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Category: Programming

NSFileHandle permission errors

Published December 4, 2017

In a macOS programming project I have a command that allows the user to open a file containing some configuration options. This file is a property list, so the contents were being read using NSDictionary(contentsOf:); this method simply returns nil if an error occurs. I wanted to provide more details about the failure than simply “The file could not be read”, so I looked for a Foundation method that would throw an NSError. I settled on FileHandle(forReadingFrom:) and inserted the following code:

    do {
        _ = try FileHandle(forReadingFrom: url)
    }
    catch let error as NSError {
        let alert = NSAlert()
        alert.messageText = "An error occurred while reading the options file.".localized()
        alert.informativeText = error.localizedDescription
        alert.beginSheetModal(for: mainWindow) { (response) in
            if response == NSAlertSecondButtonReturn {
                OperationQueue.main.addOperation { self.resetOptions(nil) }
            }
        }
        return
    }

Then I set up a few test cases, including a file that was owned by another user (with no read permission). To my surprise, the error description implied that I was unable to write the file:

[application icon redacted]

…and while that’s also true, that’s certainly not the error I expected to get.

I looked at the error object in the debugger, and sure enough, the error code was 513 (NSFileWriteNoPermissionError) and not 257 (NSFileReadNoPermissionError). Weird.

Well, I know I’m trying to read the file, and I don’t want to confuse the user, so I changed the catch clause to handle this case:

    catch var error as NSError {
        if error.code == NSFileWriteNoPermissionError {
            error = NSError(domain: NSCocoaErrorDomain, code: NSFileReadNoPermissionError, userInfo: error.userInfo)
        }
        let alert = NSAlert()
        //...
        return
    }

…and that seemed to do nicely. But I can’t find any documentation on whether this is expected behavior or what I might be doing wrong.

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AppKit bug in 10.12.2: NSTextField with tabs in text displays incorrectly

Published December 20, 2016

UPDATE: The bug described below is fixed in macOS 10.12.4 and the workaround described is no longer necessary (except for backward compatibility).

In a recent macOS project I have an NSTextField that contains text with tab characters in order to display a data table. (The text is an NSAttributedString to which I apply an NSParagraphStyle with its tabStops property set to an array containing a number of right-aligned NSTextTabs.)

The text field was drawn as intended (with the text aligned to the tab stops) before upgrading to 10.12.2. After upgraded, the text was no longer aligned to the tab stops, and when using monospacedDigitSystemFont there were cosmetic glitches displayed as well:

Under time pressure for a release, I implemented the workaround of replacing the NSTextField with an NSTextView. (I did this dynamically since it is not obvious how to create “bare” NSTextViews in Xcode’s Interface Builder.) Using an NSTextView I was able to get the data table to display as I had intended.

When writing up this bug (#29758498; Radar, Open Radar) I tried to create a simple test case that would illustrate the issue, but on my first try to recreate the problem from scratch, the issue didn’t appear. Puzzled, I copied the code and .xib file from my application into the test project, and was able to reproduce the issue.

By examining the text of the .xib files, I discovered that my newly created .xib file has a content view with wantsLayer=”YES”, and in this case the NSTextField works as intended under 10.12.2; but the .xib file from my application (which was created before Xcode 8 was released) did not have a layer set on the content view, and this did not work under 10.12.2 (although it had worked fine on 10.12.1 and earlier). Setting a layer on the application’s window’s content view (in the View Effects tab of the Assistant Editor in Xcode) was enough to fix the issue and make the table display as intended:

(Yes, bridge players, those point counts are bogus; my test program is generating random values to insert into the table.)

My wider questions are:

  1. What happened? We know there were a lot of graphic updates in 10.12.2, but it’s odd that the problem only occurs with the system font. I haven’t investigated whether the font file itself has changed.
  2. I can’t believe I’m the only person that’s run into this (although my usual googling produced no other hits). Is this really an uncommon case? Or is this a(nother) gap in Apple’s test suite?
  3. If Apple is expecting that the default case for content views is to have wantsLayer set, then it should certainly make this more widely known. I couldn’t find any references to this issue in the AppKit release notes.
2 Comments

Reading text files with arbitrary line endings in Swift 3

Published November 21, 2016

It’s easy enough in Swift to get an array of strings representing the lines of a text file:

  let contents = try! String(contentsOf: textFileURL)
  let lines = contents.components(separatedBy: .newlines)

…but what if the input might be a DOS format text file? CharacterSet.newlines contains both "\r" and "\n", so the code above will divide the text at each of them, and the array of strings will contain an extra empty string between every line.

