I've found this function in an assemblyscript project for a NEAR contract:
export function assert_single_promise_success(): void {
const x = ContractPromise.getResults()
assert(x.length == 1, "Expected exactly one promise result")
assert(x[0].succeeded, "Expected PromiseStatus to be successful")
}
What does ContractPromise.getResults() do exactly? How should implement the same thing in rust?
here is something similar in Rust from one of the examples in the SDK repo
require!(env::promise_results_count() == 2);
let data0: Vec<u8> = match env::promise_result(0) {
PromiseResult::Successful(x) => x,
_ => env::panic_str("Promise with index 0 failed"),
};
I'm going to give an answer, comments taken directly from the implementation of ContractPromise.getResults(), which can be found here. The implementation also has an example on how to use the function, which may be useful.
Method to receive async (one or multiple) results from the remote
contract in the callback.
#returns An array of results based on the number of promises the
callback was created on. If the callback using then was scheduled
only on one result, then one result will be returned.
Related
Is using a variable from outside an observable within an operator considered a (significantly) bad practice?
createObservableExample1(parameter1: string, obs$: Observable<string>): Observable<string> {
return obs$.pipe(
map( x => {
const returnValue = `${parameter1}, ${x}`;
return returnValue;
})
);
}
I understand you can do something like this:
createObservableExample2(parameter1: string, obs$: Observable<string>): Observable<string> {
return combineLatest([
of(parameter1),
obs$
]).pipe(
map( (x, y) => {
const returnValue = `${x}, ${y}`;
return returnValue;
})
);
}
But is it worth it?
Does this just come down to accessing variables from outside the scope of anonymous function? Would this force the context of the enclosing method to exist for longer than it should? I remember a code tool I used to use for C# complaining about something similar to this. I have found somewhat related topics by searching for, "anonymous functions and closures", but as of yet, nothing really discussing the scenario explained above.
I ask because I have been creating some relatively complex observables that have enormous operator chains, and constantly adding the needed variables, using combineLatest and of, from the parent scope can make the code even harder to follow.
When I teach Reactive programming to neophytes, I try to make them grasp : Do not break the reactivity by having uneccessary side effects :
no input that from a state (for example using a class or instance property
no storing outside value.
There is none of these red flags in your example. Your function is pure & idempotent with both implementation, go with what ever you like and if possible be consistant within your code base !
Trying to learn RxJs, and I found what looks like a nice tutorial on that topic at https://www.learnrxjs.io.
I'm going through their primer section, and I am not clear on what the return statement in the pipe() function actually does or what it means. These are a couple of screen shots from their tutorial:
In traditional programming, I've always understood a return statement to be an exit - if function A calls function B, and function B has the line return 1, then control goes back to function A.
Is that what happens here? If so, in either of these two examples, where am I returning to???
Or what if I don't want to return anywhere but act on the data immediately? For example, in the error handling example, instead of return makeRequest..., I want to do something like const result = makeRequest.... Can I do that?
In general I'm having some conceptual difficulties around all the returns I've seen used with observables, and any help in explaining what they do/are would be appreciated. So would any other tutorial sites on RxJs.
These are all very similar constructs in javascript
function adder0(a,b){
return a + b
}
const adder1 = (a,b) => {
return a + b
}
const adder2 = (a,b) => a + b
console.log(adder0(3,7)) // 10
console.log(adder1(3,7)) // 10
console.log(adder2(3,7)) // 10
lets re-write the code from that example but with explicit function definitions (instead of arrow syntax).
function maker(value){
return makeRequest(value).pipe(
catchError(handleError)
);
}
function actioner(value){
// take action
}
source.pipe(
mergeMap(maker)
).subscribe(actioner)
The thing to notice is that you never actually call these functions. RxJS does that for you. You give them a function and they'll call it when they're good and ready based on the spec for a given operator.
I have tried to unsubscribe within the subscribe method. It seems like it works, I haven't found an example on the internet that you can do it this way.
