I've got a message map at the beginning of my program that looks like the following:
BEGIN_MESSAGE_MAP(SoftwareDlg, CDialog)
//{{AFX_MSG_MAP(SoftwareDlg)
ON_WM_SYSCOMMAND()
ON_WM_PAINT()
ON_WM_QUERYDRAGICON()
ON_WM_TIMER()
ON_WM_DESTROY()
...
ON_COMMAND(ID_TOOLS_UPLOADDATA, UploadData)
...
//}}AFX_MSG_MAP
END_MESSAGE_MAP()
And here the function that the ID_TOOLS_UPLOADDATA menu option calls when clicked:
void UploadData()
{
string apiEndpoint = "/api/stuff";
upload_data(apiEndpoint);
}
My problem is that that I want my UploadData() function to be able to take the string apiEndpoint as a parameter so that I can call it from multiple locations in my program for multiple operations (not just when the user clicks the menu button). Like so:
void UploadData(string apiEndpoint = "/api/stuff")
{
upload_data(apiEndpoint);
}
I took a look at ON_COMMAND_EX, but the only usage example I was able to find does not appear to be what I'm looking for... Anyone have any ideas?
As #Iinspectable said, you can’t.
Try for example to add a member variable and set it before calling the function.
I get the following error:
A value of type 'Future<int>' can't be assigned to a variable of type 'int'
It might be another type instead of int, but basically the pattern is:
A value of type 'Future<T>' can't be assigned to a variable of type 'T'
So:
What exactly is a Future?
How do I get the actual value I want to get?
What widget do I use to display my value when all I have is a Future<T>?
In case you are familiar with Task<T> or Promise<T> and the async/ await pattern, then you can skip right to the "How to use a Future with the widgets in Flutter" section.
What is a Future and how do I use it?
Well, the documentation says:
An object representing a delayed computation.
That is correct. It's also a little abstract and dry. Normally, a function returns a result. Sequentially. The function is called, runs and returns it's result. Until then, the caller waits. Some functions, especially when they access resources like hardware or network, take a little time to do so. Imagine an avatar picture being loaded from a web server, a user's data being loaded from a database or just the texts of the app in multiple languages being loaded from device memory. That might be slow.
Most applications by default have a single flow of control. When this flow is blocked, for example by waiting for a computation or resource access that takes time, the application just freezes. You may remember this as standard if you are old enough, but in today's world that would be seen as a bug. Even if something takes time, we get a little animation. A spinner, an hourglass, maybe a progress bar. But how can an application run and show an animation and yet still wait for the result? The answer is: asynchronous operations. Operations that still run while your code waits for something. Now how does the compiler know, whether it should actually stop everything and wait for a result or continue with all the background work and wait only in this instance? Well, it cannot figure that out on it's own. We have to tell it.
This is achieved through a pattern known as async and await. It's not specific to flutter or dart, it exists under the same name in many other languages. You can find the documentation for Dart here.
Since a method that takes some time cannot return immediately, it will return the promise of delivering a value when it's done.
That is called a Future. So the promise to load a number from the database would return a Future<int> while the promise to return a list of movies from an internet search might return a Future<List<Movie>>. A Future<T> is something that in the future will give you a T.
Lets try a different explanation:
A future represents the result of an asynchronous operation, and can have two states: uncompleted or completed.
Most likely, as you aren't doing this just for fun, you actually need the results of that Future<T> to progress in your application. You need to display the number from the database or the list of movies found. So you want to wait, until the result is there. This is where await comes in:
Future<List<Movie>> result = loadMoviesFromSearch(input);
// right here, you need the result. So you wait for it:
List<Movie> movies = await result;
But wait, haven't we come full circle? Aren't we waiting on the result again? Yes, indeed we are. Programs would be utterly chaotic if they did not have some resemblence of sequential flow. But the point is that using the keyword await we have told the compiler, that at this point, while we want to wait for the result, we do not want our application to just freeze. We want all the other running operations like for example animations to continue.
