我想允许用户以任何线都不能与另一条线甚至自身相交的方式绘制曲线。绘制曲线没问题，我什至发现我可以通过前后跟踪线的节点然后关闭路径来创建一条闭合的路径，但仍然很像线。

遗憾的是，iOS 只提供了一个点是否包含在闭合路径中的测试(containsPoint: 和CGPathContainsPoint)。不幸的是，用户可以很容易地快速移动他们的手指，使触摸点落在现有路径的两侧，而实际上并没有被该路径所包含，因此测试触摸点是毫无意义的。

我找不到路径方法的任何“交集”。

关于如何完成这项任务还有其他想法吗？

最佳答案

好吧，我确实想出了一个办法来做到这一点。它并不完美，但我认为其他人可能希望看到该技术，因为这个问题被多次提出。我使用的技术将所有要测试的项目绘制到一个位图上下文中，然后将进度行的新段绘制到另一个位图上下文中。这些上下文中的数据使用按位运算符进行比较，如果发现任何重叠，则声明命中。

此技术背后的想法是针对所有先前绘制的线条，甚至针对同一线条的较早部分，测试新绘制线条的每一段。换句话说，该技术将检测一条线何时与另一条线交叉以及何时与自身交叉。

演示该技术的示例应用程序可用:LineSample.zip .

HitTest 的核心是在我的 LineView 对象中完成的。这里有两个关键方法:

- (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: http://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.x, 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，只检查新片段的边界矩形而不是整个 View 会快得多。

关于ios - 在 iOS 中画线时命中检测，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/6482362/