I know that there are many other possibilities to unsubscribe the method or to limit it with pipes. Please do not suggest any other solution, but answer why you shouldn't do that or is it a possible way ?
example:
let localSubscription = someObservable.subscribe(result => {
this.result = result;
if (localSubscription && someStatement) {
localSubscription.unsubscribe();
}
});
The problem
Sometimes the pattern you used above will work and sometimes it won't. Here are two examples, you can try to run them yourself. One will throw an error and the other will not.
const subscription = of(1,2,3,4,5).pipe(
tap(console.log)
).subscribe(v => {
if(v === 4) subscription.unsubscribe();
});
The output:
1
2
3
4
Error: Cannot access 'subscription' before initialization
Something similar:
const subscription = of(1,2,3,4,5).pipe(
tap(console.log),
delay(0)
).subscribe(v => {
if (v === 4) subscription.unsubscribe();
});
The output:
1
2
3
4
This time you don't get an error, but you also unsubscribed before the 5 was emitted from the source observable of(1,2,3,4,5)
Hidden Constraints
If you're familiar with Schedulers in RxJS, you might immediately be able to spot the extra hidden information that allows one example to work while the other doesn't.
delay (Even a delay of 0 milliseconds) returns an Observable that uses an asynchronous scheduler. This means, in effect, that the current block of code will finish execution before the delayed observable has a chance to emit.
This guarantees that in a single-threaded environment (like the Javascript runtime found in browsers currently) your subscription has been initialized.
The Solutions
1. Keep a fragile codebase
One possible solution is to just ignore common wisdom and continue to use this pattern for unsubscribing. To do so, you and anyone on your team that might use your code for reference or might someday need to maintain your code must take on the extra cognitive load of remembering which observable use the correct scheduler.
Changing how an observable transforms data in one part of your application may cause unexpected errors in every part of the application that relies on this data being supplied by an asynchronous scheduler.
For example: code that runs fine when querying a server may break when synchronously returned a cashed result. What seems like an optimization, now wreaks havoc in your codebase. When this sort of error appears, the source can be rather difficult to track down.
Finally, if ever browsers (or you're running code in Node.js) start to support multi-threaded environments, your code will either have to make do without that enhancement or be re-written.
2. Making "unsubscribe inside subscription callback" a safe pattern
Idiomatic RxJS code tries to be schedular agnostic wherever possible.
Here is how you might use the pattern above without worrying about which scheduler an observable is using. This is effectively scheduler agnostic, though it likely complicates a rather simple task much more than it needs to.
const stream = publish()(of(1,2,3,4,5));
const subscription = stream.pipe(
tap(console.log)
).subscribe(x => {
if(x === 4) subscription.unsubscribe();
});
stream.connect();
This lets you use a "unsubscribe inside a subscription" pattern safely. This will always work regardless of the scheduler and would continue to work if (for example) you put your code in a multi-threaded environment (The delay example above may break, but this will not).
3. RxJS Operators
The best solutions will be those that use operators that handle subscription/unsubscription on your behalf. They require no extra cognitive load in the best circumstances and manage to contain/manage errors relatively well (less spooky action at a distance) in the more exotic circumstances.
Most higher-order operators do this (concat, merge, concatMap, switchMap, mergeMap, ect). Other operators like take, takeUntil, takeWhile, ect let you use a more declarative style to manage subscriptions.
Where possible, these are preferable as they're all less likely to cause strange errors or confusion within a team that is using them.
The examples above re-written:
of(1,2,3,4,5).pipe(
tap(console.log)
first(v => v === 4)
).subscribe();
It's working method, but RxJS mainly recommend use async pipe in Angular. That's the perfect solution. In your example you assign result to the object property and that's not a good practice.
If you use your variable in the template, then just use async pipe. If you don't, just make it observable in that way:
private readonly result$ = someObservable.pipe(/...get exactly what you need here.../)
And then you can use your result$ in cases when you need it: in other observable or template.
Also you can use pipe(take(1)) or pipe(first()) for unsubscribing. There are also some other pipe methods allowing you unsubscribe without additional code.
There are various ways of unsubscribing data:
Method 1: Unsubscribe after subscription; (Not preferred)
let localSubscription = someObservable.subscribe(result => {
this.result = result;
}).unsubscribe();
---------------------
Method 2: If you want only first one or 2 values, use take operator or first operator
a) let localSubscription =
someObservable.pipe(take(1)).subscribe(result => {
this.result = result;
});
b) let localSubscription =
someObservable.pipe(first()).subscribe(result => {
this.result = result;
});
---------------------
Method 3: Use Subscription and unsubscribe in your ngOnDestroy();
let localSubscription =
someObservable.subscribe(result => {
this.result = result;
});
ngOnDestroy() { this.localSubscription.unsubscribe() }
----------------------
Method 4: Use Subject and takeUntil Operator and destroy in ngOnDestroy
let destroySubject: Subject<any> = new Subject();
let localSubscription =
someObservable.pipe(takeUntil(this.destroySubject)).subscribe(result => {
this.result = result;
});
ngOnDestroy() {
this.destroySubject.next();
this.destroySubject.complete();
}
I would personally prefer method 4, because you can use the same destroy subject for multiple subscriptions if you have in a single page.
