我有这个功能:
- (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context
{
TileOverlay *tileOverlay = (TileOverlay *)self.overlay;
NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];
CGContextSetAlpha(context, tileAlpha);
for (ImageTile *tile in tilesInRect)
{
__block UIImage * image;
CGRect rect = [self rectForMapRect:tile.frame];
NSString *path = [[NSString alloc] initWithFormat:@".../%@.png", tile.imagePath];
NSLog(@"Loading tile from URL %@", path);
image =[UIImage imageWithData: [NSData dataWithContentsOfURL:[NSURL URLWithString: path]]];
CGContextSaveGState(context);
CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));
CGContextScaleCTM(context, 1/zoomScale, 1/zoomScale);
CGContextTranslateCTM(context, 0, image.size.height);
CGContextScaleCTM(context, 1, -1);
CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), [image CGImage]);
CGContextRestoreGState(context);
}
}
如您所知,
dataWithContentsOfURL
直到完成阻塞线程。我想将图像加载块添加到GCD
部分。我试图这样做:
- (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context
{
TileOverlay *tileOverlay = (TileOverlay *)self.overlay;
NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];
CGContextSetAlpha(context, tileAlpha);
for (ImageTile *tile in tilesInRect)
{
__block UIImage * image;
CGRect rect = [self rectForMapRect:tile.frame];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW,0), ^{
NSString *path = [[NSString alloc] initWithFormat:@".../%@.png", tile.imagePath];
NSLog(@"Loading tile from URL %@", path);
image =[UIImage imageWithData: [NSData dataWithContentsOfURL:[NSURL URLWithString: path]]];
CGContextSaveGState(context);
CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));
CGContextScaleCTM(context, 1/zoomScale, 1/zoomScale);
CGContextTranslateCTM(context, 0, image.size.height);
CGContextScaleCTM(context, 1, -1);
CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), [image CGImage]);
CGContextRestoreGState(context);
});
}
}
但是我遇到上下文错误。请帮我这个东西。
如何在GCD块中使用上下文操作?
最佳答案
我的第一个注意事项是MKOverlayView已贬值。您应该考虑切换到MKOverlayRenderer。
在任何情况下,都不应在-draw__方法中使用GCD。其中包括MKOverlayView -drawMapRect:zoomScale:inContext:和UIView -drawRect:。相反,您应该将NSOperationQueue与-canDrawMapRect:zoomScale:zoomScale和setNeedsDisplayInMapRect:结合使用。
这是一些sudo代码:
- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale
{
BOOL hasAtLeastOneTile = NO;
TileOverlay *tileOverlay = (TileOverlay *)self.overlay;
NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];
for (ImageTile *tile in tilesInRect) {
if ([tile isAvailable]) {
hasAtLeastOneTile = hasAtLeastOneTile || YES;
} else {
// Add operation to NSOperationQueue to fetch tile
__weak MKOverlayView *weakOverlay = self; // Weak ref to prevent retain cycles
NSOperation *op = [NSBlockOperation blockOperationWithBlock: ^{
//TODO: Load Tile
[weakOverlay setNeedsDisplayInMapRect:mapRect];
}];
[self.operationQueue addOperation:op];
}
}
return hasAtLeastOneTile;
}
然后在-drawMapRect:zoomScale:inContext中:绘制可用的图块,并跳过不可用的图块。