var callback = null;
addEventListener(Event.MouseDown, callback = function(e:Event){blabla....});
Can I assign arguments when calling a function?
Of course. Try it this way instead.
var callback = function(e:Event, arg1, arg2) { blabla....};
addEventListener(Event.MouseDown, function(e) { callback(e, 0, 1); } );
Or If I misunderstood your question, perhaps this way:
var callback = null;
addEventListener(Event.MouseDown,
callback = function(e:Event) { anotherfunction(arg1, arg2); });
Related
The listener function can listen to any parameter type(not only listener type). This has nothing related to widgets (this should work successfully in https://dartpad.dev without using the flutter).
ex.
int a = 0;
listener((a>0)=>print("A = $a"));
a= 1; //A = 1
a= -1; //
a= 2; //A = 2
You can use ValueNotifier for this. It's a ChangeNotifier that is triggered when the value is replaced with something that is not equal to the old value as evaluated by the equality operator ==.
Here is a nice tutorial about this approach.
The basic method is to create the function for updating the parameter that you want to add to the listener.
void test() {
int a = 0;
void updateA(newA) {
if(newA is! int) return;
a = newA;
if (a > 0) print("A = $a");
}
updateA(1);
updateA(-1);
updateA(2);
}
A better way is to create parameters with class.
void main() {
ParameterWithListener a = ParameterWithListener(data: 0);
a.listener = () {
if (a.data is int && a.data > 0) print("A = ${a.data}");
};
a.update(1);
a.update(-1);
a.update(2);
}
class ParameterWithListener {
ParameterWithListener({this.data, this.listener});
dynamic data;
Function()? listener;
Future update(data) async {
this.data = data;
if (listener is Function()) await listener!();
}
}
result:
A = 1
A = 2
I'm struggling with how to invoke a top level function in dart. I'd like to be able to annotate a function with #Route, find all the Route annotations, then check that the annotation is on a method, and then call that method by its symbol.
What I have so far is:
class Route {
final String url;
const Route(this.url);
}
#Route('/')
void handle() {
print('Request received');
}
void main() {
var mirrorSystem = currentMirrorSystem();
var lm = mirrorSystem.isolate.rootLibrary;
for (var mirror in lm.declarations.values) {
var metadata = mirror.metadata;
for (var im in metadata) {
if (im.reflectee is Route) {
print('Route found');
// how to invoke the function handle associated with the route annotation?
}
}
}
}
From this point i'm not sure how I would then call the method. If it was a class then I could use invoke and pass in the Symbol for the method, but that doesn't work as it's not a class.
Can anyone give me some clues? Information about the mirrors library in dart is fairly sparse unfortunately.
I worked this out. For anyone who finds this, you use the LibraryMirror on rootLibrary to invoke the top level function:
void main() {
var mirrorSystem = currentMirrorSystem();
var lm = mirrorSystem.isolate.rootLibrary;
for (var mirror in lm.declarations.values) {
var metadata = mirror.metadata;
for (var im in metadata) {
if (im.reflectee is Route && mirror is MethodMirror) {
lm.invoke(mirror.simpleName, []);
// prints 'Request received'
}
}
}
}
i have a function called foo which is listening to the stdout, what i want is to return some string which i got from stdout. here is my function;
dynamic foo(process) {
return (
process.stdout.transform(UTF8.decoder).listen((data) {
String s = data.toString();
// print(s);
if (s.contains("received event of")) {
var s1 = s.split(":");
print("${s1[1]}");
return s1[1];
}
}));
}
I want to return s1 to the calling function
here a callback function do the trick
foo(process, callback) {
process.stdout.transform(UTF8.decoder).listen((data) {
String s = data.toString();
if (s.contains("received event of")) {
String message = s.split(":")[1];
callback(message);
}
});
}
and here i am calling the method and printing the data which i get get from stream.
foo(process,(data){print(data);})
This should do what you want
Future<String> dynamic foo(process) {
return process.stdout.transform(UTF8.decoder).map((data) {
String s = data.toString();
// print(s);
if (s.contains("received event of")) {
var s1 = s.split(":");
print("${s1[1]}");
return s1[1];
} else {
return null;
}
}).where((val) => val != null).first;
}
Your custom code either returns a valid value or null.
