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What is the proper use of UIResponder.undoManager (Swift 4)
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Main-storyboard.xml
<?xml version="1.0" encoding="UTF-8"?>
<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="14113" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" useSafeAreas="YES" colorMatched="YES" initialViewController="gUO-Uk-rVh">
<device id="retina4_7" orientation="portrait">
<adaptation id="fullscreen"/>
</device>
<dependencies>
<deployment identifier="iOS"/>
<plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="14088"/>
<capability name="Safe area layout guides" minToolsVersion="9.0"/>
<capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
</dependencies>
<scenes>
<!--Item 2-->
<scene sceneID="B6D-zg-AL2">
<objects>
<viewController id="3ub-SL-PrQ" sceneMemberID="viewController">
<view key="view" contentMode="scaleToFill" id="orx-mp-Y7e">
<rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
<autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
<color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="genericGamma22GrayColorSpace"/>
<viewLayoutGuide key="safeArea" id="mo3-06-6kI"/>
</view>
<tabBarItem key="tabBarItem" title="Item 2" id="Lfa-0g-MH8"/>
</viewController>
<placeholder placeholderIdentifier="IBFirstResponder" id="lC5-np-aH3" userLabel="First Responder" sceneMemberID="firstResponder"/>
</objects>
<point key="canvasLocation" x="1030" y="455"/>
</scene>
<!--My SubclassedTVC-->
<scene sceneID="1Px-hQ-8Z2">
<objects>
<tableViewController id="aYp-Ls-x3G" customClass="MySubclassedTVC" customModule="TestProject" customModuleProvider="target" sceneMemberID="viewController">
<tableView key="view" clipsSubviews="YES" contentMode="scaleToFill" alwaysBounceVertical="YES" dataMode="static" style="plain" separatorStyle="default" rowHeight="-1" estimatedRowHeight="-1" sectionHeaderHeight="28" sectionFooterHeight="28" id="wcH-hv-H3C">
<rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
<autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
<color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="genericGamma22GrayColorSpace"/>
<sections>
<tableViewSection headerTitle="MySubclassedTVC" id="Wq8-eO-PWV">
<cells>
<tableViewCell clipsSubviews="YES" contentMode="scaleToFill" preservesSuperviewLayoutMargins="YES" selectionStyle="default" indentationWidth="10" reuseIdentifier="row1" textLabel="nCW-ld-pIh" style="IBUITableViewCellStyleDefault" id="vbN-Iv-5QF">
<rect key="frame" x="0.0" y="28" width="375" height="44"/>
<autoresizingMask key="autoresizingMask"/>
<tableViewCellContentView key="contentView" opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="center" preservesSuperviewLayoutMargins="YES" insetsLayoutMarginsFromSafeArea="NO" tableViewCell="vbN-Iv-5QF" id="XCh-y4-Y1a">
<rect key="frame" x="0.0" y="0.0" width="375" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<subviews>
<label opaque="NO" multipleTouchEnabled="YES" contentMode="left" insetsLayoutMarginsFromSafeArea="NO" text="Row 1 (initiate segue)" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" id="nCW-ld-pIh">
<rect key="frame" x="16" y="0.0" width="343" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<fontDescription key="fontDescription" type="system" pointSize="17"/>
<nil key="textColor"/>
<nil key="highlightedColor"/>
</label>
</subviews>
</tableViewCellContentView>
<connections>
<segue destination="So6-9U-ErM" kind="show" id="z0W-ti-r34"/>
</connections>
</tableViewCell>
<tableViewCell clipsSubviews="YES" contentMode="scaleToFill" preservesSuperviewLayoutMargins="YES" selectionStyle="default" indentationWidth="10" reuseIdentifier="row2" textLabel="3mJ-aS-ejI" style="IBUITableViewCellStyleDefault" id="MSe-Gh-vGZ">
<rect key="frame" x="0.0" y="72" width="375" height="44"/>
<autoresizingMask key="autoresizingMask"/>
<tableViewCellContentView key="contentView" opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="center" preservesSuperviewLayoutMargins="YES" insetsLayoutMarginsFromSafeArea="NO" tableViewCell="MSe-Gh-vGZ" id="L8Q-kb-DQE">
<rect key="frame" x="0.0" y="0.0" width="375" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<subviews>
<label opaque="NO" multipleTouchEnabled="YES" contentMode="left" insetsLayoutMarginsFromSafeArea="NO" text="Row 2" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" id="3mJ-aS-ejI">
<rect key="frame" x="16" y="0.0" width="343" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<fontDescription key="fontDescription" type="system" pointSize="17"/>
<nil key="textColor"/>
<nil key="highlightedColor"/>
</label>
</subviews>
</tableViewCellContentView>
