Objective-C-文本基线位于左下角

Question

When drawing a string in IOS, by default the text is baselined by the upper left corner. This can cause problems when using multiple strings with different font-sizes with the same y-coordinate.

Question;
How do I baseline a string by the lower left corner of the first row .

How do I achieve this?

Br,
Tim

Answer 1

I finally found the solution to my problem.
By subtracting the UIFont ascender property from the rect.origin.y, I could baseline the text to the lower left corner.

Citing the UIFont documentation of the ascender property:

"The top y-coordinate, offset from the baseline, of the receiver's longest ascender." https://developer.apple.com/library/ios/documentation/uikit/reference/UIFont_Class/Reference/Reference.html#//apple_ref/occ/instp/UIFont/ascender

- (void)drawTextBaseLined:(NSString *)text
                     rect:(CGRect)rect
                     font:(UIFont *)font
{
  if ([text length] == 0) return false;

  NSDictionary *attributes = @{NSFontAttributeName : font};

  rect.origin.y -= font.ascender;
  [text drawWithRect:rect
             options:NSStringDrawingUsesLineFragmentOrigin
          attributes:attributes
             context:nil];
}

This method will work with one or multiple lines of text and with different font-sizes. I hope my solution helps someone else too.

Br,
Tim

Answer 2

Use - (CGRect)boundingRectWithSize:(CGSize)size options:(NSStringDrawingOptions)options attributes:(NSDictionary *)attributes context:(NSStringDrawingContext *)context;

When you have the height of your label just do the math to choose the right origin of it to have the below left corner always at the same place.

NSDictionary * attributes = @{NSFontAttributeName:[UIFont systemFont]};

CGRect rect = [[NSString string] boundingRectWithSize:CGSizeMake(MAXFLOAT, MAXFLOAT) options:NSStringDrawingUsesLineFragmentOrigin attributes:attributes context:nil];

If you want your label below left corner to be always at 50pix the origin of your label will be at (50 - rect.size.height).

Change the size to adapt to your constraints.