Turns Out™ that Foundation has a number of APIs that understand all the ways line endings are represented in text files. The easiest way to read a text file with arbitrary line endings is to use the enumerateSubstrings(in:options:body:) method, specifying EnumerationOptions.byLines. This uses getLineStart(_:end:contentsEnd:for:) to recognize the line endings (see the documentation thereof for details) and is powerful enough to handle files with mixed line endings.

(In Objective-C these are the -enumerateSubstringsInRange:options:usingBlock: and -getLineStart:end:contentsEnd:forRange: methods of NSString.)

It’s straightforward to wrap this into an extension on String:


extension String {
func separatedIntoLines() -> [String] {
var lines: [String] = []
let wholeString = self.startIndex..<self.endIndex
self.enumerateSubstrings(in: wholeString, options: .byLines) {
(substring, range, enclosingRange, stopPointer) in
if let line = substring {
lines.append(line)
}
}
return lines
}
}

view raw

String+SeparatedIntoLines.swift

hosted with ❤ by GitHub

The name separatedIntoLines() seems like the best choice to describe the work being done; the alternative of a computed property simply called lines seems a little too terse to me. UPDATE 2018-02-13: After reviewing the Swift 4 API, perhaps splitIntoLines() would be a better name.

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VisualEffectPlayground in Swift

Published April 22, 2016

When searching for information on implementing vibrancy effects and NSVisualEffectView, developers often find references to the VisualEffectPlayground sample code presented by Apple at WWDC 2014. However, they soon quickly discover that Apple pulled the sample code from the Apple Developer site. Perhaps the application was thrown together for WWDC and didn’t meet the usual standard for Apple sample code; however, to date Apple has not seen fit to post any replacement. And so the search continues.

During my own search, I discovered that a Xamarin engineer named Oleg Demchenko had ported the sample code to Mono and posted it to the Xamarin samples site. In order to get a working sample application, I decided to port this code back to Swift and posted it to GitHub.

DISCLAIMER: I don’t know C# and I’m still learning Swift. Nevertheless, the resulting application appears to perform as expected (although I haven’t made an exhaustive test). I will certainly consider any pull requests.

1 Comment

Updating the cover of the Swift iBook

Published June 17, 2014

Apple updated the cover of The Swift Programming Language to match the new book in the series, but deleting and redownloading the book within iBooks in OS X Mavericks didn’t update the cover — it continued to display the original cover.

The solution was to quit iBooks, delete the iBooks cache located at ~/Library/Containers/com.apple.iBooksX/Data/Library/Caches/com.apple.iBooksX, and restart iBooks.

You’re welcome, Internet.

5 Comments

Improved cwd restoration in tcsh

Published December 19, 2013

My previous post illustrating a technique for restoring the current working directory in Apple’s Terminal for tcsh users was discovered by Khoo Yit Phang, who suggested an improved version down in the comments. I’m reproducing it here for greater visibility:

if ("$?TERM_PROGRAM") then
  if ("$TERM_PROGRAM" == "Apple_Terminal") then
    alias cwdcmd '( set echo_style = both; echo -n "33]7;file://${host}${cwd:ags/ /%20/}a" )'
    cwdcmd
  endif
endif

Compared to my approach, it has two improvements: (1) by using cwdcmd (of which I was previously unaware) rather than precmd, it only runs when you actually change directories; and (2) by using a suitably configured echo, it avoids running an external command (printf is not built into tcsh, but is present in /usr/bin).

Thanks, Yit.

Update: Of course, with this technique you have to explicitly run the command once, to restore the working directory to the title bar for the first time when you start up. Fixed above.

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Current working directory restoration in Mavericks Terminal with tcsh

Published October 25, 2013

In John Siracusa’s excellent review of OS X Mavericks, he (originally) mentioned that the Terminal application is able to restore the current working directory of open shells across launches.

