I have no idea why I get this error.
The problem code is here
for i in 0..<itemDataJson?.count {
imageUrls.append(appDelegate.itemDataJson![i]["image_url"].string!)
}
When I print(itemDataJson?.count) it prints Optional(1).
What am I doing wrong?
Thank you.
It's printing Optional(1) because the variable itemDataJson is nullable, so the count would therefore have to be nullable, because we don't know if itemDataJson actually has a value.
The main problem that I see in your code is that you are force-unwrapping variables. Force-unwrapping a variable is a code smell (usually, although I do it myself from time to time, but you need to be careful).
When you force unwrap a variable, you need to ask yourself the question, "Do I want the app to crash here if this variable is nil?". If the answer is yes, then using a force unwrap is acceptable, otherwise, you should create a non-nullable variable or if that is not possible, you should use the guard statement in swift.
This could be used like this:
guard let itemDataJson = itemDataJson else {
// itemDataJson was null do something
return
}
You can use the if let construct as well, but be careful of the pyramid of doom if you don't use the if let construct correctly. See here for using it correctly, or use a guard statement.
I would recommend checking out the documentation on optionals if you have not done so already.
I would change the code to this version:
if (itemDataJson != nil) {
for i in 0..<itemDataJson!.count {
imageUrls.append(appDelegate.itemDataJson![i]["image_url"].string!)
}
}
You should check all optionals before you try to access the underlying value. Best course of action would be to use if let statements.
if let itemDataJSON = itemDataJSON {
for i in 0..<itemDataJSON.count {
if let items = appDelegate.itemDataJSON {
if let imageURL = items[i]["imageURL"].string {
imageUrls.append(imageURL)
}
}
}
}
Other than that, it's a bad practice to store data in AppDelegate. You should create your own data layer that is not dependent of AppDelegate to make data handling more efficient and easier.
instead of checking for nil you should try this.
if let item = itemDataJson {
for i in 0..<item.count {
imageUrls.append(appDelegate.itemDataJson![i]["image_url"].string!)
}
}
Related
Just learning about the Swift guard keyword, I'm wondering what advantage it shall accomplish.
For example, let's say I have the following code:
func getEmail(email: String?) -> String {
guard email != "" else {
return ""
}
return email! + "#somewhere.com"
}
print(getEmail(email: "test123"))
I could write ...
if email == "" {
return ""
}
return email! + "#somewhere.com"
... as well.
So what's the advantage of having the guard-keyword?
Is it just having (a little bit) of syntactic sugar?
I don't think that is a very good example of using guard, it is more common to use it with variables that might be nil (aka optional) or functions that might return nil. I suggest you read about the guard statement in the Swift Programming Language book (just scroll down a bit to "Early Exit")
We could make a better example from your code that is lacking some validation
func getEmail(email: String?) -> String? {
guard let input = email, !input.isEmpty else {
return nil
}
return input + "#somewhere.com"
}
Here we use guard to check that the parameter email is not nil by assigning it to a local variable input. If it is nil the function will return nil and otherwise it will check if it is empty and then it will also return.
If it is ok the function will continue and create and return an email address. Note that the function is declared to return an optional string since I think it is much clearer if a function like this returns nil rather than an empty string if it fails.
Guard statement is early exit statement the main use of guard statement is to transfer program control out of a scope on certain conditions. These statements are similar with if statements which executes statements based on certain condition but unlike if, the guard statements only run when certain conditions are not met.
i strongly recommended to read this article Swift guard statement
If your code needs to decide whether to exit early, then you use the guard construct. A guard statement consists of a condition and an else block. The else block must leave the current scope, by any of the means that Swift provides, such as return, break, continue, throw, or fatalError. In case of failure of the condition, execution absolutely will not proceed within the block that contains the guard construct. Most of the time the construct is used to check Optional values, similar to if let statement. Unlike if let and/or while let, guard let doesn’t declare the bound variable for a nested scope, it declares it for this scope. Guys gave good examples on their answers about that. I would like to pay more attention on other possible cases as well.
