This code is not working, how do I repair it?
I try to handle the string and return it, but no result. printing does not output anything to the console
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let x = "hello"
let viewm = viewmodel()
viewm.handler(x)
viewm.handler = { item in
print(item)
}
viewm.execute { (tt) in
print(tt)
}
}
class viewmodel {
var handler:(String) -> Void = {
(data: String) in
}
func execute(complete: #escaping (String) -> Void) {
self.handler = complete
}
}
It is not clear what you are trying to achieve here, but if your issue is that print(item) is not printing anything, that is happening because this code by itself is not going to print anything:
viewm.handler = { item in
print(item)
}
handler property is a closure that receives a String parameter and returns Void. On your viewDidLoad method, you are assigning a closure that complies with those characteristics and, in addition to that, is going to print the passed parameter.
Given that closures are self-contained blocks of functionality, you need to call it in order to execute its block of functionality; but you need to call it after it has been defined; otherwise, it is not going to print anything.
If you write your code in this order, "hello" is going to be printed.
viewm.handler = { item in
print(item)
}
viewm.handler(x)
Related
I am new to RxSwift and RxCocoa
I need to any advice for learning
After result of Checking Id Validation, expect no word in label
But it is updating label and no entering in break point at bind function
What’s problem my code…?
var disposeBag: DisposeBag = DisposeBag()
let viewModel = ViewModel()
override func viewDidLoad() {
super.viewDidLoad()
let input: Signal<String> = userIDTextField.rx.text.orEmpty
.asSignal(onErrorSignalWith: .empty())
let output: Driver<String> = viewModel.bind(input)
disposeBag.insert(
output.drive(userIDLabel.rx.text)
)
}
struct ViewModel {
func checkUserIDFromDB(id: String) -> Signal<Bool> {
return .just(false).asSignal()
}
func bind(_ input: Signal<String>) -> Driver<String> {
let validState = input
.map { _ in self.checkUserIDFromDB(id:)}
.withLatestFrom(input)
return validState.asDriver(onErrorDriveWith: .empty())
}
}
This line: .map { _ in self.checkUserIDFromDB(id:)} produces a Signal<(String) -> Signal<Bool>> which is likely not what you wanted.
I'm going to assume that the goal here is to pass the entered string to the network request and wait for it to emit. If it emits true then emit the string to the label, otherwise do nothing...
Further, let's simplify things by using the Observable type instead of Signals and Drivers:
final class ViewController: UIViewController {
let userIDTextField = UITextField()
let userIDLabel = UILabel()
let disposeBag = DisposeBag() // this should be a `let` not a `var`
let viewModel = ViewModel()
override func viewDidLoad() {
super.viewDidLoad()
let input = userIDTextField.rx.text.orEmpty
let output = viewModel.bind(input.asObservable())
disposeBag.insert(
output.bind(to: userIDLabel.rx.text)
)
}
}
struct ViewModel {
func checkUserIDFromDB(id: String) -> Observable<Bool> { .just(false) }
func bind(_ input: Observable<String>) -> Observable<String> {
input.flatMapLatest { id in // Note this should be `flatMapLatest` not `map`
Observable.zip( // zip up the text with its response
Observable.just(id),
self.checkUserIDFromDB(id: id) // you weren't actually making the network call. This makes it.
.catchAndReturn(false) // if the call fails, emit `false`.
)
}
.compactMap { $0.1 ? $0.0 : nil } // if the response is true, emit the text, else nothing
}
}
The biggest concern I have with this code is what happens if the user continues to type. This will fire after every character the user enters which could be a lot of network requests, the flatMapLatest will cancel ongoing requests that are no longer needed, but still... Consider putting a debounce in the stream to reduce the number of requests.
Learn more about the various versions of flatMap from this article.
Edit
In response to your comment. In my opinion, a ViewModel should not be dependent on RxCocoa, only RxSwift. However, if you feel you must use Driver, then something like this would be appropriate:
func bind(_ input: ControlProperty<String>) -> Driver<String> {
input.asDriver()
.flatMapLatest { id in
Driver.zip(
Driver.just(id),
self.checkUserIDFromDB(id: id)
.asDriver(onErrorJustReturn: false)
)
}
.compactMap { $0.1 ? $0.0 : nil }
}
Using Signal doesn't make much sense in this context.
