I'm trying out Dart for the first time and I can't get the error handling to work for me. Here's some information about it.
Resources:
Gist with HTML, CSS and Dart: gist.github.com/enjikaka/8164610
ZIP with the project: ge.tt/6StW4cB1/v/0?c
JavaScript version on CodePen: codepen.io/enjikaka/pen/giurk
How I want it:
Making an instance of MinecraftSkin should throw an StateError if the image source returns a 403 error code. The exception should be handled in the generateHead() function where the instance of MineCraft skin is attempted to be made.
How it is:
If an image representing the skin of a MineCraft player does not exist (when the image source does not exist and returns 403) the code stops on line 22 (onError; where I throw the StateError) and prints to console "Breaking on exception: Bad state: User has no skin".
However, in the catch on generateHead, nothing gets executed. It doesn't print the StateError message when I prompt it to, neither does it insert the StateError message to the selected element in the DOM.
Code
import 'dart:html';
class MinecraftSkin {
String user;
CanvasElement ce = new CanvasElement();
void _generateCanvas(Event e) {
CanvasRenderingContext2D ctx = ce.getContext('2d');
ctx.imageSmoothingEnabled = false;
ctx.drawImageScaledFromSource((e.target as ImageElement),8,8,8,8,0,0,ce.width,ce.height);
}
CanvasImageSource getHead() => ce;
String name() => user;
MinecraftSkin(String minecraftUser, num size) {
user = (minecraftUser == null) ? 'Notch' : minecraftUser;
ce.width = size;
ce.height = size;
ImageElement img = new ImageElement()
..onLoad.listen(_generateCanvas)
..onError.listen((_) => throw new StateError('User has no skin'));
img.src = "http://s3.amazonaws.com/MinecraftSkins/"+user+".png";
}
}
void generateHead(Event e) {
MinecraftSkin ms;
try {
ms = new MinecraftSkin((querySelector('#userName') as InputElement).value, 128);
} on StateError catch(se) {
print(se.message);
querySelector('#status').text = se.message;
}
CanvasElement cems = ms.getHead();
cems.id = "mc" + ms.name();
cems.title = "mc" + ms.name();
document.body.append(cems);
querySelector('#status').text = "Got head!";
}
void main() {
querySelector('#generateHead').onClick.listen(generateHead);
}
Thanks in advance!
Sincerely, Jeremy
The image listeners (onLoad, onError) are asynchronous. The MincraftSkin instantiation is completed without any errors, and only after the image resource is loaded or an error is received, is the StateError thrown, probably several hundred milliseconds later. The constructor does not wait around to see if the image will properly load or not.
Related
I am failing to understand, why the error thrown from addItem method in below code is not caught in the try-catch block
void main() async {
var executor = Executor();
var stream = Stream.fromIterable([0, 1, 2, 3, 4, 5, 6, 7]);
try {
await for (var _ in stream) {
executor.submit(() => demoMethod());
}
await executor.execute();
} catch (e) {
print(e);
}
}
Future<void> demoMethod() async {
var list = [1, 2, 3, 1, 4, 5];
var executor = Executor();
var test = Test();
for (var element in list) {
executor.submit(() => test.addItem(element));
}
await executor.execute();
test.list.forEach(print);
}
class Test {
var list = <int>[];
Future<void> addItem(int i) async {
if (list.contains(i)) {
throw Exception('Item exists');
}
list.add(i);
}
}
class Executor {
final List<Future<void>> _futures = [];
bool _disposed = false;
void submit(Future<void> Function() computation) {
if (!_disposed) {
_futures.add(computation());
} else {
throw Exception('Executor is already disposed');
}
}
Future<void> execute() async {
await Future.wait(_futures, eagerError: true);
_disposed = true;
}
}
but below code is able to catch the error properly
void main() async {
var executor = Executor();
try {
for (var i = 0; i < 10; i++) {
executor.submit(() => demoMethod());
}
await executor.execute();
} catch (e) {
print(e);
}
}
I am guessing it has something to do with the stream processing.
It's the stream.
In your other examples, you synchronously run through a loop a and call Executor.submit with all the computations, then immediately call executor.execute().
There is no asychronous gap between calling the function which returns a future, and Future.wait starting to wait for that future.
In the stream code, each stream events starts an asynchronous computation by calling Executor.submit. That creates a future, stores it in a list, and goes back to waiting for the stream.
If that future completes, with an error, before the stream ends and Future.wait gets called, then there is no error handler attached to the future yet. The error is then considered unhandled, and is reported to the current Zone's uncaught error handler. Here that's the root zone, which means it's a global uncaught error, which may crash your entire program.