However, this section has subsequently disappeared from the online version, perhaps for the reasons given below (update: see below). Try looking here for a cached version of the original; scroll down to page/slide number 94. Sorry, John. Please purchase the full review on the iBooks store or the Amazon Kindle store to support John’s work.

Of course I was excited to try this out as, in the course of my work as a software developer, I often have several shell windows open in specific directories. So I tried it, and it didn’t work. It turns out the built-in support supplied by Apple assumes that you’re using the bash shell, but as an old-timer I’m stuck on tcsh instead:

Well, that’s no good, I thought. So I looked in the supporting code’s implementation (/etc/bashrc), and figured out how to do the same thing in tcsh. Simply place the following in your .tcshrc file:

if ("$?TERM_PROGRAM") then
 if ("$TERM_PROGRAM" == "Apple_Terminal") then
  alias precmd 'printf "33]7;%sa" "file://$host$cwd:ags/ /%20/"'
 endif
endif

This uses the appropriate escape sequence to send the URL of the current working directory to the terminal before each command; Terminal detects this information and uses it to update the state of the window (including a proxy icon in the window’s title bar for the corresponding folder). It turns out that the key is doing it using precmd (tcsh’s equivalent of PROMPT_COMMAND), and not sending the sequence in the prompt string itself as one might think. (The outer if statement is necessary to avoid an error when remotely logging in, as with ssh. It has to be a separate statement because of the variable expansion rules in tcsh.)

I’ve notified John via Twitter, so perhaps he add restore this information back to his review. In any case, it’s documented here for future generations. Drop me a note below if you have any issues with it.

Update: It turns out John removed it because he discovered it was a feature in Mountain Lion as well, which had happened not to work on his machine:

If you listen to John’s podcasts, you’ll realize that makes sense: he wouldn’t let something not new in Mavericks stand in the Mavericks review. Regardless, I wasn’t aware of the feature before and it still wouldn’t have worked in tcsh, so this post is presumably still useful. (Note that I haven’t tried it on Mountain Lion, and I’ve already upgraded my working machines to Mavericks; I don’t know why this technique wouldn’t work on Mountain Lion, but use it at your own risk.)

Update 2: OK, so it turns out the feature was first implemented in Lion, and other people have already figured out how to port it to other shells; see here for an implementation in zsh, for example. How did I manage to not learn about this for the last two years? Oh, well…

5 Comments

Debugging software for laymen

Published November 28, 2009

Brent Simmons provides what is possibly the best description for laymen of the joy/agony of debugging software that I’ve ever seen:

If it took 10 minutes to reduce memory usage by 96MB, then — were there a linear relationship — it should take under half a minute, less than 30 seconds, to go the rest of the way.

That’s the way things work in the real world, after all. If you have 100 bags of leaves to carry out to the curb, each bag will take about the same amount of time as the others.

Instead, it will probably take me about two hours, maybe more, to get rid of that last 4MB.

It’s almost as if you carried out 96 bags of leaves easily and quickly, then realized you can’t get those last four bags, even though they’re exactly the same as all the others, without pouring a new driveway first.

Which is crazy, right? If the real world operated like that all the time, we’d go completely nuts.

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OCPython, anyone?

Published December 5, 2008

As dynamic scripting languages become more widely used, many developers are interested in using them to write “native” Mac OS X software, rather than having to learn Objective-C (which, of course, is a dynamic language in its own right). And in fact it is often possible to do so, using Apple’s BridgeSupport to generate the necessary metadata. Apple explicitly supports bridges for Ruby (RubyCocoa) and Python (PyObjC).

Nevertheless, there is necessarily an impedance mismatch between the runtime models — when passing objects to Cocoa, the bridge must convert the object from the dynamic language’s runtime to an Objective-C object derived from NSObject, and vice versa. This introduces a certain amount of inefficiency, which is not normally a big problem, but it would be desirable to avoid it.

Now, over the last few months, I’ve seen a variety of projects to do this by using the Objective-C 2.0 runtime to implement the object model for the dynamic language. In other words, objects in the dynamic language are subclasses of NSObject and no conversion is necessary.

These range from the venerable F-Script, which is even closer to Smalltalk than Objective-C, to Tim Burks’ original language Nu, which uses a Lisp-like syntax. I believe I’ve seen a reference to a JavaScript implementation as well, but I can’t seem to find it now.

