ios – CALayer上的绘制文本在运行时不会出现或崩溃

我有一个CALayer,我首先绘制一些东西,然后是文本:

- (void)drawInContext:(CGContextRef)context
{
    CGContextSaveGState(context);

    // draw things, everything displays correctly ...

    CGSize expectedCreditSize = [[gpData.credits stringValue] sizeWithFont:[UIFont 
                                                          systemFontOfSize:self.fontSize]];

    rect = CGRectMake(self.bounds.origin.x, self.bounds.size.height/2,
                      expectedCreditSize.width, expectedCreditSize.height);

    CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);

    [self.creditString drawInRect:rect
                         withFont:[UIFont systemFontOfSize:self.fontSize]
                    lineBreakMode:NSLineBreakByWordWrapping
                        alignment:NSTextAlignmentRight];

    CGContextRestoreGState(context);
}

所有图形内容都正确显示,但文本根本没有显示.我究竟做错了什么?

我还尝试将CATextLayer添加为subLayer,但因此我在运行时收到错误消息.

CATextLayer *creditsTextLayer = [[CATextLayer alloc] init];
[creditsTextLayer setFrame:self.frame];
[creditsTextLayer setPosition:self.position];
[creditsTextLayer setString:self.creditString];
[creditsTextLayer setFontSize:self.fontSize];
[creditsTextLayer setAlignmentMode:kCAAlignmentLeft];
[creditsTextLayer setForegroundColor:[[UIColor whiteColor] CGColor]];
[self addSublayer:creditsTextLayer];

唯一有效的是这个解决方案:

CGContextSelectFont (context,
                     "Helvetica-Bold",
                     self.fontSize,
                     kCGEncodingMacRoman);
CGContextSetTextDrawingMode (context, kCGTextFill);

CGContextSetRGBFillColor (context, 0, 1, 0, .5);
CGContextSetRGBStrokeColor (context, 0, 0, 1, 1);
CGContextShowTextAtPoint (context, 40, 0, self.creditString, 9);

但这非常不舒服.

有没有人有想法,我能做些什么才能让我的第一个建议起作用?我错过了显示文字的内容吗?

提前致谢!

编辑:

根据Seamus的回答,我现在正在使用它

- (void)drawInContext:(CGContextRef)context
{
    CGContextSaveGState(context);

    // draw things like circles and lines, 
    // everything displays correctly ...

    // now drawing the text
    UIGraphicsPushContext(context);

    CGSize expectedCreditSize = [[gpData.credits stringValue] sizeWithFont:[UIFont 
                                                          systemFontOfSize:self.fontSize]];

    rect = CGRectMake(self.bounds.origin.x, self.bounds.size.height/2,
                       expectedCreditSize.width, expectedCreditSize.height);

    CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);

    [self.creditString drawInRect:rect
                         withFont:[UIFont systemFontOfSize:self.fontSize]
                    lineBreakMode:NSLineBreakByWordWrapping
                        alignment:NSTextAlignmentRight];

    UIGraphicsPopContext();
    CGContextRestoreGState(context);
}

这意味着我只需按下并弹出文本的图形上下文.这有意义吗?为什么它可以用于绘制其他CGContext的东西?也许有人可以解释……

最佳答案 观察-drawInRect:withFont:lineBreakMode:alignment:不接受CGContextRef参数 – 它绘制到“当前”图形上下文.您没有向我们展示您如何获得您正在绘制的上下文,但通常NSString方法将在-drawRect方法中使用,其中获取的上下文如下所示:

CGContextRef context = UIGraphicsGetCurrentContext();

您可以使用UIGraphicsPushContext()将您的上下文设置为“当前上下文”(但请确保您还调用UIGraphicsPopContext()):

- (void)drawInContext:(CGContextRef)context
{
    UIGraphicsPushContext(context);
    CGContextSaveGState(context);

    // draw things, everything displays correctly ...

    CGSize expectedCreditSize = [[gpData.credits stringValue] sizeWithFont:[UIFont 
                                                          systemFontOfSize:self.fontSize]];

    rect = CGRectMake(self.bounds.origin.x, self.bounds.size.height/2,
                      expectedCreditSize.width, expectedCreditSize.height);

    CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);

    [self.creditString drawInRect:rect
                         withFont:[UIFont systemFontOfSize:self.fontSize]
                    lineBreakMode:NSLineBreakByWordWrapping
                        alignment:NSTextAlignmentRight];

    CGContextRestoreGState(context);
    UIGraphicsPopContext();
}
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