iphone - CGContextRef 损坏了吗？

Question

当我调用函数 CGContextStrokePath(下面代码的最后一行)时，我的程序崩溃了。上下文是否会以某种方式损坏？ (它只有在 expression(它是一个 NSArray)中有特定值时才会崩溃)。它应该绘制 expression 中的任何内容的图表。例如，如果 expression 具有以下对象:x, cos(表示为字符串)，它将绘制余弦曲线。这是代码:

- (double) yValueFromExpression:(id)anExpression atPosition:(double)xValue
{
    NSDictionary *aDictionary = [NSDictionary dictionaryWithObject:[NSNumber numberWithDouble:xValue] forKey:@"%x"];
    return [CalculatorBrain evaluateExpression:anExpression usingVariableValues:aDictionary];
}

#define PRECISION 500

- (void)drawRect:(CGRect)rect 
{
    double scale = [self.delegate scaleForGraphView:self];
    id expression = [self.delegate expressionForGraphView:self];

    CGPoint origin; 
    origin.x = (self.bounds.origin.x + self.bounds.size.width) / 2;
    origin.y = (self.bounds.origin.y + self.bounds.size.height) / 2;

    [AxesDrawer drawAxesInRect:self.bounds originAtPoint:origin scale:scale];

    // -150/scale to 150/scale is the range of x values that axesDrawer (drawAxesInRect) displays.
    double leftMostXValue = -150 / scale;
    double rightMostXValue = 150 / scale;
    double increment = (rightMostXValue - leftMostXValue) / PRECISION;

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextBeginPath(context);
    CGContextMoveToPoint(context, self.bounds.origin.x, origin.y - 
                     [self yValueFromExpression:expression atPosition:leftMostXValue] * scale); 

    for (int i = 1; i <= PRECISION; ++i) {
        double currentXValue = leftMostXValue + i * increment;
        CGContextAddLineToPoint(context, self.bounds.origin.x + (self.bounds.size.width / PRECISION) * i, 
                            origin.y - [self yValueFromExpression:expression atPosition:currentXValue] * scale);
    }
    CGContextStrokePath(context);
}

这是我在调用 CGContextStrokePath 时收到的错误消息:程序收到信号:“EXC_BAD_ACCESS”。

答案:我需要在 CGContextAddLineToPoint() 周围放置一个保护装置，以确保它在 rect 的范围内绘制:

for (int i = 1; i <= PRECISION; ++i) {
    double currentXValue = leftMostXValue + i * increment;
    double xPoint = self.bounds.origin.x + (self.bounds.size.width / PRECISION) * i;
    double yPoint = origin.y - [self yValueFromExpression:expression atPosition:currentXValue] * scale;
    if (xPoint < (self.bounds.origin.x + self.bounds.size.width) && xPoint > 0 &&
        yPoint < (self.bounds.origin.y + self.bounds.size.height) && yPoint > 0) {
        CGContextAddLineToPoint(context, xPoint, yPoint);
    }
}

Answer 1

EXC_BAD_ACCESS 通常意味着你有内存问题。

如果非要我猜的话，expressionForGraphView 有时可能会返回垃圾。