Line spacing in multi-language layout with Core Text

Question

The attributed string has only one attribute - 17 point Helvetica NeueUI font - covering the whole string. Line 1~3 are purely English, line 4~6 are mixtures of English and Chinese, line 7~8 are purely Chinese.

It is then layouted with CTFramesetter and resultant frame is drawn with CTFrameDraw.

// UIView subclass
- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor whiteColor];
        CTFontRef font = CTFontCreateWithName(CFSTR("Helvetica NeueUI"), 17.f, NULL);
        _text = [[NSAttributedString alloc] initWithString:string attributes:
                 [NSDictionary dictionaryWithObject:(id)font forKey:(id)kCTFontAttributeName]];
        CFRelease(font);
        _framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)_text);
        CGMutablePathRef path = CGPathCreateMutable();
        CGPathAddRect(path, NULL, self.bounds);
        _frame = CTFramesetterCreateFrame(_framesetter, CFRangeMake(0, 0), path, NULL);
        CGPathRelease(path);
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);
    // Flip the coordinate system.
    CGContextTranslateCTM(context, 0.f, self.bounds.size.height);
    CGContextScaleCTM(context, 1.f, -1.f);
    
    CGContextSetTextMatrix(context, CGAffineTransformIdentity);
    
    CTFrameDraw(_frame, context);
    CGContextRestoreGState(context);
}

The problem is that the space between line 7 and 8 (purely Chinese lines) is much smaller than others.

For contrast, I put a UILabel below it, with the same font and text. Ignoring the rendering difference of single characters, you can see the line spaces in UILabel are uniform including the last one between two Chinese lines.

_{(source: skitch.com)}

Notice: this results above is got on a real device. If you run the same code on the simulator, you'll get very different result for the Core Text view — spaces between lines containing Chinese are much larger than others.

1. Why is the space between Chinese lines smaller(larger)?
2. How can I layout multi-language text with uniform line height using Core Text?

PS: here is the sample project you can try yourself.

Answer 1

Similarly to the space between each letter in European languages, Chinese writing uses a narrow space between characters, though it does not observe the equivalent to the wider space between words, except in rare occasions. (In this respect, it may somewhat resemble a form of scriptio continua.) When space is used, it is also fullwidth. One instance of its usage is as an honorific marker. A modern example in Taiwan, is found in the reference to Chiang Kai-shek as 先總統　蔣公 (Former President, Lord Chiang), in which the preceding space serves as an honorific marker for 蔣公. This use is also still current in very formal letters or other old-style documents. When Chinese is written in transliterated form (as in Hanyu Pinyin), spaces are used to assist in reading. source:http://en.wikipedia.org/wiki/Chinese_punctuation

So its because in chinese as space is used a bit differently than in english.