为什么drawRect留下图像的一部分？

这是我的情况：

我有一个包含餐厅列表的表，每个条目在使用drawRect绘制的记分栏中显示一段时间内的检查结果。

当用户向下然后向上滚动表格时，将显示带有黄色和红色正方形的记分栏，以前的记分栏将显示这些正方形。日期也被绘制到以前的记分栏中。

我的问题在于摆脱旧广场。不应在每次调用drawRect时将其擦除吗？

我在下面放置了三张图片，希望可以解释我的意思。

我认为表格行缓存自定义单元格不是问题吗？在此，一旦UITableCell建立单元出队，便会绘制记分条。单元格中的所有其他视图都是正确的，这使我认为问题在于drawRect无法清除图形上下文。

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
  if (indexPath.row < self.establishments.count) {
      return [self establishmentCellForIndexPath:indexPath];
  } else if (self._noResultsFound) {
      return [self noResultsCell];
  } else {
      return [self loadingCell];
  }
}

- (UITableViewCell *)establishmentCellForIndexPath:(NSIndexPath *)indexPath {

  static NSString *CellIdentifier = @"EstablishmentCell";

  DSFEstablishment *establishment = [self.establishments objectAtIndex:[indexPath row]];

  DSFEstablishmentCell *cell = [self.tableView dequeueReusableCellWithIdentifier:CellIdentifier];

  [cell setEstablishment:establishment];
  [cell updateCellContent];

  return cell;
}

在updateCellContent期间，我们尝试清除旧的scorebarview（如果存在），但是代码从未找到DSFScorebarView类的其他子视图。最后，创建了scorebarView并将其添加到视图的子视图中。

- (void)updateCellContent {
NSLog(@"updateCellContent: %@", self.establishment.latestName);

self.name.text = self.establishment.latestName;
self.address.text = self.establishment.address;
self.type.text = self.establishment.latestType;
if (self.establishment.distance) {
    self.distance.text = [NSString stringWithFormat:@"%.2f km", self.establishment.distance];
} else {
    self.distance.text = @"";
}

// Clear out old scorebarView if exists
for (UIView *view in self.subviews) {
    if ([view isKindOfClass:[DSFScorebarView class]]) {
        NSLog(@">>>removeFromSuperview");
        [view removeFromSuperview];
    }
}

DSFScorebarView *scorebarView = [[DSFScorebarView alloc] initWithInspections:self.establishment.inspections];
[self addSubview:scorebarView];

}

使用initWithInspections中的drawRect绘制得分栏视图，并确保得分栏的长度不超过17个正方形（最近的检查）。每次检查都给定一个绿色=低|黄色=中等=高=的正方形，并在该正方形下方绘制年份。

我设置了self.clearsContextBeforeDrawing = YES，但是它不能解决问题。

// Custom init method
- (id)initWithInspections:(NSArray *)inspections {
CGRect scorebarFrame = CGRectMake(20, 38, 273, 22);
if ((self = [super initWithFrame:scorebarFrame])) {
    self.inspections = inspections;

    self.clearsContextBeforeDrawing = YES;

    [self setBackgroundColor:[UIColor clearColor]];
}
return self;
}

- (void)drawRect:(CGRect)rect {
CGContextRef ctx = UIGraphicsGetCurrentContext();

float xOffset = 0; // Keep track of position of next box -- start at 1
float yOffset = 0; // Fixed Y offset. Adjust to match shadows
NSString *previousYear = nil;
UIFont *yearFont = [UIFont fontWithName:@"PFTempestaFiveCompressed" size:8.0];

// take subset/slice of inspections. only the last 17, so it will fit on screen.
int inspections_count = (int)[self.inspections count];

int startIndex = inspections_count > 17 ? inspections_count - 17 - 1 : 0;
int subarrayLength = inspections_count > 17 ? 17 : inspections_count;
NSArray *inspectionsSlice = [self.inspections subarrayWithRange:NSMakeRange(startIndex, subarrayLength)];

for (id inspection in inspectionsSlice) {

    // Top of box
    NSMutableArray *pos0RGBA = [inspection colorForStatusAtPositionRGBA:0];
    CGFloat topColor[] = ...
    CGContextSetFillColor(ctx, topColor);
    CGRect box_top = CGRectMake(xOffset, yOffset, kScoreBoxWidth, kScoreBoxHeight / 2);
    CGContextAddRect(ctx, box_top);
    CGContextFillPath(ctx);

    // Bottom of box
    NSMutableArray *pos1RGBA = [inspection colorForStatusAtPositionRGBA:1];
    CGFloat bottomColor[] = ...
    CGContextSetFillColor(ctx, bottomColor);
    CGRect box_bottom = CGRectMake(xOffset, kScoreBoxHeight / 2 + yOffset, kScoreBoxWidth, kScoreBoxHeight / 2);
    CGContextAddRect(ctx, box_bottom);
    CGContextFillPath(ctx);

    // Year Text
    NSString *theYear = [inspection dateYear];
    if (![theYear isEqualToString:previousYear]) {
        CGFloat *yearColor = ...
        CGContextSetFillColor(ctx, yearColor);
        // +/- 1/2 adjustments are positioning tweaks
        CGRect yearRect = CGRectMake(xOffset+1, yOffset+kScoreBoxHeight-2, kScoreBoxWidth+50, kScoreBoxHeight);
        [theYear drawInRect:yearRect withFont:yearFont];
        previousYear = theYear;
    }

    xOffset += kScoreBoxWidth + kScoreBoxGap;
}

• 表载荷，绘制4行

• 用户向下滚动；再画8行

• 用户滚动回到顶部；Grace Market / Maple Pizza（第2行）的记分栏已更改。

在实现下面的@HaIR解决方案之后，点击表格行后，我得到了灰色背景的白色条。在将多余的宽度添加到“涂白”年线之后，这非常丑陋。

- (void)drawRect:(CGRect)rect {
  ...
  CGFloat backgroundColour[] = {1.0, 1.0, 1.0, 1.0};
  CGContextSetFillColor(ctx, backgroundColour);
  CGRect fullBox = CGRectMake(xOffset, yOffset, kScoreBoxWidth * 17, 2 * (kScoreBoxHeight-2));
  CGContextAddRect(ctx, fullBox);
  CGContextFillPath(ctx);

  for (id inspection in inspectionsSlice) {
  ...