iOS获取UIView上某点的颜色值

项目需求中遇到获取UIView上某个坐标点的RGB颜色值的需求，现在把自己找到的解决方案简单总结记录一下，遇到了下面的情况:

• 不可移动的UIView
• 旋转式的UIView
• 滑条式的UIView

不可移动的UIView

如下图所示，有一个圆形的颜色板，当手指在颜色板上移动时，UIViewController的backgroundColor将会设置成手指在颜色板上触点的颜色值:

实现该功能的方案搜索至stackoverflow, 出处点击这里.

核心代码如下:

#import "UIView+ColorOfPoint.h"
#import <QuartzCore/QuartzCore.h>

@implementation UIView (ColorOfPoint)

- (UIColor *)colorOfPoint:(CGPoint)point {
    unsigned char pixel[4] = {0};
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(pixel, 1, 1, 8, 4, colorSpace, (CGBitmapInfo)kCGImageAlphaPremultipliedLast);

    CGContextTranslateCTM(context, -point.x, -point.y);

    [self.layer renderInContext:context];

    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);

    UIColor *color = [UIColor colorWithRed:pixel[0]/255.0 green:pixel[1]/255.0 blue:pixel[2]/255.0 alpha:pixel[3]/255.0];

    return color;
}

该功能封装成UIView的一个Category了，主要注意头文件QuartzCore.h的引入以及坐标系的转换.

最终，当手指在颜色板上滑动时，获取ColorPanel中颜色值的调用如下:

#import "ImmovableColorPanel.h"
#import "UIView+ColorOfPoint.h"

@implementation ImmovableColorPanel

- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    CGPoint location = [touch locationInView:self];
    self.color = [self colorOfPoint:location];
    [self sendActionsForControlEvents:UIControlEventValueChanged];
    return YES;
}

@end

旋转式的UIView

现在假设上图所示的颜色板是可旋转的，获取颜色值的方式修改为获取色环中间正上方某点的颜色值，如下图所示，获取的是下三角所指某点的颜色值(下三角不会随着颜色板的旋转而旋转).

先看看颜色板随着手指的滑动而旋转是如何实现的:

#import "RoundColorPanel.h"

@interface RoundColorPanel ()

/**
 *  ColorPanel中心点的坐标值
 */
@property (nonatomic) CGPoint centerPoint;

@end

@implementation RoundColorPanel

- (void)awakeFromNib {
    [super awakeFromNib];

    self.centerPoint = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
}

- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {

    CGPoint previousLocation = [touch previousLocationInView:self];
    CGPoint location = [touch locationInView:self];
    CGFloat previousRadian = [self radianToCenterPoint:self.centerPoint withPoint:previousLocation];
    CGFloat curRadian = [self radianToCenterPoint:self.centerPoint withPoint:location];
    CGFloat changedRadian = curRadian - previousRadian;
    [self rotateByRadian:changedRadian];

    [self sendActionsForControlEvents:UIControlEventValueChanged];
    return YES;
}

/**
 *  以ColorPanel的anchorPoint为坐标原点建立坐标系，计算坐标点|point|与坐标原点的连线距离x轴正方向的夹角
 *
 *  @param centerPoint 坐标原点坐标
 *  @param point       某坐标点
 *
 *  @return 坐标点|point|与坐标原点的连线距离x轴正方向的夹角
 */
- (CGFloat)radianToCenterPoint:(CGPoint)centerPoint withPoint:(CGPoint)point {
    CGVector vector = CGVectorMake(point.x - centerPoint.x, point.y - centerPoint.y);
    return atan2f(vector.dy, vector.dx);
}

/**
 *  将图层旋转radian弧度
 *
 *  @param radian 旋转的弧度
 */
- (void)rotateByRadian:(CGFloat)radian {
    CGAffineTransform transform = self.layer.affineTransform;
    transform = CGAffineTransformRotate(transform, radian);
    self.layer.affineTransform = transform;
}

核心代码就是rotateByRadian:消息，使用CALayer的仿射变换完成。

可能你会发现，在continueTrackingWithTouch:消息中没有看到UIView的分类消息colorOfPoint:的发送，其实如果你尝试这样做，会发现这样取出的颜色值是旋转开始时下三角所指点referPoint的颜色值，随着ColorPanel的旋转，该颜色值不会改变，这是因为我们对ColorPanel的旋转是通过仿射变换实现，而仿射变换会改变图层的坐标系，也就是说，referPoint现在所处的坐标系是已做过仿射变换的坐标系，而不再是以图层左上角为坐标原点，向右为x轴正方向，向下为y轴正方向的坐标系。当然你可以计算ColorPanel总的旋转角度totalChangedRadian，然后通过使referPoint旋转-totalChangedRadian角度来计算该点在新坐标系中的坐标值，如下代码所示:

#import "RoundColorPanel.h"
#import "UIView+ColorOfPoint.h"

