I've enabled the dart 2.8 experimental null saftey.
I have the following exiting code.
StreamSubscription<String> subscription;
subscription =
response.transform(Utf8Decoder()).transform(LineSplitter()).listen(
(line) async {
result += line;
},
onDone: () async {
unawaited(subscription.cancel());
completer.complete(result);
},
);
With null saftey enabled I get a error in the 'onDone' method where it calls subscription.cancl
"The expression is nullable and must be null-checked before it can be used.
Try checking that the value isn't null before using it.",
I can fix the problem by putting a conditional before the call to cancel, but this seems unnecessary as in reality subscription can never be null.
Is there a coding pattern that allows subscription to be declared as non-null?
The problem here is that the read of subscription happens at a place where it's still potentially unassigned. It isn't, actually, but we only know that because the listen method promises not to call any of the callbacks before returning. The compiler can't see that. So, you need to move the reading to after the assignment.
What I'd do to make this listen call work:
var buffer = StringBuffer(result);
var subscription = response
.transform(Utf8Decoder())
.transform(LineSplitter())
.listen((line) {
buffer.write(line);
});
subscription.onDone(() {
completer.complete(buffer.toString());
});
I removed the async from the callbacks because it is not needed. All it does to make these functions async is to return a future that no-one would ever look at.
In general, the callbacks on Stream and Future should have non-async callbacks.
I also removed the subscription.cancel from the onDone event handler. If you get a "done" event, the subscription is done, there is no need to cancel it.
I also added a string buffer to avoid the quadratic time and space complexity of repeated string concatenation.
Looking at the code, you seem to be concatenating lines right after splitting them, maybe all you need is:
response.transform(Utf8Decoder()).join("").then(completer.complete);
I'll assume for now that the splitting+joining is necessary.
In that case, what I'd actually prefer to do instead is of using listen is:
var buffer = StringBuffer();
response
.transform(Utf8Decoder())
.transform(LineSplitter())
.forEach((line) {
buffer.write(line);
}).then(() {
completer.complete(buffer.toString());
}, onError: (e, s) {
completer.completeError(e, s);
});
or, if in an async function:
try {
var buffer = StringBuffer();
await for (var line in response.transform(Utf8Decoder()).transform(LineSplitter())) {
buffer.write(line);
}
completer.complete(buffer.toString());
} catch(e, s) {
completer.completeError(e, s);
}
Related
Say I have this class:
void main() async {
final example = ExampleClass();
await example.waitOne();
await example.waitOne();
print('finished');
}
class ExampleClass {
Future<void> waitOne() async {
await Future.delayed(Duration(seconds: 1));
print('1 second');
}
}
This code works exactly as I expect it to. It's output is as follows:
1 second
1 second
finished
Then we have this code:
void main() async {
final example = ExampleClass();
await example
..waitOne()
..waitOne();
print('finished');
}
This code now has cascading operators (..) and the output seems strange:
finished
1 second
1 second
The code skips the two futures and prints "finished" to the console first, then "1 second" gets printed twice at the same time (like Future#wait would do).
Why does Dart act in this way?
In your example with the cascading operator adding await doesn't do anything since the cascade operation doesn't return anything hence there is no future to be awaited and then finished is printed right away
Remember that the result of the cascade operator is the original object that you used it on. That is, for var result = object..x()..y()..z(), result will be assigned the value of object, regardless of what x, y, or z return. The values returned by x(), y(), and z() are ignored. It's the equivalent of:
object.x();
object.y();
object.z();
var result = object;
Your case, which involves Futures, is no different:
final example = ExampleClass();
await example
..waitOne()
..waitOne();
So you're doing the equivalent of:
final example = ExampleClass();
example.waitOne(); // The returned Future is ignored.
example.waitOne(); // The returned Future is ignored.
await example; // Incorrectly using await on a non-Future.
(Note that enabling the unawaited_futures and await_only_futures lints would catch this mistake.)
To properly wait, you can't use the cascade operator and will need to explicitly await the individual operations. Also see the issue Prefix await is cumbersome to work with. which discusses possible changes to the language to support using await with member or cascade operators.
I'm making a Future method that lives inside a seperate class, that fetches a bunch XKCD comics, and puts them in a List and returns it.
And that is all fine and dandy, but I would like to notify back when a single comic has been fetched, so I can show a progress dialog, on how far we are.
