虽然你的问题还不清楚你究竟想要什么将标签置于饼图中心。我尝试了解你在上面发布的问题中所理解的内容。

首先使用 UIBezierPath 而不是 CGContext上。我使用

我试过如果还有什么不清楚请覆盖我答案中的所有内容请随时发表评论。我还附上我的示例项目，以节省时间。

希望它有所帮助！

I want add Text in Piechart.

UIGraphicsPushContext(context);
CGContextMoveToPoint(context, newCenterX, newCenterY);
CGContextAddArc(context, newCenterX, newCenterY, radius,arcOffset-(myAngle/2),arcOffset+(myAngle/2), 0);
UIColor *color = [PIECHART_COLORS objectAtIndex:i%6];
CGContextSetFillColorWithColor(context, color.CGColor);
CGContextClosePath(context);
CGContextFillPath(context);
CGMutablePathRef arcPath = CGPathCreateMutable();
CGPathAddArc(arcPath, NULL, self.frame.size.width/2, self.frame.size.height/2, radius, arcOffset-(myAngle/2), arcOffset+(myAngle/2),0);
CGRect rect =CGPathGetBoundingBox(arcPath);

rect = CGRectMake(rect.origin.x+rect.size.width/2, rect.origin.y+rect.size.height/2, 80, 20);

// For InnerCircle Empty

CGContextMoveToPoint(context, newCenterX, newCenterY);
float innercircleRadius = IS_IPAD ? 70 : 40;
CGContextAddArc(context, newCenterX, newCenterY, radius-innercircleRadius,arcOffset-(myAngle/2),arcOffset+(myAngle/2), 0);
CGContextSetFillColorWithColor(context, [UIColor blackColor].CGColor);
CGContextClosePath(context);
CGContextFillPath(context);

From above I will able to draw PieChart with Inner part White space but I am unable to add text in particular piechart slice. Can anyone help with some good example?

My pie chart is displaying perfectly but text align is not coming in proper way.

//For Text I will Try This Code

NSString *text = [NSString stringWithFormat:@"%.2f %%",[[[myArray objectAtIndex:i] objectForKey:@"Count"] floatValue]];
[text drawInRect: rect withAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:16], NSForegroundColorAttributeName:[UIColor redColor]} ];

Although your question is quite unclear that how exactly you want to center the label to pie slice. I tried with what I understand from the question you posted above.

First thing is use UIBezierPath instead of CGContext. I used Parametric equaction of circle to calculate the boundary points of my circle.

CGPoint newPoints = CGPointMake(a + r * cos(theta), b + r * sin(theta));
// theta = angle
// r = radius
// a = centerX
// b = centerY

It will give you the points where you need to show your label. Now you just need to calculate the correct angle (center of slice) to achieve your goal.

UIBezierPath has a method [bezierPath addArcWithCenter:center radius:radius startAngle:startAngle endAngle:endAngle clockwise:YES]; which creates an arc.

Now I'm going to share the code step by step:

Two macros:

#define TO_DEGREE(x) (x * 180.0)/M_PI
#define TO_RADIAN(x) (x * M_PI)/180.0

viewDidLoad: method to initialize the dummy data for my chart.

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    chartView.backgroundColor = [UIColor clearColor];

    /// Assuming dummy data for our chartView
    NSArray *gendersData = @[@{@"color": [UIColor redColor], @"title": @"Boys", @"percentage": @40},
                         @{@"color": [UIColor greenColor], @"title": @"Girls", @"percentage": @45},
                         @{@"color": [UIColor purpleColor], @"title": @"Unknown", @"percentage": @15}];

    [self preparePieChartForData:gendersData];