A guard construct will also come in handy in conjunction with try?. Let’s presume we can’t proceed unless String(contentsOfFile:) succeeds. Then we can do the following:
guard let text = try? String(contentsOfFile: path) else {return}
There is also a guard case construct, forming the logical inverse of if case, e.g.:
guard case let .number(n) = error else { return }
There is one more interesting problem that you can solve using guard case construct. Suppose we have a function whose returned value we want to check in a guard statement:
guard howMany() > 10 else { return }
All looks good, but suppose that in the next line you want to use the value returned from that function. You may not want to call that function over again, because that might be time-consuming or it may have any side effects. You can't capture the result of calling the function with with guard let, because that requires an Optional, and the function howMany() doesn’t return an Optional.
In this case, you can use guard case construct in following way:
guard case let amount = howMany(), amount > 10 else { return }
I don't think for email id it would be that beneficial,
But consider example where there is a function where among students,
for one student you have to perform some crucial operation but what if
the student does not exist then there is no point in doing anything so
there comes your here as early exist which you can do using guard
statement.
let student = studentArr.filter{$0.id == "1234"}
guard let stdnt = student[0], stdnt != nil else {
return nil
}
// calculate marks of this student for all years' maths subject to prepare analytics
if let and guard let serve similar, but distinct purposes.
The "else" case of guard must exit the current scope. Generally that
means it must call return or abort the program. guard is used to
provide early return without requiring nesting of the rest of the
function.
if let nests its scope, and does not require anything special of it.
It can return or not.
In general, if the if-let block was going to be the rest of the
function, or its else clause would have a return or abort in it, then
you should be using guard instead. This often means (at least in my
experience), when in doubt, guard is usually the better answer. But
there are plenty of situations where if let still is appropriate.
This question already has answers here:
Check if optional array is empty
(8 answers)
Closed 4 years ago.
I have a array of my custom model, and I want to check if it is not nil and its size is greater then 0.
Following is my array with custom object
var listCountries : [Countries]? = nil
now In viewDIdLoad I want to make a check on it. I am new to Swift. I have good experience in working in Java.
I have read out Optional values concept and guard, if let statements. But I am unable to understand how efficiently they may be used. I have read too much SO questions but failed to figure out.
for example , if I want to check the upper given array in java I have only to do
if(listCountries != null && listCountries.size()>0){
//DO something
}
So to summarize my question:
How to make the upper given(Java code) check in to swift 4.? What is more smooth and reliable way.
What is a use of if let , guard, guard let statements. if I declare a variable (array, string) as optional I have to bear optional check like force wrapping each and every place. This is for me so making too much confusion.
Please help. I know this question has been asked in different ways. But this has some different context.
Just use ??.
if !(listCountries ?? []).isEmpty {
However, since you want to probably use listCountries in the if block, you should unwrap
if let listCountries = self.listCountries, !listCountries.isEmpty {
Ideally, if nil and empty means the same to you, don't even use an optional:
var listCountries: [Countries] = []
I would do it something like...
if let list = listCountries, !list.isEmpty { // Can also use list.count > 0
// do something
}
Even though you are not using the list inside the braces you are still using the list in the condition.
Or, like Sulthan said... make it non-optional to begin with if it makes no difference.
Obviously, I would assume that you are able to recognize the difference between nil array and empty array.
So, if we tried to implement a literal translation to your question:
I want to check if it is not nil and its size is greater then 0
For the first condition:
// "I want to check if it is not nil":
if let unwrappedList = listCountries {
// ...
}
and for the second condition:
// "I want to check if it is not nil":
if let unwrappedList = listCountries {
// "and its size is greater then 0":
if !unwrappedList.isEmpty {
// ...
}
}
However, you could combine both of the conditions by using the comma to achieve the multi-clause condition:
// I want to check if it is not nil and its size is greater then 0
if let unwrappedList = listCountries, !unwrappedList.isEmpty {
// ...
}
Or by using guard statement:
// I want to check if it is not nil and its size is greater then 0
guard let unwrappedList = listCountries, !unwrappedList.isEmpty else {
return
}
if let list = listCountries {
if(!list.isEmpty && list.count > 0) {
//value
}
}
So, I am new to cloudKit and to working with multiple threads in general, which I think is the source of the problem here, so if I simply need to research more, please just comment so and I will take that to heart.
Here is my question:
I am working in Swift 3 Xcode 8.1
I have in my view controller this variable:
var contactsNearby: [String:CLLocation]?
