When converting text to image, the converted images were different between macOS and iOS. The main differences are font weight and line height.

Font Weight

Thought the font weight and the fonts are the same, on macOS the font result is always thicker. I don't know why. But in my experience, if you use "HelveticaNeue-Light" for iOS, use "HelveticaNeue-Thin" for macOS.

Line Height

The line height of font is even tricky.

Equation

In Apple's doc, Apple gives below graph. We could draw a simple equation from the graph.

line height = ascent + decent + line gap (leading)

So I did two tests on both iOS and macOS in Playgound.

// macOS
func getFontInfo(_ name:String) {
    let font = NSFont(name: name, size: 17.0)!
    print(font.ascender) // 13.09033203125
    print(font.descender) // -3.90966796875
    print(font.leading) // 0.0
    print(font.ascender - font.descender + font.leading) // 17.0
    
    let layoutManager = NSLayoutManager()
    print(layoutManager.defaultLineHeight(for: font)) // 20.0
}

getFontInfo("Helvetica")

// iOS
func getFontInfo(_ name:String) {
    let font = UIFont(name: name, size: 17.0)!
    print(font.ascender) // 15.64033203125
    print(font.descender) // -3.90966796875
    print(font.leading) // 0.0
    print(font.ascender - font.descender + font.leading) // 19.55
    print(font.lineHeight) // // 19.55
}

getFontInfo("Helvetica")

From the two tests, we could draw two conclusions:

1. The equation on iOS was balanced, but on macOS was not.
2. For the same font with the same weight, the ascender were different.

I didn't know why those happened. So I sent an "Apple Developer Technical Support". Here was the reply from Apple.

According to Apple, if I wanted to use the equation, I should use Core Text framework. But in fact Apple didn't provide line height in Core Text.

Then I did another two tests.

// macOS
func getLineHeightForFontName(_ name:String) {
    let font = CTFontCreateWithName(name as CFString, 17.0, nil)
    
    print(CTFontGetAscent(font)) // 13.09033203125
    print(CTFontGetDescent(font)) // 3.90966796875
    print(CTFontGetLeading(font)) // 0.0
}

getLineHeightForFontName("Helvetica")

// iOS
func getLineHeightForFontName(_ name:String) {
    let font = CTFontCreateWithName(name as CFString, 17.0, nil)
    
    print(CTFontGetAscent(font)) // 13.09033203125
    print(CTFontGetDescent(font)) // 3.90966796875
    print(CTFontGetLeading(font)) // 0.0
}

getLineHeightForFontName("Helvetica")

From all four tests, we could get the conclusions:

1. Though on iOS, the equation was balanced. The ascent property was modified by Apple.
2. On macOS, the line height was modified by Apple.
3. From the above two conclusions, both NSFont and UIFont were not trusted. The only trusted line height was something we get from Core Text.

Line Height

#if os(macOS)
func getLineHeight(_ font:NSFont) -> CGFloat {
    let ctFont = CTFontCreateWithName(font.fontName as CFString, font.pointSize, nil)
    return CTFontGetAscent(ctFont) + CTFontGetDescent(ctFont) + CTFontGetLeading(ctFont)
}
#else
func getLineHeight(_ font:UIFont) -> CGFloat {
    let ctFont = CTFontCreateWithName(font.fontName as CFString, font.pointSize, nil)
    return CTFontGetAscent(ctFont) + CTFontGetDescent(ctFont) + CTFontGetLeading(ctFont)
}
#endif

Others

NSTextView Best Practice

Text Programming Guide for iOS

Cocoa Text Architecture Guide

Core Text - Calculating line heights

Initiate Font

Unlike UITextView, you cannot assign font in NSTextView with empty string in Interface Builder. There are many ways to set the font, like to set the font of NSTextView.textStorage.font or set the attributedString of NSTextView.textStorage or just set the attributes. However, those methods are all imperfect.

Working With Input Method

The answers above are all not working with input method. The cursor indicator will still be the unset font's style and the font only applied when you have finished inputing.

The only correct way is to set the typingAttributes property of NSTextView.

inputTextView.typingAttributes = [
    .font : NSFont.userFont(ofSize: 17.0) ?? NSFont.systemFont(ofSize: 17.0),
    .foregroundColor : NSColor.labelColor
]

Don't forget to set the foregroundColor as well, or you textview won't work with light/dark mode switching.

Undo

It is easy to turn on undo in Interface Builder. However, you should know that every time you set string or attributedString property, the undo operations are clear.

You may notice that if the textview is in a modal window, its view controller is presented by presentAsModalWindow(_:), the undo operations will not work as expected.

NSTextDelegate And NSTextStorageDelegate

Both NSTextDelegate and NSTextStorageDelegate work with NSTextView, however there are slightly differences between those functions.

// NSTextDelegate
func textDidBeginEditing(_ notification: Notification)
// only runs once when a user first starts editing.

func textDidChange(_ notification: Notification) 
// only runs when a user edits in the textview.

// NSTextStorageDelegate
textStorage(_ textStorage: NSTextStorage, willProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int)
// and
func textStorage(_ textStorage: NSTextStorage, didProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int)
// runs every time a user edits, also runs when the developer sets string property in code.

Mixing Fonts Causing the Whole Line Moves

This issue happens when using texts with emoji or texts in different languages. Both TextEdit.app and Pages.app have this issue. Changing text size over 22 may solve this issue. There is no other method to solve this issue yet.

Others

Weight and Line Height of Font between macOS and iOS

对于使用RestAPI的应用，微博强制用户必须每30天登录一次。我原本的思路是类似这样的：

设备1：@小明 x号登录，x+27天后开始提醒。
设备2：@小明 y号登录，y+27天后开始提醒。(x < y)

但是实际使用中我发现，当x+30之后，在不重新登录账户@小明的情况下，还是可以通过@小明的验证信息发送微博。

分析

表面上，设备1上的授权是过期的，但是为什么还能继续发微博呢？我使用微博提供的API查询了token的信息。结果显示，实际上的设备1上的授权时间，要比x+30要长。

我分析，可是这样的。我原本以为微博服务器会记录每次用户登录的授权，即

设备1：@小明 x号登录，x+30授权结束。
设备2：@小明 y号登录，y+30授权结束。 (x<y)

但实际上，为了方便，微博根本没有记录每次的过期时间，而是每次小明通过同一个应用的授权进行登录时，就自动延长了该授权对应的时间。所以实际上发生的，可能是这样的：

设备1：@小明 x号登录，x+30授权结束。
设备2：@小明 y号登录，y+30授权结束。 (x<y)
因为x<y,所以设备1和2上的@小明，都变成y+30授权结束。

结论

根据上面的分析，新算法就变成了，当x+27时，先向微博服务器进行查询，查看剩余时间是否不足3天，如果超出，则先不进行提示，而是更新下一个时点，这样用户就可以少登录几次了。