Apple Devo | Contra Corner | Bob Peterson

Debugging SwiftUI

Fri, 21 Jan 2022 14:43:38 -0500

To print why a view is being rebuilt, use Self._printChanges(). You can add it as a line of code in the body property like this:

let _ = Self._printChanges()

(Remember you now need to explicitly return the computed view.)

You can also add it as a debugger breakpoint withoit modifying code:

The output is something like:

DanceStack: @self, @identity, _vm, _compact changed.
() $R6 = {}

(The register dump can be ignored.)

CPU Temperature

Sun, 21 Jun 2020 14:25:51 -0400

This is a useful built-in command for checking hardware metrics, especially CPU temperature.

sudo powermetrics

[…]

**** SMC sensors ****
CPU Thermal level: 99
GPU Thermal level: 50
IO Thermal level: 50
Fan: 5219.96 rpm
CPU die temperature: 57.20 C
GPU die temperature: 35.00 C
CPU Plimit: 0.00
GPU Plimit (Int): 0.00
Number of prochots: 0

See:

AskDifferent/StackExchange: Can I get the CPU temperature and fan speed from the command line in OS X?

BBEdit Filters

Mon, 25 May 2020 11:19:16 -0400

Some useful filters. Put filters in ~/Library/Application Support/BBEdit/Text Filters. They must be bash scripts.

XML Format.sh

#!/bin/bash
xmllint --format -

JSON Format.sh

#!/bin/bash
python -m json.tool

Contrary to another suggestion, I did not like using plutil as it is fussy about the kind of XML it formats.

Second Life ExternalEditor

Tue, 12 May 2020 20:40:09 -0400

OK, this post has nothing to do with developing for Apple devices. Instead it’s about using an external editor with Second Life text objects such as Notecards and Scripts.

Here’s what Linden Lab says:

Opening scripts in another editor directly from the viewer:
Enable: Preferences > Advanced > Show Advanced_menu.
Go to: Advanced > Show Debug_Settings.
Edit: "ExternalEditor”.

As the description tells you, enter 3 params:
Path to editor (enclose in quotes, if path has spaces).
Optional command line params (you can ignore these if you like).
"%s" will paste script path automatically (dont change this).

Here’s what you really put into the setting value:

ExternalEditor

/usr/bin/open -a bbedit "%s"

I tried other text and it failed:

/usr/local/bin/bbedit "%s"

This is dumb. What the heck? Then, one Second Life JIRA hinted me:

/usr/bin/open "%s"

That opened the file in Apple’s own TextEdit! Close, but no cigar. The man page for open said -a might work so I tried it.

What’s extra nice is you can keep the document open and keep re-saving it. When I sigure out how to to keep versions backed up with each save I’ll add that here.

See:

https://jira.secondlife.com/browse/STORM-1949

Playgrounds, Timers, and Async Callbacks

Wed, 25 Apr 2018 10:20:00 -0400

This is a simple example of using Timer in Swift. I’ve seen some Stack Overflow advice that says Timer performs badly but had no backup to support this assertion.

Swift 4.1

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

print("Starting")
let t = Timer.scheduledTimer(withTimeInterval: 5.0, repeats: false) { (timer) in
    print("TIMER FIRED")
}
//t.fire()
print("Waiting...")

You must invoke scheduledTimer on the main thread. If you don’t, iOS will quietly never fire the timer.

Useful reading:

How do I run Asynchronous callbacks in Playground
Session 213 at WWDC 2016
There a note "Be sure to start timers in the main thread” in Timer.scheduledTimer not firing

Caching CKRecords

Fri, 02 Mar 2018 10:59:08 -0500

It’s not possible to update an existing record in CloudKit without having saved specific data from a CKRecord given to you by CloudKit. You can’t simply create a new CKRecordID and use the last changeToken and create a CKRecord yourself usingt those. If you do, CloudKit thinks you’re tryingt o create a brand new record. It will given an error if the record name already exists in that zone.

So you must either

fetch a record, change data in the CKRecord you got, and give that CKRecord to the modify operation,

save the record in whole or part, change data in that CKRecord, and give that CKRecord to the modify operation.

Method is 2 is necessary if you’re coordinating data across devices, because then the modify will correctly fail if another device modified the record since your last caching of it. You can save cache size by saving only the necessary parts of the CKRecord.

Caching CKRecord

The canonical way to cache the salient record data is CKRecord.encodeSystemFields. The prescribed method is to archive using an NSCoder, however JSONEncoder does not implement that.

Further reading

JSON and Codable

Wed, 07 Feb 2018 10:43:21 -0500

Her are some tips for using JSON and Codable.

Codable is a compound protocol containing Decodable & Encodable. When your type implements these, instances can be created from Decoder objects, and instances can be encoded out using Encoder objects.

