How to get *real* height of CFAttributedString - swift

Question

I create a simple CFAttributedString with Futura-Bold typeface and size 100px:
let font = NSFont(name: "Futura-Bold", size: 100.0)!
When I render that string on a CGContext (with CTFramesetterCreateFrame) I get the following image:
Now the question is how to get real height of this text? As you can see with the above example we're looking at 85px.
When querying font object for various attributes I get the following values:
font.pointSize // 100.0
font.ascender // 103.90
font.descender // -25.99
font.capHeight // 75.40
font.leading // 2.99
font.boundingRectForFont // (-22.7, -34.3994140625, 168.6, 144.29931640625)
Does anyone know to calculate real pixel size of rendered string?

Answer 1

One solution that gives you the value you are seeking is to use the NSString boundingRect(with:options:attributes:) method. By passing in the proper options you get the desired result:
let font = NSFont(name: "Futura-Bold", size: 100)!
let text: NSString = "Hello World!"
let rect = text.boundingRect(with: NSSize(width: 0, height: 0), options: [ .usesDeviceMetrics ], attributes: [ .font: font ], context: nil)
print("Height of \"\(text)\" is \(rect.height)")
Output:
Height of "Hello World!" is 85.1
This works with NSAttributedString as well.
let font = NSFont(name: "Futura-Bold", size: 100)!
let attrStr = NSAttributedString(string: "Hello World!", attributes: [ .font: font ])
let rect2 = attrStr.boundingRect(with: NSSize(width: 0, height: 0), options: [ .usesDeviceMetrics ])
print("Height of \"\(attrStr)\" is \(rect2.height)")
Output:
Height of "Hello World!{
NSFont = "\"Futura-Bold 100.00 pt. P [] (0x7ff6eae563b0) fobj=0x7ff6eaf1ea50, spc=34.00\"";
}" is 85.1
If needed, you can cast a CFAttributedString to NSAttributedString.
let attrStr: CFAttributedString = ... // some CFAttributedString
let rect2 = (attrStr as NSAttributedString).boundingRect(with: NSSize(width: 0, height: 0), options: [ .usesDeviceMetrics ])

Answer 2

In addition to rmaddys's excellent answer I found one more solution on my own which also gives desired results. The trick is to use CTLineGetImageBounds.
let font = NSFont(name: "Futura-Bold", size: 100.0)!
let text = NSAttributedString(string: "Hello World!", attributes: [.font:font])
let line = CTLineCreateWithAttributedString(text)
print(CTLineGetImageBounds(line, textContext))
where textContext is a CGContext on which you render your text. Per Apple docs:
This is required because the context could have settings in it that
would cause changes in the image bounds.
The above code gives the following result:
(7.9, -2.1, 664.2, 85.1)
^^^^