}

preparePieChartForData: method

- (void)preparePieChartForData:(NSArray *)data {

    /// We will use Parametric equaction of circle to get the boundary of circle
    /*
     * Parametric equation of circle
     * x = a + r cos t
     * y = b + r sin ⁡t
     * a, b are center of circle
     * t (theta) is angle
     * x and y will be points which are on circumference of circle
     *
               270
                |
            _   |   _
                |
     180 -------o------- 360
                |
            +   |   +
                |
                90
     *
     */

    /// Thats why starting from 270.0
    CGFloat lastAngle = 270.0;
    for (NSDictionary *genderData in data) {

        /// Getting data from dictionary
        CGFloat percentage = [genderData[@"percentage"] floatValue];
        UIColor *color = genderData[@"color"];
        NSString *title = genderData[@"title"];

        /// Calculating the angle from percentage, 360 is full circle.
        CGFloat angle = lastAngle + (360 * percentage)/100.0;
        [self makeSliceStartFrom:lastAngle endAt:angle radius:80.0 color: color title:title];

        /// Updating lastAngle so that next angle can start just after this
        lastAngle = angle;
    }
}

Making pie slice method

- (void)makeSliceStartFrom:(CGFloat)startAngle
                     endAt:(CGFloat)endAngle
                    radius:(CGFloat)radius
                     color:(UIColor *)color
                     title:(NSString *)title {

    /// Converting degree to radians as bezierPath accept angle in radians
    CGFloat endAngleInRadians   = TO_RADIAN(endAngle);
    CGFloat startAngleInRadians = TO_RADIAN(startAngle);
    if (endAngle >= -180.0 && endAngle < -90.0) {
        /// This is because of above diagram
        startAngleInRadians = TO_RADIAN(-90);
    }

    /// This is the center of chartView
    CGPoint center = CGPointMake(chartView.bounds.size.width/2.0, chartView.bounds.size.height/2.0);

    /// Initializing Bezeir path
    UIBezierPath *bezierPath = [UIBezierPath bezierPath];
    [bezierPath addArcWithCenter:center radius:radius startAngle:startAngleInRadians endAngle:endAngleInRadians clockwise:YES];

    /// Line width of pie chart
    CGFloat lineWidth = 30.0;
    CGPathRef slicePath = bezierPath.CGPath;

    /// Making shape layer from the path
    CAShapeLayer *sliceLayer = [CAShapeLayer layer];
    sliceLayer.path = slicePath;
    sliceLayer.strokeColor = color.CGColor;
    sliceLayer.lineWidth = lineWidth;
    sliceLayer.fillColor = [UIColor clearColor].CGColor;
    [chartView.layer addSublayer:sliceLayer];

    /*
     * ------------- LABEL PART -------------
     * Adding label at center of the slice
     */

    /// Creating an empty label
    UILabel *lbl = [[UILabel alloc] init];
    lbl.font = [UIFont systemFontOfSize:10.0];
    lbl.text = title;
    [lbl sizeToFit];

    /// theta is the center (middle) angle of this slice
    CGFloat theta = startAngleInRadians + (endAngleInRadians - startAngleInRadians)/2.0;

    /// Adding lineWith and 10.0 extra in radius so that label can visible outside of the circle
    CGFloat r = radius + lineWidth + 10.0;
    CGFloat a = center.x;
    CGFloat b = center.y;

    /// Calculating points from theta and angle by using parametric equation of cricle
    CGPoint newPoints = CGPointMake(a + r * cos(theta), b + r * sin(theta));
    CGRect  frame = lbl.frame;

    /// Recalculating the origin so that label can be exact center of slice. newPoints are (0, 0) position of label.
    frame.origin = CGPointMake(newPoints.x - frame.size.width/2.0, newPoints.y - frame.size.height/2.0);
    lbl.frame = frame;
    [chartView addSubview:lbl];
}

chartView is a UIView adding on self.view in storyboard. Its size is (300.0, 300.0) and placed at center of the screen. Below is the output:

I tried to cover everything in my answer if still anything is unclear please feel free to comment. I'm also attaching my sample project for time saving.

Hope it helps!