JSONEncoders, using Inversion of Control, will create an Encoder and call your type’s encode method. JSONDecoders likewise will create instances using the special init(from decoder: Decoder) method. Inversion of control means entire data graphs can be coded witha single call. Nicely automatic!

Here’s a sample round trip encode and decode:

Swift 4

// Encode a Thing
let jencoder = JSONEncoder()
do {
    let d1: Data = try jencoder.encode(thing)
    // d1 is now a Data which is a String in utf8 encoding.
	
    // Decode a Data back to a Thing
    let jdecoder = JSONDecoder()
    let sameAsThing = try jdecoder.decode(Thing.self, from: d1)
    // sameAsThing is now a Thing
}
catch let err {
    print(err.localizedDescription)
}

Here what the type looks like:

Swift 4

struct Thing: Codable {
    let parts: [String]
    init(_  parts: [String]) { self.parts = parts }
    init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        var parts = [String]()
        while !container.isAtEnd {
            let part = try container.decode(String.self)
            parts.append(part)
        }
        self.init(parts)
    }
    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        for part in parts {
            try container.encode(part)
        }
    }
}

Another way to encode an array is:

Swift 4

struct Thing: Codable {
    let parts: [String]
    init(_  parts: [String]) { self.parts = parts }
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let parts = try container.decode([String].self)
        self.init(parts)
    }
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.parts)
    }
}

However, a top-levle JSON cannot be a scalar. It must be an encoded array or dictionary.

Swift 4

struct Thing: Codable {
    let parts: [String]
    init(_  parts: [String]) { self.parts = parts }
    init(from decoder: Decoder) throws {
        let raw = try decoder.singleValueContainer().decode(String.self)
        self.init(raw.split(separator: " ").map({ String($0) }))
    }
    func encode(to encoder: Encoder) throws {
        var ven = encoder.singleValueContainer()
        let raw = parts.joined(separator: " ")
        try ven.encode(raw)
    }
}

If you try to encode this, it tries to generate a top-level String, which is scalar and creates illegal JSON. You’ll get an invalidValue error:

debugDescription: "Top-level Text encoded as string JSON fragment.”
localizedDescription: "The data couldn’t be written because it isn’t in the correct format.”

Enum Craziness

Tue, 06 Feb 2018 16:04:53 -0500

Swift 4 enums kinda drove me crazy until I learned to love the bomb. Let’s look at this example enum type:

Swift 4

enum Color {
    case red(Int8)
    case green(Int8)
    case blue(Int8)
}

There’s no way to enumerate all values in the enum. That’s probably because of associated values and the lack of a case attribute. Enums can have associated values, and that turns each case into a static factory function:

Suppose there were a Case type and case attribute. The Case type might be a kind of associated type of the enum (though I don’t think that strictly applies here, but go with it a moment).

Then Color.Cases would be [Color.Case], or perhaps a set of Color.Case if it’s more sensible for the values to have no order.

More valuable would be a way to ask “is this Color value some red?” Right now you have to write “if case .red = value { … }”. Even more valuable is a way to ask “Are these two the same Color case?” You cannot write an expression for that today.

What about the associated values? That’s complicates the notion of equality to the point Swift cannot make any useful assumptions for you.

What I have done is this, on a type by type basis, as needed.

Swift 4

extension Color : Equatable {
    /// Operator is true if the two enums are the same vaue, ignoring the associated values.
    /// See .identical(rhs) if you want to compare the associated values as well.
    static func ==(lhs: Color, rhs: Color) -> Bool {
        if case .red = lhs, case .red = rhs { return true }
        if case .green = lhs, case .green = rhs { return true }
        if case .blue = lhs, case .blue = rhs { return true }
        return false
    }
    
    static func ===(lhs: Color, rhs: Color) -> Bool {
        if case .red(let lhsv) = lhs, case .red(let rhsv) = rhs, lhsv == rhsv { return true }
        if case .green(let lhsv) = lhs, case .green(let rhsv) = rhs, lhsv == rhsv { return true }
        if case .blue(let lhsv) = lhs, case .blue(let rhsv) = rhs, lhsv == rhsv { return true }
        return false
    }
}

let c1 = Color.red(64)
let c2 = Color.red(64)
let c3 = Color.red(5)
let d4 = Color.green(64)
c1 == c2    // true
c1 === c2   // true
c1 == c3    // true
c1 === c3   // false
c1 == d4    // false
c1 === d4   // false

Add a document type

Sat, 17 Dec 2016 18:49:07 -0500

Supported file types and extensions are registered in the Info.plist file. Everything you read will say to edit that file directly, but there is a better way. Quoting the Q&A page: To add the document type do the following:

In your Xcode project, select the target you want to add the document type to.
Select the Info tab.
Click on the disclosure button for Document Types to open the document types.
Click the “+” button.
In the newly created document type :
- Type the name of the document type.
- In the “Types” section fill in the UTI for the new type.
- Provide an icon for the document.
Click the disclosure triangle to open Additional document type properties.
Click in the table to add a new key and value.
- For the key value type: CFBundleTypeRole.
- For the value type: Editor.
Click the + button to add another key/value pair.
- For the key value type: LSHandlerRank.
- For the value type: Owner.