Then at the end of ViewDidLoad I call one of my view controllers methods let's call it:
populateContactsNearby()
inside that method I call:
container.discoverAllIdentities(completionHandler: { (identities, error) in
for userIdentity in identities! {
self.container.publicCloudDatabase.fetch(withRecordID: userIdentity.userRecordID!, completionHandler: { (userRecord, error) in
let contactsLocation = userRecord?.object(forKey: "currentLocation")
if self.closeEnough(self.myLocation!, contactLocation: contactsLocation as! CLLocation) {
var contactsName = ""
contactsFirstName = userIdentity.nameComponents?.givenName
if contactsName != "" && contactsLocation != nil {
self.contactsNearby?["\(contactsName)"] = contactsLocation as? CLLocation
}
}
})
}
})
}
I apologize if I am missing or have an extra bracket somewhere. I have omitted some error checking code and so forth in order to get this down to bare-bones. So the goal of all that is to populate my contactsNearby Dictionary with data from CloudKit. A name as the key a location as the value. I want to use that data to populate a tableview. In the above code, the call to closeEnough is a call to another one of my view controllers methods to check if the contact from CloudKit has a location close enough to my user to be relevant to the apps purposes. Also myLocation is a variable that is populated before the segue. It holds the CLLocation of the app users current location.
The Problem:
The if statement:
if contactsName != "" && contactsLocation != nil { }
Appears to succeed. But my view controllers variable:
var contactsNearby: [String:CLLocation]?
Is never populated and I know there is data available in cloudKit.
If it's relevant here is some test code that I have in cellForRowAtIndexPath right now:
let contact = self.contactsNearby?.popFirst()
let name = contact?.key
if name != nil {
cell.textLabel?.text = name
}else {
cell.textLabel?.text = "nothing was there"
}
My rows alway populate with "nothing was there". I have seen answers where people have done CKQueries to update the UI, but in those answers, the user built the query themselves. That seems different from using a CloudKit function like discoverAllIdentities.
I have tried to be as specific as possible in asking this question. If this question could be improved please let me know. I think it's a question that could benefit the community.
Okay, I need to do some more testing, but I think I got it working. Thank you Paulw11 for your comment. It got me on the right track.
As it turns out there were 2 problems.
First, as pointed out I have an asynchronous call inside a for loop. As recommended, I used a dispatchGroup.
Inside the cloudKit call to discoverAllIdentities I declared a dispatchGroup, kind of like so:
var icloudDispatchGroup = DispatchGroup()
Then just inside the for loop that is going to make an async call, I enter the dispatchGroup:
icloudDispatchGroup.enter()
Then just before the end of the publicCloudDatabase.fetch completion handler I call:
icloudDispatchGroup.leave()
and
icloudDispatchGroup.wait()
Which, I believe, I'm still new to this remember, ends the dispatchGroup and causes the current thread to wait until that dispatchGroup finishes before allowing the current thread to continue.
The Above took care of the multithreading issue, but my contactsNearby[String:CLLocation]? Dictionary was still not being populated.
Which leads me to the 2nd problem
At the top of my view controller I declared my Dictionary:
var contactsNearby: [String: CLLocation]?
This declared a dictionary, but does not initialize it, which I don't think I fully realized, so when I attempted to populate it:
self.contactsNearby?["\(contactsName)"] = contactsLocation as? CLLocation
It quietly failed because it is optional and returned nil
So, in viewDidLoad before I even call populateContactsNearby I initialize the dictionary:
contactsNearby = [String:CLLocation]()
This does not make it cease to be an optional, which Swift being strongly typed would not allow, but merely initializes contactsNearby as an optional empty Dictionary.
At least, that is my understanding of what is going on. If anyone has a more elegant solution, I am always trying to improve.