</tableViewCell>
<tableViewCell clipsSubviews="YES" contentMode="scaleToFill" preservesSuperviewLayoutMargins="YES" selectionStyle="default" indentationWidth="10" reuseIdentifier="row3" textLabel="8X7-0k-gW2" style="IBUITableViewCellStyleDefault" id="1hF-90-LNx">
<rect key="frame" x="0.0" y="116" width="375" height="44"/>
<autoresizingMask key="autoresizingMask"/>
<tableViewCellContentView key="contentView" opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="center" preservesSuperviewLayoutMargins="YES" insetsLayoutMarginsFromSafeArea="NO" tableViewCell="1hF-90-LNx" id="aZt-ZZ-AdL">
<rect key="frame" x="0.0" y="0.0" width="375" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<subviews>
<label opaque="NO" multipleTouchEnabled="YES" contentMode="left" insetsLayoutMarginsFromSafeArea="NO" text="Row 3" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" id="8X7-0k-gW2">
<rect key="frame" x="16" y="0.0" width="343" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
<fontDescription key="fontDescription" type="system" pointSize="17"/>
<nil key="textColor"/>
<nil key="highlightedColor"/>
</label>
</subviews>
</tableViewCellContentView>
</tableViewCell>
</cells>
</tableViewSection>
</sections>
<connections>
<outlet property="dataSource" destination="aYp-Ls-x3G" id="0Tf-3x-ZFe"/>
<outlet property="delegate" destination="aYp-Ls-x3G" id="USF-qY-dKN"/>
</connections>
</tableView>
<navigationItem key="navigationItem" id="osl-fH-wOu"/>
</tableViewController>
<placeholder placeholderIdentifier="IBFirstResponder" id="onQ-Z1-gxz" userLabel="First Responder" sceneMemberID="firstResponder"/>
</objects>
<point key="canvasLocation" x="1959" y="-210"/>
</scene>
<!--My Sub SubclassedTVC-->
<scene sceneID="npu-qz-94D">
<objects>
<tableViewController id="So6-9U-ErM" customClass="MySubSubclassedTVC" customModule="TestProject" customModuleProvider="target" sceneMemberID="viewController">
<tableView key="view" clipsSubviews="YES" contentMode="scaleToFill" alwaysBounceVertical="YES" dataMode="static" style="plain" separatorStyle="default" rowHeight="-1" estimatedRowHeight="-1" sectionHeaderHeight="28" sectionFooterHeight="28" id="V7k-LQ-pkk">
<rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
<autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
<color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="genericGamma22GrayColorSpace"/>
<sections>
<tableViewSection headerTitle="MySubSubclassedTVC" id="DC5-O3-Y4i">
<cells>
<tableViewCell clipsSubviews="YES" contentMode="scaleToFill" preservesSuperviewLayoutMargins="YES" selectionStyle="default" indentationWidth="10" id="Kss-5J-a4B">
<rect key="frame" x="0.0" y="28" width="375" height="44"/>
<autoresizingMask key="autoresizingMask"/>
<tableViewCellContentView key="contentView" opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="center" preservesSuperviewLayoutMargins="YES" insetsLayoutMarginsFromSafeArea="NO" tableViewCell="Kss-5J-a4B" id="Epw-KJ-VU7">
<rect key="frame" x="0.0" y="0.0" width="375" height="43.5"/>
<autoresizingMask key="autoresizingMask"/>
</tableViewCellContentView>
</tableViewCell>
</cells>
</tableViewSection>
</sections>
<connections>
<outlet property="dataSource" destination="So6-9U-ErM" id="0vY-8g-c6A"/>
<outlet property="delegate" destination="So6-9U-ErM" id="15t-qo-URt"/>
</connections>
</tableView>
</tableViewController>
<placeholder placeholderIdentifier="IBFirstResponder" id="Fi9-A8-mP0" userLabel="First Responder" sceneMemberID="firstResponder"/>
</objects>
<point key="canvasLocation" x="2942" y="-210"/>
</scene>
<!--My Tab BarVC-->
<scene sceneID="Ei6-s2-GoC">
<objects>
<tabBarController id="gUO-Uk-rVh" customClass="MyTabBarVC" customModule="TestProject" customModuleProvider="target" sceneMemberID="viewController">
<tabBar key="tabBar" contentMode="scaleToFill" insetsLayoutMarginsFromSafeArea="NO" id="NjD-wU-fg3">
<rect key="frame" x="0.0" y="0.0" width="375" height="49"/>
<autoresizingMask key="autoresizingMask"/>
<color key="backgroundColor" white="0.0" alpha="0.0" colorSpace="custom" customColorSpace="genericGamma22GrayColorSpace"/>
</tabBar>
<connections>
<segue destination="6Lg-yF-lMB" kind="relationship" relationship="viewControllers" id="oCO-J8-zSr"/>
<segue destination="3ub-SL-PrQ" kind="relationship" relationship="viewControllers" id="Lgv-aG-XL7"/>
</connections>
</tabBarController>
<placeholder placeholderIdentifier="IBFirstResponder" id="UEB-ST-kDp" userLabel="First Responder" sceneMemberID="firstResponder"/>
</objects>
<point key="canvasLocation" x="-41" y="121"/>
</scene>
<!--Item-->
<scene sceneID="2Iq-t0-rlN">
<objects>
