void main() {
A one = new A(1);
A two = new A(2);
var fnRef = one.getMyId; //A closure created here
var anotherFnRef = two.getMyId; //Another closure created here
}
class A{
int _id;
A(this._id);
int getMyId(){
return _id;
}
}
According to the dart language tour page referencing methods like this creates a new closure each time. Does anyone know why it does this? I can understand creating closures when defining a method body as we can use variables in an outer scope within the method body, but when just referencing a method like above, why create the closure as the method body isn't changing so it can't use any of the variables available in that scope can it? I noticed in a previous question I asked that referencing methods like this effectively binds them to the object they were referenced from. So in the above example if we call fnRef() it will behave like one.getMyId() so is the closure used just for binding the calling context? ... I'm confused :S
UPDATE
In response to Ladicek. So does that mean that:
void main(){
var fnRef = useLotsOfMemory();
//did the closure created in the return statement close on just 'aVeryLargeObj'
//or did it close on all of the 'veryLargeObjects' thus keeping them all in memory
//at this point where they aren't needed
}
useLotsOfMemory(){
//create lots of 'veryLarge' objects
return aVeryLargeObj.doStuff;
}
Ladicek is right: accessing a method as a getter will automatically bind the method.
In response to the updated question:
No. It shouldn't keep the scope alive. Binding closures are normally implemented as if you invoked a getter of the same name:
class A{
int _id;
A(this._id);
int getMyId() => _id;
// The implicit getter for getMyId. This is not valid
// code but explains how dart2js implements it. The VM has
// probably a similar mechanism.
Function get getMyId { return () => this.getMyId(); }
}
When implemented this way you will not capture any variable that is alive in your useLotsOfMemory function.
Even if it really was allocating the closure inside the useLotsOfMemory function, it wouldn't be clear if it kept lots of memory alive.
Dart does not specify how much (or how little) is captured when a closure is created. Clearly it needs to capture at least the free variables of itself. This is the minimum. The question is thus: "how much more does it capture"?
The general consensus seems to be to capture every variable that is free in some closure. All local variables that are captured by some closure are moved into a context object and every closure that is created will just store a link to that object.
Example:
foo() {
var x = new List(1000);
var y = new List(100);
var z = new List(10);
var f = () => y; // y is free here.
// The variables y and z are free in some closure.
// The returned closure will keep both alive.
// The local x will be garbage collected.
return () => z; // z is free here.
}
I have seen Scheme implementations that only captured their own free variables (splitting the context object into independent pieces), so less is possible. However in Dart this is not a requirement and I wouldn't rely on it. For safety I would always assume that all captured variables (independent of who captures them) are kept alive. I would also make the assumption that bound closures are implemented similar to what I showed above and that they keep a strict minimum of memory alive.
That's exactly right -- the closure captures the object on which the method will be invoked.
Related
Here is the function that is defined within the C code:
typedef void (*spAnimationStateListener)(spAnimationState *state, spEventType type, spTrackEntry *entry, spEvent *event);
struct spTrackEntry {
...
spAnimationStateListener listener;
...
};
And, I am trying to use it in Swift like this:
class TrackEntry {
private let ptr: UnsafeMutablePointer<spTrackEntry>!
public func addEventListener(_ eventListener: #escaping () -> ()) {
let listener: spAnimationStateListener = { (state, type, entry, event) in
eventListener() //cannot use this
}
ptr.pointee.listener = listener
}
}
I saw some suggestion like passing "self" as a parameter but I could not figure out how to do that. I cannot change the C code and it is accepting four parameters. I also tried unsafeBitCast by using convention(block) and it compiled but data were broken.
I cannot use it as a static variable because listeners are separate between different TrackEntry instances.
As mentioned, C function pointers can only be called from static or global context.
There is a good reason : in C, functions are only declared in global context - ie. in the memory dedicated to instructions storage, not in the memory dedicated to data storage. You cannot have instances of a function. A function pointer can just point to one function or another. You cannot create code on-the-fly, neither pass it by value. You can only point to one part of the instructions or another.
So, if only one listener can be used at a time, you could declare a global variable for the closure, and change it before the listener is created, pointing to this listener ; and when another listener is created replace it.
But if you have several listeners at once, well you will have to globally create a closure for each of them (if you do not have too much, and not too generic).
What is the logic here when the programmer initializes _random at once but the _streamController is initialized in the constructor?
Can all the fields be initialized without a constructor then?
RandomStore {
RandomStore() {
_streamController = StreamController<int>();
_timer = Timer.periodic(const Duration(seconds: 1),
(_) => _streamController.add(_random.nextInt(100)));
randomStream = ObservableStream(_streamController.stream);
}
late final Timer _timer;
final _random = Random();
late final StreamController<int> _streamController;
late final ObservableStream<int?> randomStream;
...