I changed listen to map to be able to use additional stream methods.
where filters invalid values (null) and returns the first non-null value.
The caller of the foo method needs to handle the returned Future (using for example async/await) to get the value when it becomes available.
Use it like
bar() async {
...
var input = await foo(proc);
print(input);
}
I think that everybody wants this:
import 'dart:io';
import 'dart:convert';
// Using system encoding:
var outputStr = await process.stdout.transform(systemEncoding.decoder).join();
// Using UTF-8 encoding:
var outputStr = await process.stdout.transform(utf8.decoder).join();
I'm trying to create a server-side Dart class that performs various data-related tasks. All of these tasks rely on the database having been first initialized. The problem is that the init of the database happens asynchronously (returns a Future). I first tried to put the init code into the constructor, but have given up on this approach as it seems to not be viable.
I am now attempting to figure out how to force the DB initialization as a first step in any method call that accesses data. So in other words, when attemptLogin() is called below, I'd like to first check if the DB has been initialized and initialize it if necessary.
However, there are two obstacles. If the database hasn't been initialized, the code is straightforward - initialize the db, then use the then() method of the returned future to do the rest of the function. If the db is not yet initialized, what do I attach my then() method to?
Second related question is what happens when a database is currently being initialized but this process is not yet complete? How can I pull in and return this "in-progress" Future?
This is the basic gist of the code I'm trying to wrangle:
class DataManager {
bool DbIsReady = false;
bool InitializingDb = false;
Db _db;
Future InitMongoDB() {
print("Initializing MongoDB");
InitializingDb = true;
_db = new Db("mongodb://127.0.0.1/test");
return _db.open().then((_) {
DbIsReady = true;
InitializingDb = false;
});
}
Future<List> attemptLogin(String username, String password) {
Future firstStep;
if ((!DbIsReady) && (!InitializingDb) {
Future firstStep = InitMongoDB()
}
else if (InitializingDb) {
// Need to return the InitMongoDB() Future that's currently running, but how?
}
else {
// How do I create a blank firstStep here?
}
return firstStep.then((_) {
users = _db.collection("users");
return // ... rest of code cut out for clarity
});
}
}
Thanks in advance for your help,
Greg
Just return
return new Future<bool>.value(true);
// or any other value instead of `true` you want to return.
// or none
// return new Future.value();
Just keep the future alive:
class DataManager {
Future _initializedDb;
Future initMongoDb() { ... }
Future<List> attemptLogin(String username, String password) {
if (_initializedDb == null) {
_initializedDb = initMongoDB();
}
return _initializedDb.then((db) {
users = db.collection("users");
return // ... rest of code cut out for clarity
});
}
}
You might need to pay attention for the error-case. It's up to you if you want to deal with errors in the initMongoDB or after it.
One of the possible solutions:
import "dart:async";
void main() {
var dm = new DataManager();
var selectOne = dm.execute("SELECT 1");
var selectUsers = dm.execute("SELECT * FROM users");
var users = selectOne.then((result) {
print(result);
return selectUsers.then((result) {
print(result);
});
});
users.then((result) {
print("Goodbye");
});
}
class Event {
List<Function> _actions = new List<Function>();
bool _raised = false;
void add(Function action) {
if (_raised) {
action();
} else {
_actions.add(action);
}
}
void raise() {
_raised = true;
_notify();
}
void _notify() {
if (_actions.isEmpty) {
return;
}
var actions = _actions.toList();
_actions.clear();
for (var action in actions) {
action();
}
}
}
class DataManager {
static const int _STATE_NOT_INITIALIZED = 1;
static const int _STATE_INITIALIZING = 2;
static const int _STATE_READY = 3;
Event _initEvent = new Event();
int _state = _STATE_NOT_INITIALIZED;
Future _init() {
if (_state == _STATE_NOT_INITIALIZED) {
_state = _STATE_INITIALIZING;
print("Initializing...");
return new Future(() {
print("Initialized");
_state = _STATE_READY;
_initEvent.raise();
});
} else if (_state == _STATE_INITIALIZING) {
print("Waiting until initialized");
var completer = new Completer();
_initEvent.add(() => completer.complete());
return completer.future;
}
return new Future.value();
}
Future execute(String query, [Map arguments]) {
return _init().then((result) {
return _execute(query, arguments);
});
}
Future _execute(String query, Map arguments) {
return new Future.value("query: $query");
}
}
Output:
Initializing...