I'm trying to save a struct in background but I get this error :
closure cannot implicitly capture a mutating self parameter
This is my code :
//MARK: Parse self methods
fileprivate mutating func ParseSave(_ completionBlock: #escaping SuccessCompletionBlock) {
let message: PFObject = PFObject(className: "Message")
if let id = self.id {
//this object exit just update it
message.objectId = id
}
// set attributes
if let text = self.text {
message["text"] = text
}
message["sender"] = PFUser(withoutDataWithObjectId: self.sender.id)
message["conversation"] = PFObject(withoutDataWithClassName: "Conversation", objectId: conversationId)
message["viewed"] = self.viewed
message.saveInBackground { (success, error) in
if success {
// the next 3 lines cause the error : (when I try to update the struct - self )
self.id = message.objectId
self.createdAt = message.createdAt ?? self.createdAt
self.updatedAt = message.updatedAt ?? self.updatedAt
}
completionBlock(success, error)
}
}
I've checked those question: 1 - 2 I've added the #escaping
but didn't work.
I think it will help if we minimally elicit the error message you're getting. (For delay, see dispatch_after - GCD in swift?.)
struct S {
var name = ""
mutating func test() {
delay(1) {
self.name = "Matt" // Error: Closure cannot ...
// ... implicitly capture a mutating self parameter
}
}
}
The reason lies in the peculiar nature of struct (and enum) mutation: namely, it doesn't really exist. When you set a property of a struct, what you're really doing is copying the struct instance and replacing it with another. That is why only a var-referenced struct instance can be mutated: the reference must be replaceable in order for the instance to be mutable.
Now we can see what's wrong with our code. Obviously it is legal for a mutating method to mutate self; that is what mutating means. But in this case we are offering to go away for a while and then suddenly reappear on the scene (after 1 second, in this case) and now mutate self. So we are going to maintain a copy of self until some disconnected moment in the future, when self will suddenly be somehow replaced. That is incoherent, not least because who knows how the original self may have been mutated in the meantime, rendering our copy imperfect; and the compiler prevents it.
The same issue does not arise with a nonescaping closure:
func f(_ f:()->()) {}
struct S {
var name = ""
mutating func test() {
f {
self.name = "Matt" // fine
}
}
}
That's because the closure is nonescaping; it is executed now, so the incoherency about what will happen in the future is absent. This is an important difference between escaping and nonescaping closures, and is one of the reasons why they are differentiated.
Also, the same issue does not arise with a class:
class C {
var name = ""
func test() {
delay(1) {
self.name = "Matt" // fine
}
}
}
That's because the class instance is captured by reference in the closure, and a class instance is mutable in place.
(See also my little essay here: https://stackoverflow.com/a/27366050/341994.)
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
isSuccess(true, success: { (name) -> String in
return "My name is \(name)"
})
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
func isSuccess(val:Bool, success: (name: String) -> String) {
if val {
success(name: "Jacky")
}
}
}
I expect it to return string "My name is Jacky",but it didn't .But if I change the isSuccess to :
func isSuccess(val:Bool, success: (name: String) -> String) {
if val {
print(success(name: "Jacky"))
}
}
}
Then it worked properly, why is that? Thanks in advance!
Your completion block returns a String.
When you invoke it by calling
success(name: "Jacky")
the completion block returns the String My name is Jacky. But you do nothing with that string. You just returned it and never used it.
In your second example, you actually used it - you took the string from the completion block, and printed it.
For example, instead the print, you could also write
let stringFromCompletionBlock = success(name: "Jacky")
That way you can see that it indeed returned a value.
And another thing is that the completion block should be call as the last thing in the function - this way you "notify" that the function has finished it's purpose, so it's not reasonable to use the value returned from a completion block inside the same function which called that completion block
First of all the closure in the function isSuccess should be declared this way. The closure should not return a String, it should just accept a String as param.
func isSuccess(val:Bool, success: (name: String) -> ()) {
if val {
success(name: "Jacky")
}
}
Next you could use that value to update the UI like follow
class ViewController: UIViewController {
weak var label:UILabel!
override func viewDidLoad() {
isSuccess(true) { (name) -> () in
dispatch_async(dispatch_get_main_queue()) {
self.label.text = "My name is \(name)"
}
}
super.viewDidLoad()
}
func isSuccess(val:Bool, success: (name: String) -> ()) {
if val {
success(name: "Jacky")
}
}
}
Thanks in advance for help!!
I'm trying to call a func from within my Class and I keep getting an error saying that:
Missing parameter for argument #1.............Read a few posts saying it's an instance vs class problem? I don't get it..I'm calling the method from within the Class??? There has to be an instance of the class if the method is being called????? right? Here is my code...Thanks
import Foundation
import Parse
class TestViewController {
let photos = getWallImages() //-----This is the line requesting an argument
func getWallImages() -> [WallPost] {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
return objects
println("We have \(objects.count)")
}
} else if let error = error {
println(error)
}
}
}
}
So the "crime" you are committing is the fact that the method is applied in an instance of the class and not as a class method. The function is expecting a self parameter (a reference to the instance). That explains the error message.
