CATextLayer 仅在一段时间后显示其内容

Question

我知道这听起来很奇怪，但问题恰恰在于：我有一个ViewController，它加载一个与 Grand Central Dispatch 异步的 CSV 文件（该文件代表一个直方图）。我还有一个名为HistogramView 的自定义视图。当控制器完成加载 CSV 文件时，它会在 HistogramView 上调用函数 invalidate。在该函数中，视图解析从文件中读取的数据并创建：

1. 一些表示直方图条形的 CAShapeLayer
2. 一些表示条形标签的 CATextLayer

对于形状层没问题，一切都很好。文本层出现问题：最初只显示背景。文本仅在几秒钟后出现：-/ 这很奇怪。这是invalidate函数的代码（这个函数，正如我所说，在自定义视图中）。

-(void)invalidate{
    float tempX = 0;
    float tempHeight = 0;
    NSArray *data;

    if([self.dataSource getHistogramData] != nil){
        data = [self.dataSource getHistogramData];
        tempX = [self getStartingX];
        [self getDataMaxValue];

        for (int i=0; i<[data count]; i+=2) {

            //THE SHAPE LAYER THAT REPRESENTS AN HISTOGRAM BAR
            tempHeight = [self uniformValue:[[data objectAtIndex:i+1] intValue]];
            CAShapeLayer *rectLayer = [CAShapeLayer layer];
            CGRect rect = CGRectMake(tempX, tempHeight, barWidth, height - tempHeight - spaceForLabels);
            rectLayer.path = CGPathCreateWithRect(rect, nil);
            [rectLayer setBackgroundColor:[UIColor colorWithRed:1 green:1 blue:1 alpha:1].CGColor];
            [self.layer addSublayer:rectLayer];


            //THE TEXT LAYER THAT REPRESENTS A LABEL FOR THE BAR
            CATextLayer *textLayer = [CATextLayer layer];
            textLayer.frame = CGRectMake(tempX, height - spaceForLabels, barWidth, spaceForLabels);
            textLayer.font = CFBridgingRetain([UIFont boldSystemFontOfSize:18].fontName);
            textLayer.fontSize = 18;
            textLayer.foregroundColor = [UIColor redColor].CGColor;
            textLayer.backgroundColor = [UIColor yellowColor].CGColor;
            textLayer.alignmentMode = kCAAlignmentCenter;
            textLayer.string = @"example";
            [self.layer addSublayer:textLayer];


            tempX += barWidth + spaceBetweenBars;
        }

    }
}

如您所见，我以相同的方式在同一点创建了条形图和标签。提前谢谢你。

编辑： 在我的视图控制器中，我以这种方式在 viewDidLoad 方法中完成工作：

- (void)viewDidLoad {
    [super viewDidLoad];

    //create the channel with which read the CSV file
    dispatch_io_t ch = dispatch_io_create_with_path(DISPATCH_IO_STREAM, [[[NSBundle mainBundle] pathForResource:@"histogram1" ofType:@"csv"] UTF8String], O_RDONLY, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), nil);

    //read the CSV file async with GCD
    [self readCSVFile:ch];

    //set the delegates for the histogram view
    self.histogramView.delegate = self;
    self.histogramView.dataSource = self;
}

-(void)readCSVFile:(dispatch_io_t)ch{

    //the string that represents the content of CSV file
    NSMutableString __block *stringFromData;

    //read the whole file
    dispatch_io_read(ch, 0, SIZE_MAX, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(bool done, dispatch_data_t dataRead, int error) {

        if(!error && !done){

            //convert from dispatch_data_t to NSString
            size_t dataSize = dispatch_data_get_size(dataRead);
            stringFromData = [[NSMutableString alloc]initWithCapacity:dataSize];
            dispatch_data_apply(dataRead, ^bool(dispatch_data_t region, size_t offset, const void *buffer, size_t size) {
                [stringFromData appendFormat:@"%.*s", (unsigned int)size, buffer];
                return true;
            });
        }else{
            dispatch_io_close(ch, 0);
            data = [stringFromData componentsSeparatedByCharactersInSet:
                       [NSCharacterSet characterSetWithCharactersInString:@"\n,"]];

            //AND NOW I CALL INVALIDATE ON THE VIEW
            [self.histogramView invalidate];
        }

    });
}

Answer 1

好的，我明白了。我要感谢 rdelmar，他的建议为我指出了问题所在。正如他所说，invalidate 不是从主线程调用的，而是从后台线程（读取文件的线程）调用的。这是错误的，因为必须从主线程更新视图。所以，我以这种方式简单地调用invalidate 方法来解决这个问题：

-(void)readCSVFile:(dispatch_io_t)ch{

    //the string that represents the content of CSV file
    NSMutableString __block *stringFromData;

    //read the whole file
    dispatch_io_read(ch, 0, SIZE_MAX, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(bool done, dispatch_data_t dataRead, int error) {

        if(!error && !done){

            //convert from dispatch_data_t to NSString
            size_t dataSize = dispatch_data_get_size(dataRead);
            stringFromData = [[NSMutableString alloc]initWithCapacity:dataSize];
            dispatch_data_apply(dataRead, ^bool(dispatch_data_t region, size_t offset, const void *buffer, size_t size) {
                [stringFromData appendFormat:@"%.*s", (unsigned int)size, buffer];
                return true;
            });
        }else{
            dispatch_io_close(ch, 0);
            data = [stringFromData componentsSeparatedByCharactersInSet:
                       [NSCharacterSet characterSetWithCharactersInString:@"\n,"]];

            //HERE IS THE SOLUTION
            [[NSOperationQueue mainQueue]addOperationWithBlock:^{
                [self.histogramView invalidate];
            }];
        }

    });
}