这种技术背后的想法是对所有先前绘制的线测试新绘制的线的每个片段,甚至对同一行的较早片段进行测试.换句话说,这种技术将检测线何时穿过另一条线,并且当它跨过自身时.

演示该技术的示例应用程序可用：LineSample.zip.

命中测试的核心是在我的LineView对象中完成.这里有两个关键的方法：

- (CGContextRef)newBitmapContext {

    // creating b&w bitmaps to do hit testing
    // based on: http://robnapier.net/blog/clipping-cgrect-cgpath-531
    // see "Supported Pixel Formats" in Quartz 2D Programming Guide
    CGContextRef bitmapContext =
    CGBitmapContextCreate(NULL,// data automatically allocated
                          self.bounds.size.width,self.bounds.size.height,8,self.bounds.size.width,NULL,kCGImageAlphaOnly);
    CGContextSetShouldAntialias(bitmapContext,NO);
    // use CGBitmapContextGetData to get at this data

    return bitmapContext;
}


- (BOOL)line:(Line *)line canExtendToPoint:(CGPoint) newPoint {

    //  Lines are made up of segments that go from node to node. If we want to test for self-crossing,then we can't just test the whole in progress line against the completed line,we actually have to test each segment since one segment of the in progress line may cross another segment of the same line (think of a loop in the line). We also have to avoid checking the first point of the new segment against the last point of the prevIoUs segment (which is the same point). Luckily,a line cannot curve back on itself in just one segment (think about it,it takes at least two segments to reach yourself again). This means that we can both test progressive segments and avoid false hits by NOT drawing the last segment of the line into the test! So we will put everything up to the  last segment into the hitProgressLayer,we will put the new segment into the segmentLayer,and then we will test for overlap among those two and the hitTestLayer. Any point that is in all three layers will indicate a hit,otherwise we are OK.

    if (line.Failed) {
        // shortcut in case a Failed line is retested
        return NO;
    }
    BOOL ok = YES; // thinking positively

    // set up a context to hold the new segment and stroke it in
    CGContextRef segmentContext = [self newBitmapContext];
    CGContextSetLineWidth(segmentContext,2); // bit thicker to facilitate hits
    CGPoint lastPoint = [[[line nodes] lastObject] point];
    CGContextMoveToPoint(segmentContext,lastPoint.x,lastPoint.y);
    CGContextAddLineToPoint(segmentContext,newPoint.x,newPoint.y);
    CGContextStrokePath(segmentContext);

    // now we actually test
    // based on code from benzado: https://stackoverflow.com/questions/6515885/how-to-do-comparisons-of-bitmaps-in-ios/6515999#6515999
    unsigned char *completedData = CGBitmapContextGetData(hitCompletedContext);
    unsigned char *progressData = CGBitmapContextGetData(hitProgressContext);
    unsigned char *segmentData = CGBitmapContextGetData(segmentContext);

    size_t bytesPerRow = CGBitmapContextGetBytesPerRow(segmentContext);
    size_t height = CGBitmapContextGetHeight(segmentContext);
    size_t len = bytesPerRow * height;

    for (int i = 0; i < len; i++) {
        if ((completedData[i] | progressData[i]) & segmentData[i]) { 
            ok = NO; 
            break; 
        }
    }

    CGContextRelease(segmentContext);

    if (ok) {
        // now that we know we are good to go,// we will add the last segment onto the hitProgressLayer
        int numberOfSegments = [[line nodes] count] - 1;
        if (numberOfSegments > 0) {
            // but only if there is a segment there!
            CGPoint secondToLastPoint = [[[line nodes] objectAtIndex:numberOfSegments-1] point];
            CGContextSetLineWidth(hitProgressContext,1); // but thinner
            CGContextMoveToPoint(hitProgressContext,secondToLastPoint.x,secondToLastPoint.y);
            CGContextAddLineToPoint(hitProgressContext,lastPoint.y);
            CGContextStrokePath(hitProgressContext);
        }
    } else {
        line.Failed = YES;
        [linesFailed addObject:line];
    }
    return ok;
}

我很乐意听到建议或看到改进.一方面,只需检查新段落的边界,而不是整个视图,将会更快.