<navigationController automaticallyAdjustsScrollViewInsets="NO" id="6Lg-yF-lMB" sceneMemberID="viewController">
<tabBarItem key="tabBarItem" title="Item" id="vKq-bx-53o"/>
<toolbarItems/>
<navigationBar key="navigationBar" contentMode="scaleToFill" insetsLayoutMarginsFromSafeArea="NO" id="N9C-Do-5iD">
<rect key="frame" x="0.0" y="20" width="375" height="44"/>
<autoresizingMask key="autoresizingMask"/>
</navigationBar>
<nil name="viewControllers"/>
<connections>
<segue destination="aYp-Ls-x3G" kind="relationship" relationship="rootViewController" id="Ebo-G6-ePb"/>
</connections>
</navigationController>
<placeholder placeholderIdentifier="IBFirstResponder" id="bek-vp-oL7" userLabel="First Responder" sceneMemberID="firstResponder"/>
</objects>
<point key="canvasLocation" x="1030" y="-210"/>
</scene>
</scenes>
</document>
Raw
MyApp.swift
//
// MyApp.swift
// TestProject
//
// Created by Frank J. Edwards on 9/8/18.
// Copyright © 2018 Frank J. Edwards. All rights reserved.
//
/*
* From this thread: https://forums.developer.apple.com/message/329975
*
* Execute this code and you'll get a tab bar with two items, the first (and default)
* is a table view. Click on the first row to initiate a segue to a second table view.
* (This represents a user wanting to edit the data represented by the row.) When the
* user is done and clicks _Done_ in the navigation bar, the `viewWillDisappear()`
* method invokes a callback to pass data back to the parent. The parent should change
* the model and register an undo operation.
*
* You'll notice in the output that all pieces of code can see the undoManager object
* except the closure in the parent. (Look for "NOT found" in the output.)
*
* Why? Shouldn't the undoManager that was accessible before still be accessible?
* If this code is wrong, what is the right way to pass data back to the parent and
* have it register the undo operation?
*/
import UIKit
class MyTabBarVC: UITabBarController {
private var _undoManager = UndoManager()
override var undoManager: UndoManager { return _undoManager }
override var canBecomeFirstResponder: Bool { return true }
}
protocol DelegateForCallback {
func callbackFunction(_ msg: String)
}
class MyTableViewVC: UITableViewController, DelegateForCallback {
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
let typ = "MyTableViewVC"
if let _ = self.undoManager {
print("\(typ).viewDidAppear: Found in 'self'")
} else {
print("\(typ).viewDidAppear: NOT found in 'self'")
}
if let _ = view.undoManager {
print("\(typ).viewDidAppear: Found in 'view'")
} else {
print("\(typ).viewDidAppear: NOT found in 'view'")
}
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
guard let vc = segue.destination as? MySubSubclassedTVC else {
fatalError()
}
vc.callbackClosure = {
self.callbackFunction("via closure")
}
vc.delegateForCallback = self
}
func callbackFunction(_ msg: String) -> Void {
let typ = "MyTableViewVC"
if let _ = self.undoManager {
print("\(typ).callbackFunction: Found in 'self' \(msg)")
} else {
print("\(typ).callbackFunction: NOT found in 'self' \(msg)")
}
if let _ = self.view.undoManager {
print("\(typ).callbackFunction: Found in 'self.view' \(msg)")
} else {
print("\(typ).callbackFunction: NOT found in 'self.view' \(msg)")
}
}
}
class MySubclassedTVC: MyTableViewVC {
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
let typ = "MySubclassedTVC"
if let _ = self.undoManager {
print("\(typ).viewDidAppear: Found in 'self'")
} else {
print("\(typ).viewDidAppear: NOT found in 'self'")
}
if let _ = view.undoManager {
print("\(typ).viewDidAppear: Found in 'view'")
} else {
print("\(typ).viewDidAppear: NOT found in 'view'")
}
}
}
class MySubSubclassedTVC: MySubclassedTVC {
var callbackClosure: (() -> Void)? = nil
var delegateForCallback: DelegateForCallback?
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
let typ = "MySubSubclassedTVC"
if let _ = self.undoManager {
print("\(typ).viewDidAppear: Found in 'self'")
} else {
print("\(typ).viewDidAppear: NOT found in 'self'")
}
if let _ = view.undoManager {
print("\(typ).viewDidAppear: Found in 'view'")
} else {
print("\(typ).viewDidAppear: NOT found in 'view'")
}
}
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
let typ = "MySubSubclassedTVC"
if let _ = self.undoManager {
print("\(typ).viewWillDisappear: Found in 'self'")
} else {
print("\(typ).viewWillDisappear: NOT found in 'self'")
}
if let _ = view.undoManager {
print("\(typ).viewWillDisappear: Found in 'view'")
} else {
print("\(typ).viewWillDisappear: NOT found in 'view'")
}
callbackClosure?()
delegateForCallback?.callbackFunction("via delegate")
}
}
@Azhrei
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Azhrei commented Sep 11, 2018 •
edited

