What does = do here?
List<Segment> totalSegments = flight.departureFlight.segments;
Do both, totalSegments and flight.departureFlight.segments point to the same memory reference or totalSegments has the same data as flight.departureFlight.segments but points to a different memory location?
My understanding was that the latter should happen since dart is pass by value and not reference. However, a very annoying bug occurred when I added this line below that one:
totalSegments.addAll(flight.returnFlight.segments);
This above line actually modified the flight variable which in turn somehow modified the AsyncSnapshot from the StreamBuilder. Although, I wasn't using the variable anywhere else and not modifying other variables mentioned.
This all happened inside build function of a Stateless Widget. Maybe that has to do something with it.
I tried reading dart documentation for it, but either I couldn't find what I am looking for or the information is simply missing there. Read this too, but according to this, my use case shouldn't happen.
When it comes to objects as in your case, you are assigning a reference to an existing object (memory location) to a new variable. While acting upon that reference, you change the same object.
If this is not what you intend, check out answers related to (deep) copying of the objects
You were mistaken by the fact that Dart passes by value, and not by reference. Actually, it is exactly the opposite: (Almost) everything is always passed by reference (Which is usually a good thing!) Therefore, it is quite logical that because you edited totalSegments your departureflight.segments got edited too. It is a synonym. One of the ways to solve your problem would be:
List<Segment> totalSegments = List();
totalSegments.addAll(flight.departureFlight.segments.toList());
List<Segment> totalSegments = flight.departureFlight.segments;
This expression does the following.
Assigns the value of the expression flight.departureFlight.segments to variable totalSegments.
This and only this and nothing more.
There is no need to know what is really happening, because this is what happens.
P.S.
What value will be obtained as a result of executing the expression flight.departureFlight.segments is another question, because it depends on the implementation of the members of the operands of the expression flight.departureFlight.segments.
Related
I'm thinking of creating a VSCode extension to help people use a library I am creating. I've looked in 50 places and can only see documentation on getting the plain text of the document in which the completion suggestions are triggered.
My library exposes a function with two parameters, both objects with the same keys, but the first one will be defined already and the 2nd one will be passed in as an object literal. For example
const obj1 = {a:1, b:2};
doLibraryThing(obj1, {a:[], b:[]});
I want to provide code completion, or a snippet, or anything that can detect that doLibraryThing is being called, and know what properties were defined in obj1, even though usually it will be imported from another file; and then use those properties to generate the suggestion with the same properties, {a:[], b:[]}.
Is this possible?
I'm using GWT 2.9 with elemental2-1.0.0-RC1.
The following code throws a ClassCastException at runtime:
DocumentRange documentRange = Js.cast(DomGlobal.document); // Fails
Range range = documentRange.createRange(); // Never reaches here
When I change to use an Js.uncheckedCast() instead, it succeeds:
DocumentRange documentRange = Js.uncheckedCast(DomGlobal.document);
Range range = documentRange.createRange(); // Works
The documentation for Js.uncheckedCast() says:
"You should always prefer regular casting over this (unless you know what you are doing!)."
I don't know why I'm having to use it, so I'm feeling nervous. Can someone explain how Js.cast() performs its type-checking and why I need to use an Js.uncheckedCast() in this instance?
Js.cast() is a way to cheat a bit, and do something that the Java language will not permit, but might actually be legal. Ignoring "how it actually works", the idea is that you can now get past issues where Java would complain, even if it turns out to be legit.
An example could be where you take a java.lang.Double or double and want to treat it as a JsNumber so you can call toPrecision(2) on it. Since java.lang.Double is final, it isn't legal to cast to an unrelated type, but Java doesn't know that in GWT, Double is really just a js Number. So, instead you can perform the cast with Js.cast(). The compiler will insert a runtime type check in there, verifying at runtime that your number is in fact a JS Number instance.
Another example could be trying to extend some native type that elemental2 provides, either to implement a workaround for a missing feature, or to do something browser-specific. Your new class may not extend the existing class - from JS's perspective this is okay, you are just describing the API that you know will exist at runtime. As such, we need to avoid the Java language check of "does this cast even make sense?", and just tell the compiler to try it.
On the other hand, you can "lie" to the compiler with Js.uncheckedCast(). This is used in cases where you are even asking the runtime to skip the check, and just pretend that it will work. This can let you do weird things, like treating Strings as if they were arrays, or solve cross-frame problems. No runtime check will be emitted, so instead you might just get a TypeError if a method/property is missing, instead of a proper ClassCastException.