You need to make sure the future doesn't consider its error unhandled.
The easiest way to do that is to change submit to:
void submit(Future<void> Function() computation) {
if (!_disposed) {
_futures.add(computation()..ignore());
} else {
throw StateError('Executor is already disposed');
}
}
The ..ignore() tells the future that it's OK to not have an error handler.
You know, because the code will later come back and call executor.execute, that any errors will still be reported, so it should be safe to just postpone them a little. That's what Future.ignore is for.
(Also changed Exception to StateError, because that's what you should use to report people using objects that have been disposed or otherwise decommissioned.)
After reading a bunch of documentation about Streams and StreamControllers in dart I tried to build a little example and was surprised of the results. All documentation I have read states that a stream starts emiting data as soon as a listener is registered. But this doesn't show any printed data:
class Order
{
String type;
Order(this.type);
}
class Pizza
{
}
void main()
{
Order order = Order("pzza");
final StreamController sc = StreamController();
sc.sink.add(order);
sc.sink.add(order);
sc.sink.add(new Order("pizza"));
Stream st = sc.stream.map((order) {
return order.type;
})
.map((orderType) {
if(orderType == "pizza")
return Pizza();
else
throw ("dude!, I don't know how to do that");
});
var sus = st.listen((pizza)
{
print("We did a pizza");
},
onError: (error)
{
print(error);
});
sus.cancel();
sc.sink.add(new Order("pizza2"));
}
I was expecting this output:
dude!, I don't know how to do that
dude!, I don't know how to do that
We did a pizza
When creating streams and adding data is all "sinked" data scheduled to be emited on the next application step?
Cheers.
You are right in that the documentation states that you listen on a stream to make it start generating events. However, streams are asynchronous so when you call the listen() method you are registering to receive events from the stream at some point in the future. Dart will then continue to run the remainder of your main function. Immediately after calling listen() you call cancel() to cancel the subscription which is why nothing is being printed.
If you remove or comment out the cancel and run it again you will see the expected output.
A slightly modified version of your code will hopefully highlight the run of events:
class Order {
String type;
Order(this.type);
}
class Pizza {}
void main() {
print("Main starts");
Order order = Order("pzza");
final StreamController sc = StreamController();
sc.sink.add(order);
sc.sink.add(order);
sc.sink.add(new Order("pizza"));
Stream st = sc.stream.map((order) {
return order.type;
}).map((orderType) {
if (orderType == "pizza")
return Pizza();
else
throw ("dude!, I don't know how to do that");
});
var sus = st.listen((pizza) {
print("We did a pizza");
}, onError: (error) {
print(error);
});
// sus.cancel();
sc.sink.add(new Order("pizza2"));
print("Main ends");
}
Running this produces the output:
Main starts
Main ends
dude!, I don't know how to do that
dude!, I don't know how to do that
We did a pizza
dude!, I don't know how to do that
I've successfully used jsctypes in the past but the latest version of firefox (32) has started to give me an odd error message
here is what used to work-
javascript content sends a message to a javascript extension in chrome (the extension uses ctypes to call a special device allocator. it then returns a cdata.finalizer to the content
later when the content is garbage collected the finaizer gets called to release the special device allocation
while this used to work fine, I'm now getting an exception ctypes.CDataFinalizer
Not allowed to define cross-origin object as property on [Object] or [Array] XrayWrapper
searching on Google did not seem to find anything related.
on the extension I have this code( the func... things are access methods for c code)
Any suggestions?
self.addEventListener("allocArray", function (event) {
var info = event.detail.info;
try {
var cBytes = ctypes.int32_t(info.bytes);
var cArrayId = ctypes.uint32_t(0);
var err = funcAllocArray(cBytes, cArrayId.address());
if(err !== 0) {
info.rtnCode = err;
info.arrayId = -1;
info.error = "Error: " + (err === 2)? "out of memory": "allocation failed";
} else {
info.rtnCode = 0;
info.arrayId = ctypes.CDataFinalizer(cArrayId.value, funcReleaseArray);
}
}
catch(exception) {
info.rtnCode = -1;
info.arrayId = -1;
info.error = report(exception);
}
}, true, true);
Could someone please explain what's wrong with the following code. I'm making two calls to the function fInputData. The first works ok, the second results in an error :
"unhandled exception"
"Bad state: Stream already has subscriber"
I need to write a test console program that inputs multiple parameters.