Obviously, the Main-storyboard.xml has to be renamed to Main.storyboard, but GitHub wouldn't let me attach a file with extension of .storyboard

Images: Storyboard and Class Diagram

@Azhrei
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Azhrei commented Sep 16, 2018 •
edited

Run the above code in an Xcode project (I'm using Xcode 9.4.1 and targeting iOS 11.4 with Swift 4.1). I'm targeting an iPhone 8 Plus, but it doesn't matter which device is used.

When the main window appears, you'll see a table with three entries and four lines of text in the debug window. The text indicates whether undoManager has a valid value or whether it contains nil. So far, it has been non-nil in the places shown:

MyTableViewVC.viewDidAppear: Found in 'self'
MyTableViewVC.viewDidAppear: Found in 'view'
MySubclassedTVC.viewDidAppear: Found in 'self'
MySubclassedTVC.viewDidAppear: Found in 'view'

Click on the first row and the view replaces itself with a view pushed onto the nav controller's stack. The following appear in the debug window. So far, everything is good and undoManager is found in all cases:

MyTableViewVC.viewDidAppear: Found in 'self'
MyTableViewVC.viewDidAppear: Found in 'view'
MySubclassedTVC.viewDidAppear: Found in 'self'
MySubclassedTVC.viewDidAppear: Found in 'view'
MySubSubclassedTVC.viewDidAppear: Found in 'self'
MySubSubclassedTVC.viewDidAppear: Found in 'view'

Now click the <Back button. The detail view's viewWillDisappear() is invoking a closure provided by the parent view's prepare(0 method. This closure attempts to access the undoManager and it fails (returns nil). See the text below:

MySubSubclassedTVC.viewWillDisappear: Found in 'self'
MySubSubclassedTVC.viewWillDisappear: Found in 'view'
MyTableViewVC.callbackFunction: NOT found in 'self' via closure
MyTableViewVC.callbackFunction: NOT found in 'self.view' via closure
MyTableViewVC.callbackFunction: NOT found in 'self' via delegate
MyTableViewVC.callbackFunction: NOT found in 'self.view' via delegate
MyTableViewVC.viewDidAppear: Found in 'self'
MyTableViewVC.viewDidAppear: Found in 'view'
MySubclassedTVC.viewDidAppear: Found in 'self'
MySubclassedTVC.viewDidAppear: Found in 'view'

I don't understand why. The documentation for UIResponder indicates that the undoManager property does a dynamic lookup, walking up the hierarchy of views until it finds one. So why does the closure not have a valid undoManager property when the same class previous did have a valid one (see the MyTableViewVC.viewDidAppear() messages)?

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