There’s more on that same page:

Adding a custom UTI. If it’s a custom UTI you need to add it. This is how you associate file extensions with the UTI.
How to test (hint: send the document as email)
Troubleshooting

Also look at the original documentation, though it doesn’t mention how XCode can help. See: Registering the File Types Your App Supports and Uniform Type Identifiers.

Detailed info keys for document file types: CFBundleDocumentTypes

h/t

Swift Neologisms

Fri, 16 Dec 2016 19:42:43 -0500

Swift evolves and stuff gets renamed. Here is a score card of the ones that trip me.

Was			Now
Name	Swift		Name	Swift
Printable	1	→	CustomStringConvertible	2
DebugPrintable	1	→	CustomDebugStringConvertible	2

Table row reordering, deleting, inserting

Sat, 26 Nov 2016 09:20:31 -0500

https://developer.apple.com/library/content/documentation/UserExperience/Conceptual/TableView_iPhone/ManageReorderRow/ManageReorderRow.html

UITabBarController in UINavigationController

Wed, 23 Nov 2016 20:57:34 -0500

It can’t be done.

You can fake it by making your own tab controller, so long as you figure out a way to honor the surrounding UINavigationController, meaning a navigation bar atop and optional (disallowed?) tool bar below all tabbed views.

Or you can use a modal segue to display the UITabBarController. That means:

Convert the class to UIViewController.
Make it implement UITabBarDelegate (if you wish). That means overridden delegate methods are no longer there to override.
Create a fake UINavigationBar since you cannot embed thi

adding a navigation bar with a Done/Close bar button and perhaps an exit segue.

h/t

UITabBarController inside a UINavigationController (2011) describes a third method, making the tab bar controller the root and then doginscreen swapping (I think).
Stack Overflow "Tab bar controller inside a navigation controller, or sharing a navigation root view” (2009) describes the first method.

Making code documentation

Wed, 23 Nov 2016 16:54:40 -0500

Here is a cheat sheet

Swift Documentation Comments

// MARK: - heading
// TODO: comment
// FIXME: comment

Block and single line comments

/**
*
*/

///
///
///

Example:

/// One line description of function.
///
/// A detailed description
/// may be placed on the remaining lines
/// - parameter something: the parameter description
/// - throws: `ErrorCode.noDevicePersister` The `something` parameter was blank.
/// - returns: a result of the computation
func think(about something: String) throws -> String {

Longer parameter lists:

/// - Parameters:
/// - x: The *x* component of the vector.
/// - y: The *y* component of the vector.
/// - z: The *z* component of the vector.
func magnitude3D(x: Double, y: Double, z: Double) -> Double {

Description fields headers

Algorithm/Safety Information: Precondition, Postcondition, Requires, Invariant, Complexity, Important, Warning

Metadata: Author, Authors, Copyright, Date, SeeAlso, Since, Version

General Notes & Exhortations: Attention, Bug, Experiment, Note, Remark, ToDo

Code blocks

Indent code block by at least 4 spaces.

/**
The area of the `Shape` instance.

Computation depends on the shape of the instance.
For a triangle, `area` will be equivalent to:

let height = triangle.calculateHeight()
let area = triangle.base * height / 2
*/

Fence unindented code blocks using triple backtick (```) or tilde (~):

```
// Circles:
let perimeter = circle.radius * 2 * CGFloat(M_PI)

// Other shapes:
let perimeter = shape.sides.map { $0.length }
.reduce(0, combine: +)
```

For further reading:

How to Generate Beautiful Apple-Style Documentation in Xcode 5

There is no enumeration for all cases on an enum [Swift 3.0.1]

Sun, 13 Nov 2016 20:29:40 -0500

My practice is to define a static constant array inside the enum which enumerates them. This is error prone when a case is added and the array isn’t updated.

#ThingsMomNeverToldMe

Halting test after XCTAssert fails

Sun, 06 Nov 2016 22:41:45 -0500

While it can be useful to have a test continue after an assertion failure, if you have code following the XCTAssert then it mustn’t depend on the condition being successful.

