When Supplying a PageStorageKey, does it really matter what the argument value is?
Asking another way, are two instances of PageStorageKey with the same string argument unique or equivalent?
In other languages, things like this use their storage reference to disambiguate, so the actual text linked to the object doesn't matter.
I'm trying to determine if I need to ensure that all PageStorageKeys in my application have to have unique strings as a seed, or can I just boilerplate them all to the same value?
var psKey1 = PageStorageKey<String>('Am I the same or different than my sibling?');
var psKey2 = PageStorageKey<String>('Am I the same or different than my sibling?');
It appears two PageStorageKeys with the same String initializer are considered the same.
I wrote a short example and ran it under debugging
Test 1
var psKey1 = PageStorageKey('am I a clone');
var psKey2 = PageStorageKey('am I a clone');
var equivalent = psKey1 == psKey2;
equivalent = true;
Test 2
var psKey1 = PageStorageKey('am I a clone');
var psKey2 = PageStorageKey('am I not a clone');
var equivalent = psKey1 == psKey2;
equivalent = false;
I will be proceeeding under the presumption that all PageStorageKeys need to be unique within a given scope.
Related
var result = (dimensione-base)/2
dimensione & base are two "Double?" number, how can i solve this?
i want to calculate this, but he always gives me this error:"Binary operator - cannot be applied to two "double?" operands.
Double? is an optional type. That means a variable of this type can be nil. So before making an operation with them, you need to make sure these variables actually hold a value :
if let dimensione = dimensione,
let base = base
{
var result = (dimensione-base)/2
// Use result here
}else{
// Do something here when dimensione or base is nil.
}
You could also prefer to assign them a default value when they are nil (but in this case it seems less secure) :
var result = ((dimensione ?? 0)-(base ?? 0))/2
Here, if nil, 0 will be used instead.
//First way
var myVar: String = " Hello"
print(myVar)
//Second way
var str = "Hello"
print(str)
I get the same output no matter which of the two I use. What's the difference between them?
These two are basically the same.
When you use var myVar: String = "Hello", you directly tell the swift compiler that your variable is type String.
When you use var myVar = "Hello", you do not specify the type of your variable, so the swift compiler has do do that for you.
Usually, you can get away without declaring your variable type and just have swift do it for you. However, in some cases, namely computed properties and custom classes/structures, you must manually declare your variable to be a specific type.
In your case, either way is fine. The end result is the same, just be aware of the difference for the future.
These two variable are same but with different names :
var myVar
var str
in swift Type doesn't matter that defined because, swift based on value it recognizes what type it is.
I am converting a program written in Pascal to Swift and some Pascal features do not have direct Swift equivalents such as variant records and defining sets as types. A variant record in Pascal enables you to assign different field types to the same area of memory in a record. In other words, one particular location in a record could be either of type A or of type B. This can be useful in either/or cases, where a record can have either one field or the other field, but not both. What are the Swift equivalents for a variant record and a set type like setty in the Pascal fragment?
The Pascal code fragment to be converted is:
const
strglgth = 16;
sethigh = 47;
setlow = 0;
type
setty = set of setlow..sethigh;
cstclass = (reel,pset,strg);
csp = ^constant; /* pointer to constant type */
constant = record case cclass: cstclass of
reel: (rval: packed array [1..strglgth] of char);
pset: (pval: setty);
strg: (slgth: 0..strglgth;
sval: packed array [1..strglgth] of char)
end;
var
lvp: csp
My partial Swift code is
let strglgth = 16
let sethigh = 47
let setlow = 0
enum cstclass : Int {case reel = 0, pset, strg}
var lvp: csp
Any advice is appreciated. Thanks in advance.
Variant records in Pascal are very simular to unions in C.
So this link will probably be helpful:
https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/using_imported_c_structs_and_unions_in_swift
In case the link ever goes dead, here's the relevant example:
union SchroedingersCat {
bool isAlive;
bool isDead;
};
In Swift, it’s imported like this:
struct SchroedingersCat {
var isAlive: Bool { get set }
var isDead: Bool { get set }
init(isAlive: Bool)
init(isDead: Bool)
init()
}
That would be more like a functional port. It does not seem to take care of the fact that Variant records are actually meant to use the same piece of memory in different ways, so if you'd have some low-level code that reads from a stream or you have a pointer to such a structure, this might not help you.
