擦除持久的drawRect

Question

我终于设法得到了一直没有清除的 DrawRect （我也有 setClearsContextBeforeDrawing:YES ）

- (void)drawRect:(CGRect)rect
{
    UIGraphicsPushContext(drawingContext);
    CGImageRef cgImage = CGBitmapContextCreateImage(drawingContext); 
    UIImage *uiImage = [[UIImage alloc] initWithCGImage:cgImage];

    CGContextSetLineWidth(drawingContext, 4.0);
    if (draw)
        CGContextSetStrokeColorWithColor(drawingContext, [UIColor  whiteColor] CGColor]);
    else
        CGContextSetStrokeColorWithColor(drawingContext, [[UIColor clearColor] CGColor]);

    CGContextMoveToPoint(drawingContext,    lastPt.x - (31.0 / self.transform.a),  lastPt.y - (31.0 / self.transform.a)  );
    CGContextAddLineToPoint(drawingContext, currPt.x - (31.0 / self.transform.a),  currPt.y - (31.0 / self.transform.a)  );
    CGContextStrokePath(drawingContext);

    UIGraphicsPopContext();
    CGImageRelease(cgImage);
    [uiImage drawInRect: rect];

    lastPt = currPt;
}

这会在绘制时留下一条线
我的问题是当 draw 是 NO [UIColor clearColor] 没有擦除我应该用什么来擦除？提前致谢

Answer 1

因为clearColor 的alpha 为零，所以用它绘图通常没有任何效果。您需要将上下文的混合模式设置为kCGBlendModeCopy 才能生效。

然而，当你想擦除时将上下文的混合模式设置为kCGBlendModeClear，当你想绘图时设置回kCGBlendModeNormal会更简单：

if (draw) {
    CGContextSetBlendMode(drawingContext, kCGBlendModeNormal);
    CGContextSetStrokeColorWithColor(drawingContext, [UIColor  whiteColor] CGColor]);
} else {
    CGContextSetBlendMode(drawingContext, kCGBlendModeClear);
}