iOS-Coretext 图文混排占位符上下偏移处理

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