However, you can only use the await keyword in functions that themselves are marked as async and return a Future<T>. Because when you await something, then the function that is awaiting can no longer return their result immediately. You can only return what you have, if you have to wait for it, you have to return a promise to deliver it later.
Future<Pizza> getPizza() async {
Future<PizzaBox> delivery = orderPizza();
var pizzaBox = await delivery;
var pizza = pizzaBox.unwrap();
return pizza;
}
Our getPizza function has to wait for the pizza, so instead of returning Pizza immediately, it has to return the promise that a pizza will be there in the future. Now you can, in turn, await the getPizza function somewhere.
How to use a Future with the widgets in Flutter?
All the widgets in flutter expect real values. Not some promise of a value to come at a later time. When a button needs a text, it cannot use a promise that text will come later. It needs to display the button now, so it needs the text now.
But sometimes, all you have is a Future<T>. That is where FutureBuilder comes in. You can use it when you have a future, to display one thing while you are waiting for it (for example a progress indicator) and another thing when it's done (for example the result).
Let's take a look at our pizza example. You want to order pizza, you want a progress indicator while you wait for it, you want to see the result once it's delivered, and maybe show an error message when there is an error:
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
/// ordering a pizza takes 5 seconds
/// and then gives you a pizza salami with extra cheese
Future<String> orderPizza() {
return Future<String>.delayed(
const Duration(seconds: 5),
() async => 'Pizza Salami, Extra Cheese');
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData.dark(),
home: Scaffold(
body: Center(
child: PizzaOrder(),
),
),
);
}
}
class PizzaOrder extends StatefulWidget {
#override
_PizzaOrderState createState() => _PizzaOrderState();
}
class _PizzaOrderState extends State<PizzaOrder> {
Future<String>? delivery;
#override
Widget build(BuildContext context) {
return Column(
crossAxisAlignment: CrossAxisAlignment.center,
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
ElevatedButton(
onPressed: delivery != null
? null
: () => setState(() {
delivery = orderPizza();
}),
child: const Text('Order Pizza Now')
),
delivery == null
? const Text('No delivery scheduled')
: FutureBuilder(
future: delivery,
builder: (context, snapshot) {
if(snapshot.hasData) {
return Text('Delivery done: ${snapshot.data}');
} else if(snapshot.hasError) {
return Text('Delivery error: ${snapshot.error.toString()}');
} else {
return const CircularProgressIndicator();
}
})
]);
}
}
This is how you use a FutureBuilder to display the result of your future once you have it.
Here's a list of analogies to Dart's Future from other languages:
JS: Promise
Java: Future
Python: Future
C#: Task
Just like in other languages Future is a special type of object which allows to use async/await syntax sugar, write asynchronous code in synchronous/linear way. You return Future from an async method rather than accept a callback as a parameter and avoid the callback hell - both Futures and callbacks solve same problems (firing some code at a latter time) but in a different way.
Future<T> returning the potential value which will be done by async work
Eg:
Future<int> getValue() async {
return Future.value(5);
}
Above code is returning Future.value(5) which is of int type, but while receiving the value from method we can't use type Future<int> i.e
Future<int> value = await getValue(); // Not Allowed
// Error
A value of type 'Future<int>' can't be assigned to a variable of type 'int'
To solve above getValue() should be received under int type
int value = await getValue(); // right way as it returning the potential value.
I hope this key point will be informative, I show it in two different Async methods:
Note the following method where showLoading(), getAllCarsFromApi() and hideLoading() are inner Async methods.
If I put the await keyword before showLoading(), the Operation waits until it's done then goes to the next line but I intentionally removed the await because I need my Loading dialog be displayed simultaneously with getAllCarsFromApi() is being processed, so it means showLoading() and getAllCarsFromApi() methods are processed on different Threads. Finally hideLoading() hides the loading dialog.
