如何使用 UIPanGestureRecognizer 在 ios 中拖动线条

Question

我正在编写一个程序，用户可以将一条线从一个 UILabel 拖到另一个 UILabel 上。我创建了一个名为 DragView 的 UIView 子类，并重写了 drawRect 方法。我将 UIView 设为 RootController 视图的子视图。 DragView 肯定是可见的，drawRect 方法肯定被调用，但没有任何线条可见。

这些是（我认为是）相关的代码片段。

//DragView.m

- (void)drawRect:(CGRect)rect
{
    if (context == nil)
    {
        context = UIGraphicsGetCurrentContext();
    }

    if (_drawLineFlag)
    {

        CGContextSetLineWidth(context,2.0); 
        CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
        CGFloat components[] = {0.0,0.0,1.0,1.0};
        CGColorRef color = CGColorCreate(colorspace, components);

        CGContextSetStrokeColorWithColor(context,color);

        CGContextMoveToPoint(context,_startX, _startY);

        CGContextAddLineToPoint(context,_currentX,_currentY);

        CGContextStrokePath(context);   
    } 
}    

DrawProgramAppDelegate

- (void) initializeUI
{
.....

    dragView = [[DragView alloc] initWithFrame:CGRectMake(0.0f,0.0f,1024.0f,768.0f)] ;
    [view addSubview: dragView];
    UIPanGestureRecognizer *panGestureRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self  action:@selector(handlePan:)];
    [dragView addGestureRecognizer:panGestureRecognizer];
.....

}

事件处理程序是：

- (void) handlePan:(UIPanGestureRecognizer *) recognizer             
{
    CGPoint point = [recognizer translationInView:dragView];

    [dragView setCurrentX:point.x];
    [dragView setCurrentY:point.y];
    [dragView setDrawLineFlag:YES];

    [view bringSubviewToFront:drawView];
    [dragView drawRect:CGRectMake (0.0f,0.0f,768.0f, 1024.0f)];
}

非常感谢您的帮助。

乔恩

Answer 1

在我看来，你正在尝试绘制一条路径（你应该看看这个 帖子）。在这里，您似乎只是从最后一个点到当前点画一条线

CGContextMoveToPoint(context,_startX, _startY);
CGContextAddLineToPoint(context,_currentX,_currentY);

，考虑到调用的频率只会画一条很小的线。

您不应该调用drawRect：

[dragView drawRect:CGRectMake (0.0f,0.0f,768.0f, 1024.0f)];

您应该告诉系统视图需要显示

[dragView setNeedsDisplay];

您还应该检查手势识别器状态，定义为

typedef enum {
   UIGestureRecognizerStatePossible,
   UIGestureRecognizerStateBegan,
   UIGestureRecognizerStateChanged,
   UIGestureRecognizerStateEnded,
   UIGestureRecognizerStateCancelled,
   UIGestureRecognizerStateFailed,
   UIGestureRecognizerStateRecognized = UIGestureRecognizerStateEnded
} UIGestureRecognizerState;

并且可能仅在状态为 UIGestureRecognizerStateChanged 或 UIGestureRecognizerStateEnded 时绘制>