In that case you might want to try just reserving some bytes, and write different getters/setters to access them. That would even work if you'd have to port more complex structures, like nested variant types.
But overall, if possible, I'd recommend to avoid porting such structures too literally, and use idioms that match Swift better.
I have created DB for my iOS mobile app using Realm, writing with Swift.
I am trying to find way to look up for matching username and password in DB
This is what I have currently, Attempting filtering and get an object with matching username
I am trying to address attribute/key called password from retrieved object
#IBAction func SignInCheck(sender: AnyObject) {
let realm = try! Realm()
var currentlogin = realm.objects(UserRecords).filter("name = LogUsernameTextField.text")
//this line causes SIGABRT to be thrown
if currentlogin.password == LogPasswordTextField.text { //This is the incorrectly addressed line
....
}
}
The issue maybe that I am not understanding how objects work in the right way, nor knowing the right syntax to address what I want.
I suspect it is in form of Result but I am still unable to find way to address desired information.
Here is the table structure for your information
class UserRecords: Object {
dynamic var username: String = ""
dynamic var password: String = ""
dynamic var latitude: Float = 0
dynamic var longtitude: Float = 0
}
I am open for suggestions better solution and ways of looking up/matching passwords in the table too, if any.
Thanks in advance
You are using a property called name in your filter string but in your UserRecords class the property is called username. Also you have to create the filter string differently:
var currentlogin = realm.objects(UserRecords).filter("username = '\(LogUsernameTextField.text!)'")
Also be aware that the filter method returns a list of UserRecord objects that match the filter and not a single object. So calling if currentlogin.password == ... will cause an error.
The list only has 1 item (because the username is unique) but it is still a list. So to access the UserRecord object you can call first:
var currentlogin = realm.objects(UserRecords).filter("name = LogUsernameTextField.text!").first
Also the text property of UITextField returns an Optional, so you have to unwrap it.
I am having trouble with the syntax here. Basically I created a simple class and hoping to add the object of that class to an Array.
class simpleClass {
var aNum = Int()
var aWord = String()
init(thisNum:Int,thisString:String)
{
aNum = thisNum
aWord = thisString
}
}
var aObj:simpleClass
var aArray:Array<simpleClass>
aObj = simpleClass(thisNum:12,thisString:"Test")
aArray.append(aObj)
As you can see I have created an object of simpleClass and trying to append it to an array of type simpleClass. However, I receive an error saying
passed by reference before being initialized
I guess I must be missing something in the syntax. Hoping someone out there could point out my mistake.
thanks,
sweekim
You need to assign an array to the array variable.
var aArray:Array<simpleClass> = []
Or if you prefer,
var aArray = Array<simpleClass>()
Or even (my preference)
var aArray: [simpleClass] = []
Or
var aArray = [simpleClass]()
Better yet you could even reorder things and do this:
var aArray = [simpleClass(thisNum:12,thisString:"Test")]
instead of the whole 4 last lines.
Incidentally, you might find it better to declare your class like this:
class simpleClass {
var aNum: Int
var aWord: String
init(thisNum:Int,thisString:String) {
aNum = thisNum
aWord = thisString
}
}
This types aNum and aWord, but does not assign them values, since you then do that in the init method. The reason being, if you ever forgot to assign a value in init the compiler will warn you, whereas if you default them, it won’t. It’s fine to default them instead, but then don’t include them in an init method – one or the other is best, both is a bit redundant and can lead to mistakes.
Change this line:
var aArray:Array<simpleClass>
To this:
var aArray:Array<simpleClass> = []
You were declaring the array type but you forgot to make any array. If you actually look at the error message, it tells you exactly that - you didn't initialize the variable.
Also I think you didn't quite declare your instance of simpleClass correctly. I did this to silence the errors:
var aArray:Array<simpleClass> = []
let aObj:simpleClass = simpleClass(thisNum:12,thisString:"Test")
aArray.append(aObj)
Note the way an instance of simpleClass is created.