问题
my project uses both Objective-C and Swift code. When a user logs in, it calls a set of apis for user preference, I have a DataCoordinator.swift class which schedules the API operation and I make this calls from UserDetailViewController.m class to load user preferences. This use to work fine before I migrated my code to Swift 4 using Xcode 9 beta 4. Now when I login it crashes by giving me this error in my DataCoordinator class. Below is a sample of my DataCoordinator and Viewcontroller class.
DataCoordinator.swift
import UIKit
@objcMembers
class DataCoordinator: NSObject {
//MARK:- Private
fileprivate var user = myDataStore.sharedInstance().user
fileprivate var preferenceFetchOperations = [FetchOperation]()
fileprivate func scheduleFetchOperation(_ operation:FetchOperation, inFetchOperations operations:inout [FetchOperation]) {
guard operations.index(of: operation) == nil else { return }
operations.append(operation)
}
fileprivate func completeFetchOperation(_ fetchOperation:FetchOperation, withError error:Error?, andCompletionHandler handler:@escaping FetchCompletionHandler) {
func removeOperation(_ operation:FetchOperation, fromOperations operations:inout [FetchOperation]) {
if operations.count > 0 {
operations.remove(at: operations.index(of: fetchOperation)!)
handler(error)
}
}
if preferenceFetchOperations.contains(fetchOperation) {
removeOperation(fetchOperation, fromOperations: &preferenceFetchOperations)
}
}
fileprivate func schedulePreferencesFetchOperation(_ serviceName:String, fetch:@escaping FetchOperationBlock){
let operation = FetchOperation(name: serviceName, fetch: fetch);
scheduleFetchOperation(operation, inFetchOperations: &preferenceFetchOperations)
}
fileprivate func runOperationsIn(_ fetchOperations:inout [FetchOperation]) {
for var operation in fetchOperations {
guard operation.isActivated == false else { continue }
operation.isActivated = true
operation.execute()
}
}
//MARK:- Non-Private
typealias FetchCompletionHandler = (_ error:Error?)->Void
var numberOfPreferencesFetchCalls:Int {
get { return preferenceFetchOperations.count }
}
// MARK: -
func fetchPreferences(_ completionHandler:@escaping FetchCompletionHandler) -> Void {
defer {
runOperationsIn(&preferenceFetchOperations)
}
schedulePreferencesFetchOperation("com.fetchPreferences.type1") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType1Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type2") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType2Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type3") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType3Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type4") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType4Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
}
}
// MARK:- Fetch Operation Struct
private typealias FetchOperationBlock = (_ operation:FetchOperation)->Void
private struct FetchOperation:Hashable {
fileprivate var runToken = 0
fileprivate let fetchBlock:FetchOperationBlock
let name:String!
var isActivated:Bool {
get {
return runToken == 0 ? false : true
}
mutating set {
if runToken == 0 && newValue == true {
runToken = 1
}
}
}
fileprivate var hashValue: Int {
get {
return name.hashValue
}
}
func execute() -> Void {
fetchBlock(self)
}
init (name:String, fetch:@escaping FetchOperationBlock) {
self.name = name
self.fetchBlock = fetch
}
}
private func ==(lhs: FetchOperation, rhs: FetchOperation) -> Bool {
return lhs.hashValue == rhs.hashValue
}
//This is how I call it in my viewcontrollers viewDidLoad method
__weak UserDetailViewController *weakSelf = self;
[self.dataCoordinator fetchPreferences:^(NSError * _Nullable error) {
if (error == nil) {
[weakSelf didFetchPrefrences];
}
else {
// handle error
}
}];
//completion response
- (void)didFetchPrefrences {
//when api calls complete load data
if (self.dataCoordinator.numberOfPreferencesFetchCalls == 0) {
//Load details
}
}
I'm not sure how to proceed on this, I saw a bug report at https://bugs.swift.org/browse/SR-5119 but it seems to be fixed in Xcode 9 beta 3. Any help is appreciated
回答1:
I think this 'bug' may be a Swift 4 'feature', specifically something they call 'Exclusive access to Memory'.