Now to fix that you have two quick options:
1. Make it a class function and call it that way too:
class TestViewController {
let photos = TestViewController.getWallImages()
class func getWallImages() -> [WallPost] {
// mumbo jumbo
}
}
This approach is problematic in case you would want to do some instance specific operations, because class func is static method and doesn't provide you with some of the object benefits.
2. Instantiate the object you are calling the method on:
class TestViewController {
let photos = TestViewController().getWallImages()
func getWallImages() -> [WallPost] {
// mumbo jumbo
}
}
This approach isn't correct with your given structure - it doesn't make sense to instantiate another view controller, but if you take the method and put it in a separate class, maybe it would then make sense.
Then of course you have multiple other ways of changing your code to make it work. Maybe you could initialize it with lazy parameter, maybe you could initialize it in the init method. Whatever suits you best. My answer is simply explaining where you've gone wrong.
There are a few ways you can set your property appropriately. You can make getWallImages() a type method:
class TestViewController {
let photos = TestViewController.getWallImages()
class func getWallImages() -> [WallPost] {
....
}
}
Or, you can keep your method an instance method and set your property upon initialization:
class TestViewController {
let photos: [WallPost]!
init() {
super.init()
photos = getWallImages()
}
func getWallImages() -> [WallPost] {
....
}
}
If you're asking a question you should reduce your code to a minimum, discarding unnecessary details.
You probably want something like this:
class MyClass {
let x = MyClass.getStuff()
static func getStuff() -> Int {
return 0
}
}
However your method getWallImages() can't do something like this, because it's returning the result asynchronous, which means you get the result much later after the function has returned.
You could do something like this though (this is how I'd be doing it):
class MyClass {
var x : Int? {
didSet {
if let x = x {
// Do something with x, here it's assigned
} else {
// x was set to nil, something failed
}
}
}
init() {
getStuffAsynchronous()
}
func getStuffAsynchronous() {
// Do your query stuff here, assign x to your objects to invoke the didSet
x = 0
// If it fails somehow, assign set x to nil
// if fail {
// x = nil
// }
}
}
I'm having an issue retrieving data from within an closure. I'm calling function called getWallImages which is supposed to return an array. I can print the contents of the array from within the closure, but outside of it the array is empty.
import Foundation
import Parse
class WallPostQuery {
var result = [WallPost]()
func getWallImages() -> [WallPost] {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
self.result = objects
//This line will print the three PFObjects I have
println(self.result)
}
}
}
//this line prints [] ...empty array?
println(result)
return self.result
}
}
Question
How do I get values out of a closure?
That is because println(result) is executed BEFORE self.results = objects. The closure is executed asynchronously so it executes afterwards. Try making a function that uses results which can be called form the closure:
var result = [WallPost]()
func getWallImages() {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
self.result = objects
//This line will print the three PFObjects I have
println(self.result)
self.useResults(self.result)
}
}
}
}
func useResults(wallPosts: [WallPost]) {
println(wallPosts)
}
}
Another solution to your problem, so that you can return it from that function is to create your own closure:
var result = [WallPost]()
func getWallImages(completion: (wallPosts: [WallPost]?) -> ()) {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
self.result = objects
//This line will print the three PFObjects I have
println(self.result)
completion(wallPosts: self.result)
} else {
completion(wallPosts: nil)
}
} else {
completion(wallPosts: nil)
}
}
}
func useResults(wallPosts: [WallPost]) {
println(wallPosts)
}
}
What is happening is that the method is returning before the closure executes.
Fundamentally, you're running into a problem with they way you are managing asynchronous callbacks.
Asynchronous vs synchronous execution, what does it really mean?
You need to create a way of notifying your caller from within your closure. You can achieve this by: requiring your own closure as an input parameters; using a delegate pattern; using a notification.
https://codereview.stackexchange.com/questions/87016/swift-ios-call-back-functions
Each has their benefits/drawbacks, and it depends on your particular situation. The simplest way to get started with async data fetches, is to pass in your own closure. From there, you can jump to another pattern such as the delegate pattern if the need arises.
I think the latter of println(result) is called before because findObjectsInBackgroundWithBlock is executed on background as its name suggests.
So, you can confirm result in the following way,
import Foundation
import Parse
class WallPostQuery {
var result = [WallPost]() {
didSet {
println(result)
}
}
func getWallImages() {
let query = WallPost.query()!
query.findObjectsInBackgroundWithBlock { objects, error in
if error == nil {
if let objects = objects as? [WallPost] {
self.result = objects
//This line will print the three PFObjects I have
println(self.result)
}
}
}
}
}