Can all the fields be initialized without a constructor ?
Yes, you can initialize all fields without having to declare a constructor, but only if you don't need a reference to the current instance (this) or if they are 'late' fields.
The determining factor in choosing where to initialize fields is whether or not you need to have the reference (even implicit) to this.
In Dart this is only available from the construcor body; this means in particular that this is not usable in the initializer list and inside the inline initializers (except for the late fields).For terminology, see Glossary below.
this is the reference to the current instance, and is required in order to read the instance fields, even if you usually omit it (e.g., in your snippet, randomStream is equivalent to this.randomStream).
For example, in your snippet, to initialize randomStream you need to be able to read the streamController field, so you have to mark it with late; thanks to late you can initialize randomStream in the constructor body or in the inline initializer (in this second case it will actually be initialized only when you try to access it for the first time; which is an advantage if its initialization is expensive and you want to avoid it as long as possible).
As an alternative to late, you could mark the field as nullable and initialize it in the constructor body (in which case the field will first be implicitly initialized with a null value; in fact this approach is not usable if the field is final and therefore cannot be reassigned).
Instead, to initialize the streamController field, you don't use a reference to this, so you could avoid the overhead of marking the field with late and you can initialize it in the initializer list or in the inline initializer (it is the same).
Example:
class RandomStore {
final StreamController<int> _streamController;
RandomStore()
: _streamController = StreamController<int>() {
}
}
Is late always a good choice? (UPDATED)
While from the above it might seem that 'late' is a great solution for most situations, the doc suggests avoiding 'late' if it is not really necessary, because:
It is less secure:
a late field (also if it has a non-nullable type) entails a risk of errors at runtime similar to that which occurred before the introduction of sound null safety, because Dart does not force you to perform any checks before reading its value (unlike nullable fields, for which access to properties requires the use of not null operator or conditional operator). Note that Dart does not offer the possibility to check if a late field has already been initialized (See Issue #324 mentioned below)
It adds overhead:
under the cover will be created a field with the indicated type, a field -presumably boolean- for keep track of whether the initialization has occurred, and a getter that at each access checks if the initialization had occurred.
Useful sources about late:
Doc Guide about null safety and late variables https://dart.dev/null-safety/understanding-null-safety#late-variables
Dart Best practice about late
https://dart.dev/guides/language/effective-dart/usage#dont-use-late-when-a-constructor-initializer-list-will-do
https://dart.dev/guides/language/effective-dart/usage#avoid-late-variables-if-you-need-to-check-whether-they-are-initialized
Dart issue #324 Should we provide a way to query the status of late variables?
a very interesting insight into 'late' (the discussion in which the Dart Team decided not to allow the final developers to check if a late field has been initialized)
Note the differences with Java:
(which personally made it difficult for me to switch from Java to Dart at first)
this in Dart is not available in inline initializers (in Java it is available)
the final fields in Dart must be initialized before the constructor body (in Java they can also be initialized in the constructor body)
Gloassary:
class MyClass {
MyClass(String v1, String v2)
: myField1 = v1, myField2 = v2 //This is the "initializer list"
//Compared to inline initializers, it allows you to initialize fields using the constructor arguments
{
//This is the "constructor body"
myField4 = myField1; //This operation require an implicit reference to `this`; it is equivalent to `myField4 = this.myField1;`
}
String myField1;
String myField2;
String myField3 = '3'; //This is the "inline initialization"
late String myField4;
}
The Error Handling chapter of the Rust Book contains an example on how to use the combinators of Option and Result. A file is read and through application of a series of combinators the contents are parsed as an i32 and returned in a Result<i32, String>.
Now, I got confused when I looked at the code. There, in one closure to an and_then a local String value is created an subsequently passed as a return value to another combinator.
Here is the code example:
use std::fs::File;
use std::io::Read;
use std::path::Path;
fn file_double<P: AsRef<Path>>(file_path: P) -> Result<i32, String> {
File::open(file_path)
.map_err(|err| err.to_string())
.and_then(|mut file| {
let mut contents = String::new(); // local value
file.read_to_string(&mut contents)
.map_err(|err| err.to_string())
.map(|_| contents) // moved without 'move'
})
.and_then(|contents| {
contents.trim().parse::<i32>()
.map_err(|err| err.to_string())
})
.map(|n| 2 * n)
}
fn main() {
match file_double("foobar") {
Ok(n) => println!("{}", n),
Err(err) => println!("Error: {}", err),
}
}
The value I am referring to is contents. It is created and later referenced in the map combinator applied to the std::io::Result<usize> return value of Read::read_to_string.