I am trying to understand the logic of promises here. But I can't wrap my head around the code.
Can someone help me understand?
What does (r)=> fn = r, means in this code.
function promise(a,b) {
return new Promise((resolve,reject) => {
if(a%2 !== 0) {
reject('ODD ');
return;
}
resolve('even');
});
}
let list_of_promise = [ promise(1),promise(2)
,promise(2),promise(2),promise(2),promise(2)
,promise(2),promise(2),promise(2),promise(2)
,promise(2),promise(2),promise(2),promise(2),promise(2)
,promise(2),promise(2),promise(2),promise(2)
,promise(2)];
function listOfPromise(list_of_promise) {
let arr = [];
let fn;
let count = 0;
list_of_promise.map( (p) => {
p.then((res)=>{
arr.push(res);
arr.length === 10 && fn(arr);
});
})
return new Promise((r,rj) => fn= r);
}
listOfPromise(list_of_promise).then((res)=>{
console.log("result set ", rest);
});
This is horrible code.
fn = r is just assigning the resolve() function from the new Promise() executor to a higher scope so it can be called from outside the executor, causing the promise to resolve. This is a very obtuse way to write the code. A design pattern for resolving a promise from the outside is typically referred to as a Deferred. 99.999% of the time, there is no need for the Deferred pattern and there's a bunch of good reasons why it was not built into the promise architecture. If you want to see a simple Deferred object, you can see here and here.
To rewrite this in a less obtuse way, I would need to understand what the objective of the code is (in terms of a real-world problem) so I could suggest the best way to solve the actual problem. Right now, it looks like demo code trying to demonstrate something, not trying to solve a real world problem. I prefer to focus coding on real world problems rather than theoretical discussions as the real world problem provides priorities for the actual coding strategy.
I am wondering if there's a way to create a promise chain that I can build based on a series of if statements and somehow trigger it at the end. For example:
// Get response from some call
callback = (response) {
var chain = Q(response.userData)
if (!response.connected) {
chain = chain.then(connectUser)
}
if (!response.exists) {
chain = chain.then(addUser)
}
// etc...
// Finally somehow trigger the chain
chain.trigger().then(successCallback, failCallback)
}
A promise represents an operation that has already started. You can't trigger() a promise chain, since the promise chain is already running.
While you can get around this by creating a deferred and then queuing around it and eventually resolving it later - this is not optimal. If you drop the .trigger from the last line though, I suspect your task will work as expected - the only difference is that it will queue the operations and start them rather than wait:
var q = Q();
if(false){
q = q.then(function(el){ return Q.delay(1000,"Hello");
} else {
q = q.then(function(el){ return Q.delay(1000,"Hi");
}
q.then(function(res){
console.log(res); // logs "Hi"
});
The key points here are:
A promise represents an already started operation.
You can append .then handlers to a promise even after it resolved and it will still execute predictably.
Good luck, and happy coding
As Benjamin says ...
... but you might also like to consider something slightly different. Try turning the code inside-out; build the then chain unconditionally and perform the tests inside the .then() callbacks.
function foo(response) {
return = Q().then(function() {
return (response.connected) ? null : connectUser(response.userData);
}).then(function() {
return (response.exists) ? null : addUser(response.userData);//assuming addUser() accepts response.userData
});
}
I think you will get away with returning nulls - if null doesn't work, then try Q() (in two places).
If my assumption about what is passed to addUser() is correct, then you don't need to worry about passing data down the chain - response remains available in the closure formed by the outer function. If this assumption is incorrect, then no worries - simply arrange for connectUser to return whatever is necessary and pick it up in the second .then.
I would regard this approach to be more elegant than conditional chain building, even though it is less efficient. That said, you are unlikely ever to notice the difference.