iphone - MKMapViewDelegate 派生类和委托(delegate)赋值

这可能更像是一个关于 iOS 的 Objective-c 问题，但我已经看到了一些类似于以下的示例代码，我想更好地理解这些代码。

@interface MyMapView : MKMapView <MKMapViewDelegate> {
// ivars specific to derived class
}

@property(nonatomic,assign) id<MKMapViewDelegate> delegate;

@end

@implementation MyMapView
- (id) initWithFrameCGRect)frame
{
    self = [super initWithFrame:frame];
    if (self)
    {
        // initialize the ivars specific to this class

        // Q1: Why invoke super on this delegate that's also a property of this class?    
        super.delegate = self;

        zoomLevel = self.visibleMapRect.size.width * self.visibleMapRect.size.height;
    }
    return self;
}
#pragma mark - MKMapViewDelegate methods
// Q2: Why intercept these callbacks, only to invoke the delegate?
- (void)mapViewMKMapView *)mapView regionWillChangeAnimatedBOOL)animated
{
    if( [delegate respondsToSelectorselector(mapView:regionWillChangeAnimated] )
    {
        [delegate mapView:mapView regionWillChangeAnimated:animated];
    } 
}

@end

我的两个问题是: 1. 为什么要调用 super.delegate 并且只将“delegate”声明为属性？ 2. 为什么要拦截所有的委托(delegate)调用，然后将它们转发回委托(delegate)？

感谢任何见解。

---

Best Answer-推荐答案

Apple 的文档明确指出您应该避免子类 MKMapView:

http://developer.apple.com/library/ios/#documentation/MapKit/Reference/MKMapView_Class/MKMapView/MKMapView.html#//apple_ref/doc/uid/TP40008205

Although you should not subclass the MKMapView class itself, you can get information about the map view’s behavior by providing a delegate object.

所以我猜这个委托(delegate)“前进”模式是用来不破坏事物的。

我对MKMapView 使用了一些不同的方法。为了尽量减少破损，我使用了两个类。一个子类 MKMapView 并且只是覆盖 init/dealloc 方法并将 delegate 属性分配/释放到另一个类的实例。另一个类是 NSObject 的子类，它实现了 MKMapViewDelegate 协议(protocol)，将是真正的工作。

MyMapView.h

@interface MyMapView : MKMapView
@end

MyMapView.m

// private map delegate class
@interface MapDelegate : NSObject <MKMapViewDelegate>
// instance is not alive longer then MKMapView so use assign to also solve
// problem with circular retain
@property(nonatomic, assign) MKMapView *mapView;
@end

@implementation MapDelegate
@synthesize mapView;

- (id)initWithMapViewReportsMapView *)aMapView {
  self = [super init];
  if (self == nil) {
    return nil;
  }

  self.mapView = aMapView;

  return self;
}

// MKMapViewDelegate methods and other stuff goes here

@end

@implementation MyMapView
- (id)init {
  self = [super init];
  if (self == nil) {
    return nil;
  }

  // delegate is a assign property
  self.delegate = [[MapDelegate alloc] initWithMapView:self];

  return self;
}

- (void)dealloc {
  ((MapDelegate *)self.delegate).mapView = nil;
  [self.delegate release];
  self.delegate = nil;

  [super dealloc];
}
@end

MapDelegate 类的 mapView 属性不是严格需要的，但如果想要对 map View 执行不是某些 MKMapViewDelegate 方法调用、定时器等

关于iphone - MKMapViewDelegate 派生类和委托(delegate)赋值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7586961/