这里说的占位符，实际就是排版时需要展示的图片，图片基于占位符填充，那么处理图片时，怎么解决占位符（图片）的上下偏移
在设置占位符属性时，我通过以下方法来实现它：

+ (NSAttributedString *)wxImageAttributeCoreTextFromPaperQuestion:(WXTKCoretextQSourceImg *)image{
    
    CTRunDelegateCallbacks callbacks;
    memset(&callbacks, 0, sizeof(CTRunDelegateCallbacks));
    callbacks.version = kCTRunDelegateVersion1;
    callbacks.getAscent = ascentCallbackPaper;
    callbacks.getDescent = descentCallbackPaper;
    callbacks.getWidth = widthCallbackPaper;
    CTRunDelegateRef delegate = CTRunDelegateCreate(&callbacks, (__bridge void *)(image));
    // 使用0xFFFC作为空白的占位符
    unichar objectReplacementChar = 0xFFFC;
    NSString * content = [NSString stringWithCharacters:&objectReplacementChar length:1];
    NSMutableDictionary * attributes = [self wxAttributesPaperImg:image];
    NSMutableAttributedString * space = [[NSMutableAttributedString alloc] initWithString:content attributes:attributes];
    CFAttributedStringSetAttribute((CFMutableAttributedStringRef)space, CFRangeMake(0, 1),
                                   kCTRunDelegateAttributeName, delegate);
    CFRelease(delegate);
    return space;
}

上述方法在引入 CTRunDelegateCallbacks 时，提供了控制占位符大小属性，即：getAscent、getDescent、getWidth

getWidth是占位符所取宽，getAscent与getDescent分别基于基准可上下偏移，一般情况，getDescent会提供返回0值，而getAscent一般是占位符（图片）的高度；下面通过设置不同数值，看下字符如何偏移；

向下不偏移，向上提供占位符高度

///占位基准上升度
static CGFloat ascentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height;
}
///占位基准下降度
static CGFloat descentCallbackPaper(void *ref){
    return 0;
}

视觉给我感觉默认不向下偏移，图片比左侧字符高一点点

向下偏移5，向上提供占位符高度 - 5

///占位基准上升度
static CGFloat ascentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height - 5;
}
///占位基准下降度
static CGFloat descentCallbackPaper(void *ref){
    return 5;
}

向下偏移10，向上提供占位符高度 - 10

///占位基准上升度
static CGFloat ascentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height - 10;
}
///占位基准下降度
static CGFloat descentCallbackPaper(void *ref){
    return 10;
}

向下偏移整个占位（图片）高度，向上提供占位符高度 0

///占位基准上升度
static CGFloat ascentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height - refP.height;
}
///占位基准下降度
static CGFloat descentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height;
}

注意有个问题，上述的 getAscent、getDescent值加起来，其实就是图片的高度，那么如果比高度大或者小的情况下，图片会被拉伸，或者压缩

向下偏移小于整个占位（图片）高度( -10)，向上提供占位符高度 0

///占位基准上升度
static CGFloat ascentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height - refP.height;
}
///占位基准下降度
static CGFloat descentCallbackPaper(void *ref){
    WXTKCoretextQSourceImg *refP = (__bridge WXTKCoretextQSourceImg *)ref;
    return refP.height - 10;
}

总结

上下偏移要处理好图片的高度值，确保getAscent + getDescent = 占位符（图片）高度即可