@interface RoundColorPanel ()

/**
 *  颜色板中心点坐标
 */
@property (nonatomic) CGPoint centerPoint;
/**
 *  获取颜色值参考点与|centerPoint|的连线与x轴正方向的夹角
 */
@property (nonatomic) CGFloat referRadian;
/**
 *  获取颜色值参考点与|centerPoint|的连线长度
 */
@property (nonatomic) CGFloat referRadius;
/**
 *  颜色板在整个旋转过程中的总和
 */
@property (nonatomic) CGFloat totalChangedRadian;

@end

@implementation RoundColorPanel

- (void)awakeFromNib {
    [super awakeFromNib];

    self.centerPoint = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
    //|referPoint|获取该点的颜色值
    CGPoint referPoint = CGPointMake(CGRectGetMidX(self.bounds), 2);
    CGVector vector = CGVectorMake(referPoint.x - self.centerPoint.x, referPoint.y - self.centerPoint.y);
    self.referRadian = atan2(vector.dy, vector.dx);
    self.referRadius = sqrt(vector.dx * vector.dx + vector.dy * vector.dy);
    self.layer.cornerRadius = CGRectGetWidth(self.bounds) / 2;
}

- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {

    CGPoint previousLocation = [touch previousLocationInView:self];
    CGPoint location = [touch locationInView:self];
    CGFloat previousRadian = [self radianToCenterPoint:self.centerPoint withPoint:previousLocation];
    CGFloat curRadian = [self radianToCenterPoint:self.centerPoint withPoint:location];
    CGFloat changedRadian = curRadian - previousRadian;
    [self rotateByRadian:changedRadian];

    self.totalChangedRadian += changedRadian;
    CGFloat radian = self.referRadian - self.totalChangedRadian;
    CGPoint referPoint1 = CGPointMake(self.centerPoint.x + self.referRadius * cos(radian), self.centerPoint.y + self.referRadius * sin(radian));
    self.color = [self colorOfPoint:referPoint1];
    [self sendActionsForControlEvents:UIControlEventValueChanged];

    return YES;
}

- (CGFloat)radianToCenterPoint:(CGPoint)centerPoint withPoint:(CGPoint)point {
    CGVector vector = CGVectorMake(point.x - centerPoint.x, point.y - centerPoint.y);
    return atan2f(vector.dy, vector.dx);
}

- (void)rotateByRadian:(CGFloat)radian {
    CGAffineTransform transform = self.layer.affineTransform;
    transform = CGAffineTransformRotate(transform, radian);
    self.layer.affineTransform = transform;
}

不过，我更趋向于使用接下来使用的方法，更加简单和理解。

将RoundColorPanel放入一个容器视图RotaryColorPanel中，RoundColorPanel的旋转改变的是自身视图的坐标系，而不会改变其父视图的坐标系，因此我们可以在其父视图的坐标系中定义需要取颜色值的坐标点，代码如下:

#import "RotaryColorPanel.h"
#import "RoundColorPanel.h"
#import "UIView+ColorOfPoint.h"
#import "XXNibBridge.h"

@interface RotaryColorPanel () <XXNibBridge>

/**
 *  可旋转的颜色板
 */
@property (weak, nonatomic) IBOutlet RoundColorPanel *roundColorPanel;
/**
 *  下三角指示标识
 */
@property (weak, nonatomic) IBOutlet UIImageView *indicator;

/**
 *  获取该点的颜色值
 */
@property (nonatomic) CGPoint referPoint;

@end

@implementation RotaryColorPanel

- (void)awakeFromNib {
    [super awakeFromNib];

    self.referPoint = CGPointMake(CGRectGetMidX(self.indicator.frame), CGRectGetMaxY(self.indicator.frame));
    [self.roundColorPanel addTarget:self action:@selector(colorPanelRotated:) forControlEvents:UIControlEventValueChanged];
}

- (void)colorPanelRotated:(id)sender {
    UIColor *color = [self colorOfPoint:self.referPoint];
    if (self.colorChangedHandler) {
        self.colorChangedHandler(color);
    }
}

@end

获取颜色值的调用变成如下:

#import "RotaryColorPanelViewController.h"
#import "RotaryColorPanel.h"

@interface RotaryColorPanelViewController ()

@property (weak, nonatomic) IBOutlet RotaryColorPanel *rotaryColorPanel;

@end

@implementation RotaryColorPanelViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    __weak typeof(self) weakSelf = self;
    self.rotaryColorPanel.colorChangedHandler = ^(UIColor *color) {
        weakSelf.view.backgroundColor = color;
    };
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

滑条式UIView

该样式与上面的区别是，在颜色条上有遮挡物滑块，获取颜色值的点坐标是滑块中心点的坐标, 如下图所示:

解决方案同第2中情况类似，就是让滑块和颜色板处在并列的层级，它们同时直属于同一superview中。

上述所有代码已上传github，可点击此处获取.