This is my code:
// This is inside my class ComicManager
Future<List<ComicModel>> generateComicList() async {
List<ComicModel> comicList = new List<ComicModel>();
ComicModel latestComic = await getLatestComic();
for (var i = 1; i <= latestComic.num; i++) {
try {
http.Response response =
await http.get('https://xkcd.com/${i}/info.0.json');
Map comicmap = json.decode(response.body);
var comic = new ComicModel.fromJson(comicmap);
comicList.add(comic);
print(comic.num);
// Notify here that we have fetched a comic
} catch (ex) {
// Comic could apparently not be parsed, skip it.
}
}
return comicList;
}
How should I solve this?
There seems no particularly elegant way to do this. From some flutter code samples, it seems using VoidCallBack listeners is an accepted way.
First register callback functions in a Set
Set<VoidCallBack> listeners
Then define the callback functions you needed. And add them to the set
void fun()
//...
listeners.add(fun);//Or you can define a method to do this or simply pass the function through the constructor of this class.
Finally, write a notifyListeners function or its equivalent and call it wherever you want
void notifyListeners(){
for(final listener in listeners){
listener();
}
}
If you want callback functions to carry an argument, just change the VoidCallBack to whatever function types.
Found a solution.
I just used Streams like so:
Stream<ComicProgressModel> getAllComicsStream() async* {
// Do what you need to do here
// This will respond back when you are listening to the stream
yield stuffToYield; // Can be anything, and you can yield as many times you want
// When you reach the end of the method, the onDone method will be called.
// So if you are running a for loop, and call yield multiple times it onDone is only called the the this method ends
}
Then I can just listen to events like so:
Stream comicStream =
ComicManager().getAllComicsStream().asBroadcastStream();
StreamSubscription comicsub = comicStream.listen((onData) {
// Do what i need
});
Super easy to be honest.
I suspect that my understanding of Streams in Dart might have a few holes in it...
I have a situation in which I'd like a Dart app to respond to intermittent input (which immediately suggests the use of Streamss -- or Futures, maybe). I can implement the behavior I want with listener functions but I was wondering how to do this in a better, more Dartesque way.
As a simple example, the following (working) program listens to keyboard input from the user and adds a div element to the document containing what has been typed since the previous space, whenever the space bar is hit.
import 'dart:html';
main() {
listenForSpaces(showInput);
}
void listenForSpaces(void Function(String) listener) {
var input = List<String>();
document.onKeyDown.listen((keyboardEvent) {
var key = keyboardEvent.key;
if (key == " ") {
listener(input.join());
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
});
}
void showInput(String message) {
document.body.children.add(DivElement()..text = message);
}
What I'd like to be able to do is to create a new Stream from the Stream that I'm listening to (in the example above, to create a new Stream from onKeyDown). In other words, I might set the program above out as:
var myStream = ...
myStream.listen(showInput);
I suspect that there is a way to create a Stream and then, at different times and places, insert elements to it or call for it to emit a value: it feels as though I am missing something simple. In any case, any help or direction to documentation would be appreciated.
Creating a new stream from an existing stream is fairly easy with an async* function.
For a normal stream, I would just do:
Stream<String> listenForSpaces() async* {
var input = <String>[];
await for (var keyboardEvent in document.onKeyDown) {
var key = keyboardEvent.key;
if (key == " ") {
yield input.join();
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
}
}
The async* function will propagate pauses through to the underlying stream, and it may potentially pause the source during the yield.
That may or may not be what you want, since pausing a DOM event stream can cause you to miss events. For a DOM stream, I'd probably prefer to go with the StreamController based solution above.
There are several methods and there is a whole package rxdart to allow all kinds of things.
Only the final consumer should use listen and only if you need to explicitly want to unsubscribe, otherwise use forEach
If you want to manipulate events like in your example, use map.
I wasn't originally planning to answer my own question but I have since found a very simple answer to this question in the dartlang creating streams article; in case it's helpful to others:
Specifically, if we'd like to create a stream that we can insert elements into at arbitrary times and places in the code, we can do so via the StreamController class. Instances of this class have an add method; we can simply use the instance's stream property as our stream.
As an example, the code in my question could be rewritten as:
import 'dart:html';
import 'dart:async';
main() async {
// The desired implementation stated in the question:
var myStream = listenForSpaces();
myStream.listen(showInput);
}
Stream<String> listenForSpaces() {
// Use the StreamController class.
var controller = StreamController<String>();
var input = List<String>();
document.onKeyDown.listen((keyboardEvent) {
var key = keyboardEvent.key;
if (key == " ") {
// Add items to the controller's stream.
controller.add(input.join());
input.clear();
} else {
input.add(key.length > 1 ? "[$key]" : key);
}
});
// Listen to the controller's stream.
return controller.stream;
}
void showInput(String message) {
document.body.children.add(DivElement()..text = message);
}
(As mentioned in the article, we need to be careful if we want to set up a stream from scratch like this because there is nothing to stop us from inserting items to streams that don't have associated, active subscribers; inserted items would in that case be buffered, which could result in a memory leak.)