The question: I thought that not marking the closure with move would borrow any referenced value by default, which would result in the borrow checker complaining, that contents does not live long enough. However, this code compiles just fine. That means, the String contents is moved into, and subequently out of, the closure. Why is this done without the explicit move?
I thought that not marking the closure with move would borrow any referenced value by default,
Not quite. The compiler does a bit of inspection on the code within the closure body and tracks how the closed-over variables are used.
When the compiler sees that a method is called on a variable, then it looks to see what type the receiver is (self, &self, &mut self). When a variable is used as a parameter, the compiler also tracks if it is by value, reference, or mutable reference. Whatever the most restrictive requirement is will be what is used by default.
Occasionally, this analysis is not complete enough — even though the variable is only used as a reference, we intend for the closure to own the variable. This usually occurs when returning a closure or handing it off to another thread.
In this case, the variable is returned from the closure, which must mean that it is used by value. Thus the variable will be moved into the closure automatically.
Occasionally the move keyword is too big of a hammer as it moves all of the referenced variables in. Sometimes you may want to just force one variable to be moved in but not others. In that case, the best solution I know of is to make an explicit reference and move the reference in:
fn main() {
let a = 1;
let b = 2;
{
let b = &b;
needs_to_own_a(move || a_function(a, b));
}
}
Just curious why F# has:
member val Foo = ... with get, set
While omitting the self identifier (e.g. this.).
This is still an instance property. Maybe I am the only one confused when using it. But just bothered me enough to query whoever knows how the language was defined.
With this syntax, the property is almost totally auto-implemented -- all you provide is the initialization code, which essentially runs as part of the constructor.
One of the best-practice guard rails F# puts in place is that it does not let you access instance members before the instance is fully initialized. (wow, crazy idea, right?).
So you would have no use for a self-identifier in auto-props, anyways, since the only code you get to write is init code that can't touch instance members.
Per the MSDN docs (emphasis mine):
Automatically implemented properties are part of the initialization of
a type, so they must be included before any other member definitions,
just like let bindings and do bindings in a type definition. Note that
the expression that initializes an automatically implemented property
is only evaluated upon initialization, and not every time the property
is accessed. This behavior is in contrast to the behavior of an
explicitly implemented property. What this effectively means is that
the code to initialize these properties is added to the constructor of
a class.
Btw, if you try to be a smartass and use the class-level self-identifier to get around this, you'll still blow up at runtime:
type A() as this =
member val X =
this.Y + 10
with get, set
member this.Y = 42
let a = A()
System.InvalidOperationException: The initialization of an object or value resulted in an object or value being accessed recursively before it was fully initialized.
at Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions.FailInit()
at FSI_0013.A.get_Y()
at FSI_0013.A..ctor()
at <StartupCode$FSI_0014>.$FSI_0014.main#()
Edit: Worth noting that in upcoming C# 6, they also now allow auto-props with initializers (more F# features stolen for C#, shocker :-P), and there is a similar restriction that you can't use the self-identifier:
class A
{
// error CS0027: Keyword 'this' is not available in the current context
public int X { get; set; } = this.Y + 10;
public int Y = 42;
public A() { }
}
Say I have an instance of a class Foo, and I want to grab a list of all of its methods that are annotated a certain way. I want to have a reference to the method itself, so I'm not looking to use reflection to invoke the method each time, just to grab a reference to it the first time.
In other words, I want to do the reflection equivalent of this:
class Foo {
a() {print("a");}
}
void main() {
var f = new Foo();
var x = f.a; // Need reflective way of doing this
x(); // prints "a"
}
I have tried using InstanceMirror#getField, but methods are not considered fields so that didn't work. Any ideas?
As far as I understand reflection in Dart, there's no way to get the actual method as you wish to. (I'll very happily delete this answer if someone comes along and shows how to do that.)
The best I can come up with to ameliorate some of what you probably don't like about using reflection to invoke the method is this:
import 'dart:mirrors';
class Foo {
a() {print("a");}
}
void main() {
var f = new Foo();
final fMirror = reflect(f);
final aSym = new Symbol('a');
final x = () => fMirror.invoke(aSym, []);
x(); // prints "a"
}
Again, I know that's not quite what you're looking for, but I believe it's as close as you can get.
Side note: getField invokes the getter and returns the result -- it's actually fine if the getter is implemented as a method. It doesn't work for you here, but for a different reason than you thought.
What you're trying to get would be described as the "closurized" version of the method. That is, you want to get the method as a function, where the receiver is implicit in the function invocation. There isn't a way to get that from the mirror. You could get a methodMirror as
reflect(foo).type.methods[const Symbol("a")]
but you can't invoke the result.