import "dart:async" as async;
import "dart:io";
void main() {
fInputData ("Enter Nr of Iterations : ")
.then((String sResult){
int iIters;
try {
iIters = int.parse(sResult);
if (iIters < 0) throw new Exception("Invalid");
} catch (oError) {
print ("Invalid entry");
exit(1);
}
print ("In Main : Iterations selected = ${iIters}");
fInputData("Continue Processing? (Y/N) : ") // this call bombs
.then((String sInput){
if (sInput != "y" && sInput != "Y")
exit(1);
fProcessData(iIters);
print ("Main Completed");
});
});
}
async.Future<String> fInputData(String sPrompt) {
async.Completer<String> oCompleter = new async.Completer();
stdout.write(sPrompt);
async.Stream<String> oStream = stdin.transform(new StringDecoder());
async.StreamSubscription oSub;
oSub = oStream.listen((String sInput) {
oCompleter.complete(sInput);
oSub.cancel();
});
return oCompleter.future;
}
void fProcessData(int iIters) {
print ("In fProcessData");
print ("iIters = ${iIters}");
for (int iPos = 1; iPos <= iIters; iPos++ ) {
if (iPos%100 == 0) print ("Processed = ${iPos}");
}
print ("In fProcessData - completed ${iIters}");
}
Some background reading:
Streams comes in two flavours: single or multiple (also known as
broadcast) subscriber. By default, our stream is a single-subscriber
stream. This means that if you try to listen to the stream more than
once, you will get an exception, and using any of the callback
functions or future properties counts as listening.
You can convert the single-subscriber stream into a broadcast stream
by using the asBroadcastStream() method.
So you've got two options - either re-use a single subscription object. i.e. call listen once, and keep the subscription object alive.
Or use a broadcast stream - note there are a number of differences between broadcast streams and single-subscriber streams, you'll need to read about those and make sure they suit your use-case.
Here's an example of reusing a subscriber to ask multiple questions:
import 'dart:async';
import 'dart:io';
main() {
var console = new Console();
var loop;
loop = () => ask(console).then((_) => loop());
loop();
}
Future ask(Console console) {
print('1 + 1 = ...');
return console.readLine().then((line) {
print(line.trim() == '2' ? 'Yup!' : 'Nope :(');
});
}
class Console {
StreamSubscription<String> _subs;
Console() {
var input = stdin
.transform(new StringDecoder())
.transform(new LineTransformer());
_subs = input.listen(null);
}
Future<String> readLine() {
var completer = new Completer<String>();
_subs.onData(completer.complete);
return completer.future;
}
}
Can we call a webservice from the scheduled periodic task class firstly, if yes,
Am trying to call a webservice method with parameters in scheduled periodic task agent class in windows phone 7.1. am getting a null reference exception while calling the method though am passing the expected values to the parameters for the webmethod.
am retrieving the id from the isolated storage.
the following is my code.
protected override void OnInvoke(ScheduledTask task)
{
if (task is PeriodicTask)
{
string Name = IName;
string Desc = IDesc;
updateinfo(Name, Desc);
}
}
public void updateinfo(string name, string desc)
{
AppSettings tmpSettings = Tr.AppSettings.Load();
id = tmpSettings.myString;
if (name == "" && desc == "")
{
name = "No Data";
desc = "No Data";
}
tservice.UpdateLogAsync(id, name,desc);
tservice.UpdateLogCompleted += new EventHandler<STservice.UpdateLogCompletedEventArgs>(t_UpdateLogCompleted);
}
Someone please help me resolve the above issue.
I've done this before without a problem. The one thing you need to make sure of is that you wait until your async read processes have completed before you call NotifyComplete();.
Here's an example from one of my apps. I had to remove much of the logic, but it should show you how the flow goes. This uses a slightly modified version of WebClient where I added a Timeout, but the principles are the same with the service that you're calling... Don't call NotifyComplete() until the end of t_UpdateLogCompleted
Here's the example code:
private void UpdateTiles(ShellTile appTile)
{
try
{
var wc = new WebClientWithTimeout(new Uri("URI Removed")) { Timeout = TimeSpan.FromSeconds(30) };
wc.DownloadAsyncCompleted += (src, e) =>
{
try
{
//process response
}
catch (Exception ex)
{
// Handle exception
}
finally
{
FinishUp();
}
};
wc.StartReadRequestAsync();
}
private void FinishUp()
{
#if DEBUG
try
{
ScheduledActionService.LaunchForTest(_taskName, TimeSpan.FromSeconds(30));
System.Diagnostics.Debug.WriteLine("relaunching in 30 seconds");
}
catch (Exception ex)
{
System.Diagnostics.Debug.WriteLine(ex.ToString());
}
#endif
NotifyComplete();
}