Waiting until initialized
Initialized
query: SELECT 1
query: SELECT * FROM users
Goodbye
I think that exist better solution but this just an attempt to answer on your question (if I correctly understand you).
P.S. EDITED at 11 July 2014
Slightly modified (with error handling) example.
import "dart:async";
void main() {
var dm = new DataManager();
var selectOne = dm.execute("SELECT 1");
var selectUsers = dm.execute("SELECT * FROM users");
var users = selectOne.then((result) {
print(result);
return selectUsers.then((result) {
print(result);
});
});
users.then((result) {
print("Goodbye");
});
}
class DataManager {
static const int _STATE_NOT_INITIALIZED = 1;
static const int _STATE_INITIALIZING = 2;
static const int _STATE_READY = 3;
static const int _STATE_FAILURE = 4;
Completer _initEvent = new Completer();
int _state = _STATE_NOT_INITIALIZED;
Future _ensureInitialized() {
switch (_state) {
case _STATE_NOT_INITIALIZED:
_state = _STATE_INITIALIZING;
print("Initializing...");
new Future(() {
print("Initialized");
_state = _STATE_READY;
// throw null;
_initEvent.complete();
}).catchError((e, s) {
print("Failure");
_initEvent.completeError(e, s);
});
break;
case _STATE_INITIALIZING:
print("Waiting until initialized");
break;
case _STATE_FAILURE:
print("Failure detected");
break;
default:
print("Aleady intialized");
break;
}
return _initEvent.future;
}
Future execute(String query, [Map arguments]) {
return _ensureInitialized().then((result) {
return _execute(query, arguments);
});
}
Future _execute(String query, Map arguments) {
return new Future.value("query: $query");
}
}
For those that are still wondering how to create a blank Future in Dart and later complete them, you should use the Completer class like in the next example.
class AsyncOperation {
final Completer _completer = new Completer();
Future<T> doOperation() {
_startOperation();
return _completer.future; // Send future object back to client.
}
// Something calls this when the value is ready.
void finishOperation(T result) {
_completer.complete(result);
}
// If something goes wrong, call this.
void _errorHappened(error) {
_completer.completeError(error);
}
}
Future<Type> is non nullable in Dart, meaning that you have to initialize it to a value. If you don't, Dart throws the following error:
Error: Field should be initialized because its type 'Future<Type>' doesn't allow null.
To initialize a Future<Type>, see the following example:
Future<String> myFutureString = Future(() => "Future String");
Here "Future String" is a String and so the code above returns an instance of Future<String>.
So coming to the question of how to create a blank/empty Future, I used the following code for initializing an empty Future List.
Future<List> myFutureList = Future(() => []);
I found this link to be quite useful in understanding Futures in Flutter and Dart: https://meysam-mahfouzi.medium.com/understanding-future-in-dart-3c3eea5a22fb
Say I have
class RestSimulator {
#Path("/var")
void functionOne() {
final Type type = this.runtimeType;
final InstanceMirror instanceMirror = reflect(this);
final ClassMirror classMirror = instanceMirror.type;
final MethodMirror methodMirror = ?????
var metadata = methodMirror.metadata;
var path = metadata.first.reflectee;
print(path.toString()):
}
}
How can I get the MethodMirror for the calling function???
[Update]
I meant without doing something like
final MethodMirror methodMirror = functions[const Symbol('functionOne')];
So probably the main question is: How do I get the Symbol for the calling / current function?
AFAIK there's no simple way to get a reference on the current function at runtime.
I say simple because you can get the name from a StackTrace but it's really ugly and has horrible performances...
class A {
m() {
var functionName;
try {
throw '';
} catch(e, s) {
functionName = parseStackTraceToGetMethod(s.toString());
}
print(functionName); // displays A.m
}
}
parseStackTraceToGetMethod(String s) =>
s.substring(8, s.indexOf("("));
main() {
new A().m();
}