If you need each test method to stop if there is a failure, set this flag to false:

Swift 3.1

self.continueAfterFailure = false

You can set this in any test and it will be reset between tests to true. That way it is in effect for that test only. You can set it for the whole test class in the setUp function:

Swift 3.1

override func setUp() {
    super.setUp()
    self.continueAfterFailure = false
}

h/t

Marek R on Stack Overflow: XCTAssertTrue doesn't stop routine.
Apple API Reference: XCTest > XCTestCase > continueAfterFailure.

Swift protocol which requires a specific type of sequence

Thu, 15 Sep 2016 20:02:53 -0400

I really can’t explain it any better than the hat tip did! What you want:

Swift 3.0

protocol HasSequenceOfInts {
    var seq : SequenceType { get }
}

What you need to write:

Swift 3.0

protocol HasSequenceOfInts {
    var seq : AnySequence { get }
}
class ArrayOfInts : HasSequenceOfInts {
    var _seq : [Int] = [0,1,2]
    var seq : AnySequence {
        get {
            return AnySequence(self._seq)
        }
    }
}

h/t

Rob Napier on Stack Overflow: Define a Swift protocol which requires a specific type of sequence.

Popping versus Unwinding

Sun, 14 Aug 2016 13:01:18 -0400

Use an unwind segue when a UINavigationController pop isn’t what you want. For example, when going back beyond the current UINavigationController stack. Note that you can pop one or more levels without needing an unwind when you are within the UINavigationController view stack.

When just using popping you will want to send the data model(s) to the child view controllers. They act on it directly.

When unwinding you can be more abstract and only pass the essential data pieces, or the model if you still want. Unwinding can communicate back since the child view controller lingers long enough to be scoured for its data.

Unwind segues in Swift

Two essential preparatory reading assignments are:

Apple Technical Note TN2298: Using Unwind Segues

Stack Overflow: What are Unwind segues for and how do you use them?

In Swift 2.3 / iOS 10.0 (I don’t know which is the critical factor) the method signature must be:

Swift 2.3

@IBAction func unwindToTeachView(withUnwindSegue unwindSegue: UIStoryboardSegue) {
        ...
    }

The critical elements are

@IBAction
func
One parameter
withUnwindSegue
UIStoryBoardSegue
No return value

I found this out on my own. I contributed this answer to another Stack Overflow question, Xcode 6 Storyboard Unwind Segue with Swift Not Connecting to Exit.

Dismiss keyboard

Sun, 14 Aug 2016 09:58:12 -0400

Dismiss keyboard after pressing Return

Set the view controller delegate to an object which implements the method textFieldShouldReturn(_) in the protocol UITextFieldDelegate.

Swift 2.3

// MARK: UITextFieldDelegate

/**
    When the user hits the return or "Done" button on a text field, we want 
    the keyboard to go away.
*/
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
    textField.resignFirstResponder()
    return true
}

h/t

Apple’s sample code “MyLife” PersonViewController.swift:

Show master view first

Sat, 13 Aug 2016 13:22:54 -0400

On an iPhone (compact layout), UISplitViewController likes to show the detail view when the app starts. To make the master view the first one shown, do this:

Swift 2.3

class MyMasterTableViewController: UITableViewController, UISplitViewControllerDelegate {
    private var collapseDetailViewController = true

    override func viewDidLoad() {
        super.viewDidLoad()

        splitViewController?.delegate = self
    }

    // MARK: - UITableViewDelegate

    override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
        collapseDetailViewController = false
    }

    // MARK: - UISplitViewControllerDelegate

    func splitViewController(splitViewController: UISplitViewController, collapseSecondaryViewController secondaryViewController: UIViewController, ontoPrimaryViewController primaryViewController: UIViewController) -> Bool {
        return collapseDetailViewController
    }
}

h/t

http://nshipster.com/uisplitviewcontroller/

UITextView not scrolled to top

Sat, 13 Aug 2016 12:30:01 -0400

When populating a UITextView by code, which is typical when the text is not meant to be editable, very long text will not be scrolled to the top. Scrolling in fact seems to be kind of random, but that’s probably because I don’t perceive the rule pattern yet.

If the view is not overlapped by a navbar, then this should do the trick:

Swift 2.3

override func viewDidLayoutSubviews() {  
    super.viewDidLayoutSubviews()  
    dispatch_async(dispatch_get_main_queue(), {  
        self.myTextView.scrollRangeToVisible(NSMakeRange(0, 0))  
    })  
}

If the view is fitted under the navbar of a UINavigationController, then try this:

Swift 2.3

override func viewDidLayoutSubviews() {  
    super.viewDidLayoutSubviews()  
    dispatch_async(dispatch_get_main_queue(), {  
        self.myTextView.contentOffset.y = -self.myTextView.contentInset.top  
    })  
}

h/t

https://forums.developer.apple.com/thread/18258