//根据指定的大小,对字符串进行分页,计算出每页显示的字符串区间(NSRange)

    - (NSArray *)pageRangeArrayWithConstrainedToSize:       (CGSize)size
    {
    NSAttributedString *attributedString = self;
NSMutableArray * resultRange = [NSMutableArray array];
CGRect rect = CGRectMake(0, 0, size.width, size.height);

//    以下方法耗时 基本再 0.5s 以内
// NSDate * date = [NSDate date];
NSInteger rangeIndex = 0;//剩余的字数
do {
    NSUInteger length = MIN(600, attributedString.length - rangeIndex);
    NSAttributedString * childString = [attributedString attributedSubstringFromRange:NSMakeRange(rangeIndex, length)];
    CTFramesetterRef childFramesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef) childString);
    UIBezierPath * bezierPath = [UIBezierPath bezierPathWithRect:rect];
    CTFrameRef frame = CTFramesetterCreateFrame(childFramesetter, CFRangeMake(0, 0), bezierPath.CGPath, NULL);
    
    CFRange range = CTFrameGetVisibleStringRange(frame);
    NSRange r = {rangeIndex, range.length};
    if (r.length > 0) {
        [resultRange addObject:[NSValue valueWithRange:r]];
    }
    rangeIndex += r.length;
    CFRelease(frame);
    CFRelease(childFramesetter);
} while (rangeIndex < attributedString.length && attributedString.length > 0);
//NSTimeInterval millionSecond = [[NSDate date] timeIntervalSinceDate:date];
//NSLog(@"耗时 %lf", millionSecond);
return resultRange;
}