如何使用 UIBezierPath 绘制跟随触摸事件的线？答案

【问题标题】：How do I use UIBezierPath to draw a line that follows a touch event?如何使用 UIBezierPath 绘制跟随触摸事件的线？
【发布时间】：2011-09-12 19:41:29
【问题描述】：

我从一个类似的问题中选择了以下代码，但我没有任何运气让它工作。

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

    NSLog(@"Check #1");

    UITouch* touch = [touches anyObject];
    self.currentPath = [UIBezierPath bezierPath];
    currentPath.lineWidth = 3.0;
    [currentPath moveToPoint:[touch locationInView:self.view]];
    [paths addObject:self.currentPath];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    [self.currentPath addLineToPoint:[touch locationInView:self.view]];
    [self.view setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect {
    [[UIColor redColor] set];
    for (UIBezierPath *path in paths) {
        NSLog(@"Check #2?");
        [path stroke];
    }
}

我从中得到的是我需要使用UIBezierPath，但我不确定如何让它跟随我的手指。我只想画一条线，在用户触摸时开始，在他们拿起手指的地方结束。

如何使用 UIBezierPath 实现这种线条绘制？

【问题讨论】：

标签： iphone objective-c ios cocoa-touch

【解决方案1】：

#import <UIKit/UIKit.h>

@interface Canvas : UIImageView

@property (nonatomic, assign) CGPoint location;

@end




#import "Canvas.h"

@implementation Canvas

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    self.location = [touch locationInView:self];
}

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint currentLocation = [touch locationInView:self];

    UIGraphicsBeginImageContext(self.frame.size);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    [self.image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetLineWidth(ctx, 5.0);
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    CGContextBeginPath(ctx);
    CGContextMoveToPoint(ctx, self.location.x, self.location.y);
    CGContextAddLineToPoint(ctx, currentLocation.x, currentLocation.y);
    CGContextStrokePath(ctx);
    self.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    self.location = currentLocation;
}

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint currentLocation = [touch locationInView:self];

    UIGraphicsBeginImageContext(self.frame.size);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    [self.image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetLineWidth(ctx, 5.0);
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    CGContextBeginPath(ctx);
    CGContextMoveToPoint(ctx, self.location.x, self.location.y);
    CGContextAddLineToPoint(ctx, currentLocation.x, currentLocation.y);
    CGContextStrokePath(ctx);
    self.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    self.location = currentLocation;
}

@end

【讨论】：

附带说明，多次使用 UIGraphicsGetCurrentContext() 可能是多余的。声明一个 CGContextRef 变量并引用它。
可能是的。这只是我在目录中找到的一个旧示例。在弄清楚 Core Graphics 的早期，我从未将它用于其他任何事情。（我已经更新了示例）
我猜UIGraphicsGetCurrentContext()应该介于UIGraphicsBeginImageContext()和UIGraphicsEndImageContext()之间，否则为NULL。
是的，可能应该。我移动了它。
不要忘记添加 imageView.userInteractionEnabled = YES。默认值为 NO，这意味着视图不响应触摸。