NSAttributedString分类-计算富文本高度

- (id)safeObjectAtIndexedSubscript:(NSUInteger)index
	{
	if (index >= self.count) {
	return nil;
	} else {
	return [self objectAtIndex:index];
	}
	}
	- (id)safeObjectAtIndex:(NSUInteger)index
	{
	if (index >= self.count) {
	return nil;
	} else {
	return [self objectAtIndex:index];
	}
	}
	+ (instancetype)safeArrayWithObject:(id)object
	{
	if (object == nil) {
	return [self array];
	} else {
	return [self arrayWithObject:object];
	}
	}
	- (NSArray *)safeSubarrayWithRange:(NSRange)range
	{
	NSUInteger location = range.location;
	NSUInteger length = range.length;
	if (location + length > self.count) {
	//超过了边界,就获取从loction开始所有的item
	if ((location + length) > self.count) {
	length = (self.count - location);
	return [self safeSubarrayWithRange:NSMakeRange(location, length)];
	}
	return nil;
	}
	else {
	return [self subarrayWithRange:range];
	}
	}
	- (NSUInteger)safeIndexOfObject:(id)anObject
	{
	if (anObject == nil) {
	return NSNotFound;
	} else {
	return [self indexOfObject:anObject];
	}
	}
	- (NSString *)toJSONStringForArray
	{
	NSData *paramsJSONData = [NSJSONSerialization dataWithJSONObject:self options:0 error:nil];
	return [[NSString alloc] initWithData:paramsJSONData encoding:NSUTF8StringEncoding];
	}