The project that seems to be getting the most buzz in the Mac community right now is MacRuby, a port of Ruby 1.9 done by the developer of RubyCocoa (who is also an Apple employee). There was even a article about it featured on the Apple developer site recently.

Conspicuous by its absence from this list is my preferred dynamic language Python. I would love to see an implementation of Python hosted on the Objective-C runtime. I’ve always felt Python’s language model was a good match for Cocoa (of course, PyObjC not only predates BridgeSupport but even Cocoa itself, going back to the days of NextStep), and I’m personally more familiar with it than Ruby or other dynamic languages. So it seems like a logical step to take.

This project couldn’t be called MacPython, because that name has already been used to denote the Mac port of Python. For now I’m calling it “OCPython”, but I’m certainly open to a cleverer name (especially if there’s one that can riff on IronPython; unfortunately the element most associated with Mac OS X is Carbon, which is entirely wrong for this project).

I’ve been thinking about which version of Python to use as a jumping-off point; originally I was leaning towards 2.5 because it’s the version shipped with Leopard, but now I’m persuaded by Bill Bumgarner’s comment to use 3.0 and take advantage of the revised, less crufty implementation. (This is based on the stated goal of the 3.0 development project; I haven’t actually looked at the source code yet.)

I wish I could say I’m prepared to get the project off the ground myself, but right now I’m between jobs, as they say, and given the current financial situation I’m actively looking for full-time employment and can’t dedicate much time to something that won’t bring in any revenue. (Of course, if anyone out there would like to fund development of OCPython, contact me immediately.)

6 Comments

Visual Studio 2005: Not Ready To Lead

Published October 26, 2008

Glenn Howe writes of an anomaly in Visual Studio 2005’s C++ compiler:

… I discovered today (Friday) that if you compile something like:

double x = (false) ? 0 : 1.4;

that x will not equal 1.4 as most people (and the gcc compiler) would think, but rather it will equal 1.0. Why? Because it sees 0, interprets it as an integer and decides that if both halves of the : have to have the same type, then that type will be integer. The fact that this is in the middle of an assignment to double means nothing.

Yes, in fact it does mean nothing; the type of the lvalue on the left-hand side of an assignment expression has no effect on the type of the rvalue on the right-hand side.

He goes on:

I’m not even saying that Visual Studio is wrong. It’s different from gcc which leads to platform specific bugs, …

Well, I was a language lawyer in a previous life, so I can’t let it rest there. (And those of you out there saying “Of course Visual Studio is wrong!” should recall that gcc doesn’t always get it right, either.)

Where were we? The type of the conditional operator, right. To find what that type should be, we have to look at the definition of the operator.

In C99, this is pretty straightforward. Section 6.5.15, which defines the conditional operator, says the result of the expression “is the value of the second or third operand (whichever is evaluated), converted to the type described below”. Looking “below”, it further says when you have two operands of arithmetic type, the result type is the type that would be determined by the “usual arithmetic conversions” applied to the two operands.

Now a simple jump to section 6.3.1.8, which defines the “usual arithmetic conversions”, tells us: “… if the corresponding real type of either operand is double, the other operand is converted, without change of type domain, to a type whose
corresponding real type is double.”

OK, so that’s not completely straightforward, talking about “real types” and “type domains”, but basically we have two numbers, and if one of them is double the other is converted to double, and so that’s that; the type of the expression should be double, as we’d expect, and so Visual Studio 2005 is wrong.

If we’re in C.

C++ is more complicated.

(What a shock, right?)

Well, C++ has to allow for the possibility that either or both operand is a class type, and if so can it be converted to the other’s type, blah blah blah (section 5.16, if you’re following along at home).

Since we don’t have class type operands, we go on, applying the “standard conversions” to the operands (none of which apply here), and then, if “the second and third operands have arithmetic or enumeration type[,] the usual arithmetic conversions are performed to bring them to a common type, and the result is of that type.”

Hey, that was pretty simple after all. No, wait: are the usual arithmetic conversions the same in C++ as they are in C? Good question; let me see… (flip, flip, …) here we are, clause 5: yes, they are.

So from where I sit, Visual Studio 2005 is wrong in C++, too.

Wrong on conditional operators.

Wrong for America.

(I can’t wait for this election to be over…)

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