.331676,-122.030725 Deltas=0.024842,0.027466 
Region DID change. Center is now 37.335224,-122.036133, Deltas=12.707505,14.062500 
changeZoom called. MapView currently is Center=37.335224,-122.036133 Deltas=12.707505,14.062500 
Calling setRegion Center=37.335224,-122.036133 Deltas=0.024841,0.027466 
Region WILL change. Center is now 37.335224,-122.036133, Deltas=12.707505,14.062500 
After setRegion Center=37.335224,-122.036133 Deltas=12.707505,14.062500 
Region DID change. Center is now 37.335224,-122.036133, Deltas=0.024841,0.027466 

因此，所有的電話我讓我的代碼進行預期和該中心不會改變，但當我監視MKMapViewDelegate方法regionDidChangeAnimated時，我看到中心被移動了。如果我設置一個斷點，我看到一個堆棧看起來像這樣：

 
#0 0x00034c4c in -[MapViewController mapView:regionDidChangeAnimated:] at MapViewController.m:209 
#1 0x01fbf7bf in -[MKMapView _didChangeRegionMidstream:] 
#2 0x003e11f5 in -[UIView(Gestures) animator:stopAnimation:] 
#3 0x003743be in -[UIAnimator stopAnimation:] 
#4 0x00374154 in -[UIAnimator(Static) _advance:] 
#5 0x0264d719 in HeartbeatTimerCallback 
#6 0x01d28ac0 in CFRunLoopRunSpecific 
#7 0x01d27c48 in CFRunLoopRunInMode 
#8 0x0264a78d in GSEventRunModal 
#9 0x0264a852 in GSEventRun 
#10 0x00307003 in UIApplicationMain 
#11 0x00002194 in main at main.m:14 

這並不能真正幫助我理解爲什麼中央會移動。我已經在模擬器和設備上運行了它，這讓我有點瘋狂。

如果任何人有任何洞察力是什麼導致中心轉移，我將不勝感激任何指針。

Answer 1

根據我的經驗，地圖的region屬性在用公共API或使用UI事件以編程方式修改後重新計算。

通過縮小，可以減少地圖中心的精度（一個像素覆蓋更大的區域）。然後，在用setRegion:[animated:]設置區域之後，（因此計算的）map.region可以不同於在setRegion中給出的區域。這可以解釋你得到的三角洲。

爲了解決您的特定問題，我寧願在用戶每次移動地圖時存儲地圖的中心，並使用存儲的值進行縮放，而不是相信map.region.center。