我正在使用自定义 MKOverlay/MKOverlayView 用我自己的异步加载的图 block 完全覆盖 Google basemap 。当我收到对覆盖 View 的 canDrawMapRect:zoomScale: 调用(并在这种情况下返回 FALSE)时，我遵循请求卸载图 block 的模式，然后调用 setNeedsDisplayInMapRect:zoomScale:一旦瓷砖可用。

这一切通常都有效，并且在模拟器中似乎完美运行。

但是，在设备上，我有时会在叠加层中看到一个“洞”——缺失的图 block 。

我可以看到该图 block 已被请求，并且请求已完成。我可以看到我调用了 setNeedsDisplayInMapRect:zoomScale:，并且我正在传递 canDrawMapRect 中提供的原始 MKMapRect 和 MKZoomScale :缩放比例:。但我也可以看到，从未要求叠加层重绘该图 block (canDrawMapRect:zoomScale: 和 drawMapRect:zoomScale:inContext: 都不再为该图 block 调用) .

我需要了解为什么会发生这种情况以及如何纠正它。

这是我的 MKOverlayView 子类的相关代码:

- (BOOL) canDrawMapRect: (MKMapRect) mapRect zoomScale: (MKZoomScale) zoomScale 
{
    NSUInteger zoomLevel = [self zoomLevelForZoomScale:zoomScale];
    CGPoint mercatorPoint = [self mercatorTileOriginForMapRect:mapRect];
    NSUInteger tilex = floor(mercatorPoint.x * [self worldTileWidthForZoomLevel:zoomLevel]);
    NSUInteger tiley = floor(mercatorPoint.y * [self worldTileWidthForZoomLevel:zoomLevel]);

    NSURL* tileUrl = [self tileURLForZoomLevel: zoomLevel tileX: tilex tileY: tiley];

    ASIHTTPRequest* tileRequest = [ASIHTTPRequest requestWithURL: tileUrl];
    tileRequest.downloadCache = [ASIDownloadCache sharedCache];
    [tileRequest setCacheStoragePolicy:ASICachePermanentlyCacheStoragePolicy];

    if ( NO == [[ASIDownloadCache sharedCache] isCachedDataCurrentForRequest: tileRequest] )
    {
        [tileRequest setCachePolicy: ASIAskServerIfModifiedWhenStaleCachePolicy];
        tileRequest.delegate = self;
        tileRequest.userInfo = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSValue value: &mapRect withObjCType: @encode( MKMapRect )],       @"mapRect",
                                [NSValue value: &zoomScale withObjCType: @encode( MKZoomScale )],   @"zoomScale",
                                [NSNumber numberWithInt: tilex], @"tilex",
                                [NSNumber numberWithInt: tiley], @"tiley",
                                nil];

        [_tileRequestStack addOperation: tileRequest];

        NSLog( @"canDrawMapRect: %d, %d - REQUESTING", tilex, tiley );

        return NO;
    }

    NSLog( @"canDrawMapRect: %d, %d - READY", tilex, tiley );

    return YES;
}

- (void) drawMapRect: (MKMapRect) mapRect zoomScale: (MKZoomScale) zoomScale inContext: (CGContextRef) context 
{
    NSUInteger zoomLevel = [self zoomLevelForZoomScale:zoomScale];

    CGPoint mercatorPoint = [self mercatorTileOriginForMapRect:mapRect];

    NSUInteger tilex = floor(mercatorPoint.x * [self worldTileWidthForZoomLevel:zoomLevel]);
    NSUInteger tiley = floor(mercatorPoint.y * [self worldTileWidthForZoomLevel:zoomLevel]);

    NSLog( @"drawMapRect:  %d, %d", tilex, tiley );

    NSURL* tileUrl = [self tileURLForZoomLevel: zoomLevel tileX: tilex tileY: tiley];
    NSData* tileData = [[ASIDownloadCache sharedCache] cachedResponseDataForURL: tileUrl];

    UIGraphicsPushContext(context);

    if ( tileData != nil )
    {
        UIImage* img = [UIImage imageWithData: tileData];

        if ( img != nil )
        {
            [img drawInRect: [self rectForMapRect: mapRect] blendMode: kCGBlendModeNormal alpha: 1.0 ];
        }
        else 
        {
            NSLog( @"oops - no image" );
        }

        CGSize s = CGContextConvertSizeToUserSpace( context, CGSizeMake( 40, 1 ));

        UIFont* f = [UIFont systemFontOfSize: s.width];

        [[UIColor blackColor] setFill];

        [[NSString stringWithFormat: @"%d,%d", tilex, tiley] drawInRect: [self rectForMapRect: mapRect] withFont: f];
    }

    UIGraphicsPopContext();
}


- (void) requestFinished: (ASIHTTPRequest *) tileRequest
{
    NSValue* mapRectValue =  [tileRequest.userInfo objectForKey: @"mapRect"];
    MKMapRect mapRect;  [mapRectValue getValue: &mapRect];

    NSValue *zoomScaleValue = [tileRequest.userInfo objectForKey:@"zoomScale"];
    MKZoomScale zoomScale; [zoomScaleValue getValue: &zoomScale];

    NSLog( @"requestFinished: %d, %d, %lf", 
          [[tileRequest.userInfo objectForKey:@"tilex"] intValue], 
          [[tileRequest.userInfo objectForKey:@"tiley"] intValue], 
          zoomScale  );

    [self setNeedsDisplayInMapRect: mapRect zoomScale: zoomScale];
}

编辑:I'm guessing that this is likely the issue.

最佳答案

我遇到的问题与此处描述的问题非常相似。在我的例子中，我无法重现所需的行为(在 http://developer.apple.com/library/ios/documentation/MapKit/Reference/MKOverlayView_class/Reference/Reference.html#//apple_ref/occ/instm/MKOverlayView/setNeedsDisplayInMapRect:zoomScale 中描述)，即使是最简单的代码也是如此:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    return NO;
}

或更接近我的原始代码:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [SomeAsynchronousRequestWithCompletionHandler:^{
        [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    }];
    return NO;
}

在这两种情况下，setNeedsDisplayInMapRect:zoomScale: 从未被调用过一次。

情况发生了变化，当我开始在 dispatch_async 中运行 setNeedsDisplayInMapRect:zoomScale: 时，dispatch_async 被调度到 canDrawMapRect 运行所在的同一队列，例如:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {

    dispatch_queue_t queue = dispatch_get_current_queue();

    NSLog(@"This should trace forever");

    dispatch_async(queue, ^{
        [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    });

    return NO;
}

或包含异步作业:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    dispatch_queue_t queue = dispatch_get_current_queue();

    [SomeAsynchronousRequestWithCompletionHandler:^{
        dispatch_async(queue, ^{
            [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
        });
    }];
    return NO;
}

使用 dispatch_async - 我可以看到 “This should trace forever” 字符串被无休止地跟踪。我原来的问题也完全消失了。

稍后更新:目前，我使用 dispatch_get_main_queue() 来调用 setNeedsDisplayInMapRect:zoomScale: 就像

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [SomeAsynchronousRequestWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
        });
    }];
    return NO;
}

关于ios - setNeedsDisplayInMapRect 不触发新的 drawMapRect : call，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/10128447/