In case you are wondering how I then update the UI, I do so with a property observer on the contactsNearby Dictionary. So the declaration of the dictionary at the top of the view controller looks like this:
var contactsNearby: [String: CLLocation]? {
didSet {
if (contactsNearby?.isEmpty)! || contactsNearby == nil {
return
}else{
DispatchQueue.main.sync {
self.nearbyTableView.reloadData()
}
}
}
}
I suppose I didn't really need to check for empty and nil. So then in cellForRowAtIndexPath I have something kind of like so:
let cell = tableview.dequeueReusableCell(withIdentifier: "nearbyCell", for: indexPath)
if contactsNearby?.isEmpty == false {
let contact = contactsNearby?.popFirst()
cell.textLabel?.text = contact?.key
}else {
cell.textLabel?.text = "Some Placeholder Text Here"
}
return cell
If anyone sees an error in my thinking or sees any of this heading for disaster, feel free to let me know. I still have a lot of testing to do, but I wanted to get back here and let you know what I have found.
I use SQLite for my persistent store.
I use a dictionary based off the primary key for an in-memory store: var localContext = [String: GrandLite]().
I use the function below to retrieve an object from the dictionary, or from the database and then store in the dictionary. This function is called frequently, and I'm trying to optimize it.
class func retrieveByKey<T: GrandLite>(aType: [T], thisKey: String) -> T? {
let thisStack = FoodysLiteStack.thisDataStack
if let thisObject = thisStack.localContext[thisKey] as? T {
return thisObject
} else {
do {
let db = thisStack.localDatabase
let thisTable = T.getTable()
if let thisRow = try db.pluck(thisTable.filter(uuidKeyLite == thisKey)) {
let thisObject = T(withRow: thisRow)
thisStack.localContext[thisKey] = thisObject
return thisObject
} else {
return nil
}
} catch {
NSLog("WARNING: Unhandled error for T retrieveByKey")
return nil
}
}
}
As I understand sqlite.swift pluck is basically a prepare with limit 1. Also, prepare compiles the SQL statement, binds the variable, and executes it. I'm trying to avoid the SQLite compile time every time this function is called.
In the sqlite.swift framework is there a way to prepare the statement once ahead of time and then bind the variable just before execution? You can do this for inserts and updates with db.prepare and db.run, but I don't see a way to bind a variable for an already prepared select statement. I may be overthinking this, the SQLite compile time on thisTable.filter(uuidKeyLite == thisKey) is probably quite small.
That framework tries its very best to abstract such details away.
The only way to keep a prepared statement around is by using the functions to execute raw SQL.
And yes, in SQLite, the prepare overhead usually is quite small; the most typical performance problem comes from not combining multiple statements into a single transaction.
I may be overthinking this, the SQLite compile time on thisTable.filter(uuidKeyLite == thisKey) is probably quite small.
I think you do. Especially that with SQLite.swift, the statement compilation time is currently negligible compared to its internal mechanisms. See Comparing the Performances of Swift SQLite libraries
I have a NSObject Subclass. Say CityWalks
class CityWalks{
var totalCount:Int?
}
How do I use this property further? Should I check the nil coalescing every time this value is accessed.
example:
let aObject =
say in one fucntion (function1()) , I need to access this value, then it would like (aObject!.totalCount ?? 0)
func function1(){
...Some Access code for the object....
(aObject!.totalCount ?? 0)
}
Similarly in every other function(function2()) , I will have to write the same code.
func function2(){
...Some Access code for the object....
(aObject!.totalCount ?? 0)
}
So, what could be a better approach for such field, considering this property might receive a value from server or might not.
If you have a default value for this property just assign this value as default value.
class YourClass {
var totalCount = 0
}
I'd recommend you avoid using an optional value if it's possible. Because optional values its a first place when you can get an error.
As stated in the comments and the other answer using an optional is not really optimal in your case. It seems like you might as well use a default value of 0.
However, to clarify, you have to check the value when unwrapping the optional.
Sometimes it's possible to pass an optional to UIElement etc and then you don't really need to do anything with them
There are pretty ways of checking for nil in optional values built into swift so you can build pretty neat code even though you work with optional.
Look in to guard let and if let if you want to know more about unwrapping values safely.
if let
if let totalWalks = aObject?.totalCount {
//operate on totalWalks
}
guard
guard let totalWalks = aObject?.totalCount else { return }
//operate on totalWalks
There are also cases where you will want to call a function on an optional value and in this case you can do so with ?
aObject?.doSomething()
Any return values this function might have will now be wrapped in an optional and you might have to unwrap them as well with an if let or guard
When working with optionals you should try to avoid forcing the unwrap with ! as even though you at the moment know that the value is not null that might after a change in the code not be true anymore.