I was writing a function in dart that would delete an object from a browser-side Indexed DB, when I discovered that I had to return an outer function value from within an inner function:
Future<bool> delete() {
Transaction tx = db.transactionStore(storeName, "readwrite");
ObjectStore os = tx.objectStore(storeName);
os.delete(_key); // returns blank future, modifies tx
// This is not correct, but shows the idea:
if (tx.onComplete) {return true;}
if (tx.onError) {return false;}
}
This function is a method for a class that I am using to save and load to the Indexed DB.
I want this function to return true or false, or a Future object containing the same, when the delete operation succeeds or fails. However, the bottleneck is the os.delete(_key); statement: it returns a future, but the actual success or failure of the delete operation is provided by tx.onComplete and tx.onError. Both of these Objects are streams, so I need to create anonymous functions that handle events from them:
tx.onComplete.listen((e){
return_to_outer_function(true);
});
tx.onError.listen((e){
return_to_outer_function(false);
});
return_to_outer_function(bool) {
return bool; // doesn't work
}
As you can see, when I create anonymous functions, the return statement no longer completes the method, but the inner function. I could have the inner functions call other functions, but then those other functions have return statements of their own that don't return a result to the whole method.
I tried the approach of setting temporary variables and periodically checking them, but it's a very inelegant solution that I don't want to have to use, not just for potential bugs, but because it would hog up the single threaded event loop.
Is it possible to return a value to an outer function from an inner function? Or is there some other, better way to get a value from the presence or absence of events from a set of streams? Or is there another way of using IndexedDB that will avoid this problem?
You can use a Completer for this.
Future<bool> delete() {
Completer completer = new Completer();
Transaction tx = db.transactionStore(storeName, "readwrite");
ObjectStore os = tx.objectStore(storeName);
tx.onError.first((e){
//return_to_outer_function(false);
completer.complete(false);
});
tx.onComplete.first(bool) {
//return bool; // doesn't work
completer.complete(true)
}
os.delete(_key); // register listeners and then do delete to be on the save side
return completer.future;
}
you then call it like
delete().then((success) => print('succeeded: $success'));
see also https://api.dartlang.org/apidocs/channels/be/dartdoc-viewer/dart:async.Completer
I defined some class to query a database.
class SqlGetData {
ConnectionPool pool;
List<String> rows;
SqlGetData(this.pool);
Future <List<String>> run(String sQuery) {
rows = new List<String>();
return readData(sQuery).then((_) {
return rows;
});
}
Future readData(String sQuery) {
return pool.query(sQuery).then((result) {
return result.forEach((row) {
String s = JSON.encode(row);
rows.add(s);
});
});
}
}
which I call like this:
var sql = new SqlGetData(pool);
sql.run('select firstName, lastName from person where id =' + s1).then((rows) {
some code here to process the data
});
If the database is not running I get an error on the return pool.query in readData, which I want to catch and pass to the client in some error message.
How and where can I code the try ... catch ... to prevent the server from dying? My problem is that I have to return futures, which is still difficult for me to grasp.
Take a look at this article Futures and Error Handling (if you haven't already).
There are two places:
.then((_) => doSomething(),
onError: (e) => doErrorHandling()).catchError((e) => doErrorHandling());
Guenter's answer is good. Here are a couple of extra tips.
It's more common to use .catchError() than the named parameter, if in doubt just use .catchError().
One problem with async code is if you forget to add a catchError handler anywhere in your codebase, and an error is triggered, it will bring your whole server down. Not good. However You can use Zones to handle uncaught errors in your code, and prevent this from happening.
There isn't much documentation about Zones at the time of writing, as it is a new feature. Florian Loitsch is working on an article which will appear here sometime soon. Here is an example of using runZoned():
runZoned(() {
var pool = new Pool.connect(...); // Don't know pool API, just making this up.
pool.query(sql).then((result) {
print(result);
});
// Oops developer forgot to add a .catchError() handler for the query.
// .catchError((e) => print('Query error: $e);
}, onError: (e) => print("Uncaught error: $e"));
This code will run without bringing down your server, despite the missing catchError() handler. Note, you will need to start the pool/connection within the same zone as the query is executed within.