In elemental2-dom 1.0.0-RC1, there is a class called DocumentRange, but it doesnt really make any sense - it is declared as a class, which means it can be type checked in JS, but the browser spec says that it should be an "interface" (which in JS-land means that it just is a description of a type, rather than something you can typecheck). https://www.w3.org/TR/DOM-Level-2-Traversal-Range/ranges.html#Level2-DocumentRange-method-createRange
This bug is inherited from closure-compiler, which claims that this has a constructor: https://github.com/google/closure-compiler/blob/6a418aa/externs/browser/w3c_range.js#L241-L251
The fix is for closure-compiler to refer to this as an interface, and for a new release of elemental2 to be made so you can use this.
There are two workarounds you can make here. The first is to cheat with Js.uncheckedCast(DomGlobal.document) and say "yes, I know that the Document is not instanceof DocumentRange, but that's because there is no such class as DocumentRange, so just pretend it worked so I can call createRange() on it". This is what you are doing already - it hides the fact there is a bug, but at the end of the day it works.
The "correct" answer is to declare your own DocumentRange, and do a Js.cast() to that instead. This is still gross - you have to keep your new interface around until closure gets fixed, and then elemental2 gets released, and then you have to remember to clean it up.
In this case, I would suggest lying to GWT and using Js.uncheckedCast() - there is only a single method on here, and it is unlikely to change in a meaningful way.
I've been stuck on this for a while now. I am trying to send the following:
boost::shared_ptr<uint8_t[]> m_data
over the wire using:
_socket.async_send_to(boost::asio::buffer(m_data), m_remote_endpoint,
boost::bind(&UDPServer::handle_send, this, message,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
I get the error "no instance of overloaded function boost::asio::buffer matches the argument list boost::shared_ptr<uint8_t[]>"
m_data is filled in another function.
I suspect this is because I actually have to use the key word new on m_data. But I can't quite figure out how to do it. I've tried a few different variations. Can anybody offer me some insight here? This is probably more a question of how to dereference a shared pointer then anything. Thanks in advance!
boost::asio::buffer has an impressive lists of consttructors, but neither of them takes shared_ptr<[]> (possibly an oversight form lib authors).
In your case, you simply need to derefence shared ptr, for example, by calling get on it.
I'm currently inserting an object into Drools working memory and running rules on it (creating a new object rather than updating the old one since I'm using Scala immutables...). From what I've understood, typically you would say something like update(myobject) and that would update the original variable inserted, letting you use the updated myObject in the main scope once the rules have been fired.
Since the objects I'm using (and inserting to memory) are immutable I can't simply modify them, and I'm having to create copies of them with slight tweaks.
Is there a way to return an object created within a rule's RHS? Perhaps by calling its FactHandle? Also open to other workarounds...
Alternatively, can I create a new object (newObject) and assign it the original's (myObject) FactHandle? Would that give me the access I need?
(Once again, I'm looking for a workaround to get Scala and Drools to work together.)
As far as I understand you problem, you could create a copy of the object with the tweaks you need and then you retract the old object and insert the new one. Something similar to:
val newObject = myObject.copy(foo = "bar");
retract(myObject);
insert(newObject);
I hope, it helps
After looking around and some prolonged trial and error I realised that FactHandles didn't quite work the way I expected. I was under the impression that they were an ID assigned to an object and that updating the object would mean that it kept its FactHandle (apparently not).
I went about this by creating a new (Scala) object and running update in the RHS of the rule as follows:
update(kcontext.getKieRuntime().getFactHandle(myObject), newObject)
While this worked fine within the rule-firing process, it was tedious to retrieve the object from the Main app afterwards. Calling ksession.getFactHandle(myObject) on the original object returned null after the rules had been fired and, as it turns out, the FactHandle of this object had indeed changed when checked by printing it out from within a rule.
The workaround for this was to save the original object's FactHandle before firing the rules (i.e. val objectFH = ksession.insert(myObject)) and to call .getObject using that FactHandle after the rules were fired (even though the FactHandle saved in this variable had changed).
In all honesty, I'm still unsure about why this works since the FactHandle changes on update, however I'm pretty sure this stems from my lack of understanding on how FactHandles work. If anyone would care to elaborate in the comments I'd be more than happy to add it in the answer for future reference.
(Thanks to #laune for helping point me back to my previous questions)
I'm using this peculiar syntax for assigning an object to an instance variable during a list iteration:
execute: =>
while #canExecute()
for #currentStack in #stacks
#executeNext()
if #currentStack.isEmpty()
#stacks.remove #currentStack
break
The important line here is for #currentStack in #stacks. This assigns this.currentStack during iteration. I like this. It makes me happy.
I'm worried though that it's an issue. I don't see it specifically documented anywhere. This code is running in several different products and I haven't found an issue myself. Anyone have experience with this?
You are using a pure javascript for...in loop, it is perfectly fine you can continue being happy. Link for documentation:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in