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当我的iPhone应用程序关闭时,我将地图区域保存为用户默认值:

MKCoordinateRegion region = mapView.region;
[[NSUserDefaults standardUserDefaults] setDouble:region.center.latitude forKey:@"map.location.center.latitude"];
[[NSUserDefaults standardUserDefaults] setDouble:region.center.longitude forKey:@"map.location.center.longitude"];
[[NSUserDefaults standardUserDefaults] setDouble:region.span.latitudeDelta forKey:@"map.location.span.latitude"];
[[NSUserDefaults standardUserDefaults] setDouble:region.span.longitudeDelta forKey:@"map.location.span.longitude"];

当应用程序再次启动时,Ш以相同的方式读取这些值,以便用户可以看到与上次完全相同的地图视图:

MKCoordinateRegion region;

region.center.latitude  = [[NSUserDefaults standardUserDefaults] doubleForKey:@"map.location.center.latitude"];
region.center.longitude = [[NSUserDefaults standardUserDefaults] doubleForKey:@"map.location.center.longitude"];
region.span.latitudeDelta  = [[NSUserDefaults standardUserDefaults] doubleForKey:@"map.location.span.latitude"];
region.span.longitudeDelta = [[NSUserDefaults standardUserDefaults] doubleForKey:@"map.location.span.longitude"];

NSLog([NSString stringWithFormat:@"Region read  : %f %f %f %f", region.center.latitude, region.center.longitude, region.span.latitudeDelta, region.span.longitudeDelta]);

[mapView setRegion:region];

NSLog([NSString stringWithFormat:@"Region on map: %f %f %f %f", mapView.region.center.latitude, mapView.region.center.longitude, mapView.region.span.latitudeDelta, mapView.region.span.longitudeDelta]);

我从用户默认值中读取的区域(毫不奇怪)与保存时完全相同.请注意,保存的内容直接来自地图,因此不会以任何方式进行转换.我用setRegion:方法将它设置在地图上,但后来却与众不同!

示例结果:

Region read  : 50.241110 8.891555 0.035683 …

请看下面的代码：

class A {
    let a: String
    let b: String

    init(a: String, b: String) {
        self.a = a
        self.b = b
    }
}

class B: A {
    let c: Bool

    private let aExpectedValue = "a"
    private let bExpectedValue = "b"

    override init(a: String, b: String) {
        c = (a == aExpectedValue && b == bExpectedValue)
        super.init(a: a, b: b)
    }
}

这会导致以下错误B.init：

但是，如果我将其更改为c = (a == aExpectedValue)或c = (b == bExpectedValue)然后正确编译。

有人知道为什么吗？