iPhone:MKMapView 缩小和返回会导致 map Region.center 漂移

Question

我有一个使用 MKMapView 的简单 iPhone 应用程序。它有一个分段控件，可以调整要缩放的区域(街道、社区、城市、州、世界)。它工作正常，但如果我缩小到州级别并回到街道，我注意到中心点已经移动。

以下是在区域更改之前和之后调用的两个委托(delegate)方法以及由分段控件调用的changeZoom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 );}

如果我运行该应用程序，它从我当前位置的街道缩放级别开始，然后单击缩小到州级别并返回到街道级别，我会在控制台中得到以下输出:

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

因此，我在代码中进行的所有调用都按预期进行，并且中心没有改变，但是当我监视 MKMapViewDelegate 方法regionDidChangeAnimated 时，我看到中心发生了移动。如果我在那里设置断点，我会看到一个如下所示的堆栈:

#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

这并不能真正帮助我理解为什么中心会移动。我已经在模拟器和设备上运行了这个，这让我有点疯狂。

如果有人对导致中心转移的原因有任何见解，我将不胜感激。

Answer 1

根据我的经验，使用公共(public) API 以编程方式或使用 UI 事件修改 map 的区域后，会重新计算 map 的 region 属性。

通过缩小，您会失去 map 中心的精确度(一个像素覆盖更大的区域)。然后，在使用 setRegion:[animated:] 设置区域后，(由此计算出的)map.region 可能与 setRegion 中给定的区域不同。这可以解释您获得的增量。

为了解决您的特定问题，我宁愿在每次用户移动 map 时存储 map 的中心并使用存储的值进行缩放，而不是信任 map.region.center。