Check out this WWDC video. Around the 50 minute mark, the long-haired speaker explains it.
https://developer.apple.com/videos/play/wwdc2017/402/?time=233
You could try turning the thread sanitizer off in your scheme settings if you're happy to ignore it. However, the debugger is trying to tell you about a subtle threading issue so it's probably a better use of your time to try to figure out why you've got something writing to your array at the same time it's being read from.
回答2:
Under the target's Build Settings. Select No Enforcement for Exclusive Access to Memory from Swift Compiler - Code Generation
回答3:
Only in Swift 4 and when using .initial option for your KVO Settings
If you check your context in observeValue method, just make your context variable static. This blog post describes this bug in detail.
回答4:
In Swift 5.0, this will be the default behavior when running your application in Release mode. Before 5.0 (Swift 4.2.1 as of today and lower) this behavior is only running when in Debug mode.
Your application will fail in release mode if you ignored this error.
Consider this example:
func modifyTwice(_ value: inout Int, by modifier: (inout Int) -> ()) {
modifier(&value)
modifier(&value)
}
func testCount() {
var count = 1
modifyTwice(&count) { $0 += count }
print(count)
}
What is the value of count, when the print(count) line is printed? Well I don't know either and the compiler gives unpredicatable results when you run this code. This isn't allowed in Swift 4.0 in debug mode and in Swift 5.0 it crashes even in runtime.
Source: https://swift.org/blog/swift-5-exclusivity/
回答5:
In my case, Swift 4 actually uncovered a kind of bug that I wouldn't have noticed until I started calling a function from more than one place. My function was passed an inout global array and it was referencing both that parameter and the global name. When I changed the function to reference only the parameter, the "simultaneous access" error went away.
回答6:
What I would do is change FetchOperation to a class instead of struct.
回答7:
The answers by @Mark Bridges and @geek1706 are good answers but I would like to add my 2 cents about this matter and give a general example.
As stated above this is a feature in Swift 4 SE-176.
The implementation should still be permitted to detect concurrent conflicting accesses, of course. Some programmers may wish to use an opt-in thread-safe enforcement mechanism instead, at least in some build configurations.
The exclusive access enforces that every write mutation of vars must be exclusive when accessing that variable. In a multithread environment, multiple threads accessing a shared var and one or more can modify it.
There's nothing like a good example:
If we try to mutate a shared value in a multi-threaded environment using an abstraction (mutation occurs on a protocol type) between 2 objects and the Exclusive Access to Memory is on, our app will crash.
protocol Abstraction {
var sharedProperty: String {get set}
}
class MyClass: Abstraction {
var sharedProperty: String
init(sharedProperty: String) {
self.sharedProperty = sharedProperty
}
func myMutatingFunc() {
// Invoking this method from a background thread
sharedProperty = "I've been changed"
}
}
class MainClass {
let myClass: Abstraction
init(myClass: Abstraction) {
self.myClass = myClass
}
func foobar() {
DispatchQueue.global(qos: .background).async {
self.myClass.myMutatingFunc()
}
}
}
let myClass = MyClass(sharedProperty: "Hello")
let mainClass = MainClass(myClass: myClass)
// This will crash
mainClass.foobar()
Since we didn't state that the Abstraction protocol is class bound, during runtime, inside myMutatingFunc, the capture of self will be treated as struct even though we injected an actual class (MyClass).
Escaping variables generally require dynamic enforcement instead of static enforcement. This is because Swift cannot reason about when an escaping closure will be called and thus when the variable will be accessed.
The solution is to bound the Abstraction protocol to class:
protocol Abstraction: class
回答8:
In my case, I changed the table height during process of build the project. At that time my device was connected with network. I deleted the derived data and it resolved the issue for me.
回答9:
Returning zero in the numberOfSections override function will cause this crash:
override func numberOfSections(in collectionView: UICollectionView) -> Int {
// This causes a crash!
return 0
}
Simple solution - return 1 in the function above and then return 0 in the collectionView(_:numberOfItemsInSection:) function.
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
来源：https://stackoverflow.com/questions/45415901/simultaneous-accesses-to-0x1c0a7f0f8-but-modification-requires-exclusive-access