Future<List<Car>> getData() async{
showLoading();
final List<Car> cars = await getAllCarsFromApi();
hideLoading();
return cars;
}
Now look at this another Async method, here the getCarByIdFromApi() method needs an id which is calculated from the getCarIdFromDatabase(), so there must be an await keyword before the first method to make the Operation wait until id is calculated and passed to the second method. So here two methods are processed one after another and in a single Thread.
Future<Car> getCar() async{
int id = await getCarIdFromDatabase();
final Car car = await getCarByIdFromApi(id);
return car;
}
A simple answer is that if a function returns its value with a delay of some time, Future is used to get its value.
Future<int> calculate({required int val1, required int val2}) async {
await Future.delayed(const Duration(seconds: 2));
return val1 + val2;
}
if we call the above function as
getTotal() async {
int result = calculate(val1: 5, val2: 5);
print(result);
}
we will get the following error:
A value of type 'Future<int>' can't be assigned to a variable of type 'int'
but if we use await before function call it will give the actual returned value from the function after a delay
getTotal() async {
int result = await calculate(val1: 5, val2: 5);
print(result);
}
the keyword async is required to use await for the Future to get returned value
I am trying to give very simple example. Suppose you have ordered something online, let it be a shirt. then you have to wait until the order is dispatched and delivered to your home. In the meanwhile you will not stop working your daily activities/work anything you do and after a day if it delivered to your home you will collect it and wear it. Now, look at the following example.
Ok, now let's make a function which handles our order delivery.(Read Comments Also)
//order function which will book our order and return our order(which is our shirt). don't focus on Order object type just focus on how this function work and you will get to know about future definitely.
Future<Order> orderSomething(){
//here our order processing and it will return our order after 24 hrs :)
await Future.delayed(const Duration(hours: 24),() => Order('data'));
}
Now
void main() {
//now here you have called orderSomething() and you dont want to wait for it to be delivered
//you are not dependent on your order to do your other activities
// so when your order arrives you will get to know
orderSomething()
wearSomething()
goingCollege()
}
Now if you are dependent on your order then you have to add await async ( i will show you where)
void main() async{
//now you're dependent on your order you want to wait for your order
await orderSomething()
wearOrderedShirt() // :)
goingCollege()
}
Now most of the times in flutter applications you will have to await for your API calls(Network), for background task for downloading/uploading, for database calls etc.
I have a function which uses Anko's async in order to call Google's Distance API and get the distance from one location to another. unfortunately i don't know how to get the data from inside the async and pass it to another function. the code looks something like this
fun getDistance(location1:LatLng,location2:LatLng){
async{
val result = URL(url).readtext()
uithread{
//Parser
//distance value
}
}
}
I'd like to also mention im really new to kotlin or android development in general, please be kind.
There are a number of ways to tackle this; pass an object to the function with your array in it that gets rearranged in your function, or go with something like:
fun getDistance(location1 : LatLng, location2 : LatLng, f: (Long) -> Unit){
doAsync{ // Anko is deprecated as I have been made aware
val result = URL(url).readtext()
val distance : Long = // parse result
uiThread{
f(distance)
}
}
}
and call that with
getDistance(loc1, loc2) { toast("The found distance was $it") }
This is by no means the only way to go; you could update a larger-scoped variable, call a listener, put your lat-longs in a class with updating functions that are called, or a bunch of other ways that I am too lazy to think about :)
I want to invoke a JavaScript function from Java, passing to the function an Object. What I hoped to do is something like:
JSObject obj = new JSObject();
obj.setString("property1", "value);
obj.setNumber("property2", number);
and pass the object to the JavaScript function via the JSFunction.invoke() method. But JSObject apparently has no default constructor, nor methods like "setString". Ditto for JSONString.
I ended up using the Google GSon library to build a JSON string that I then passed to "new JSONString()". Do you have any better way?
I wish your documentation addressed this point instead of forcing me to invent this solution. Not what I wanted to spend time on.
Here is a code sample that demonstrates how to create an object and set some properties to it:
Browser browser = new Browser();
browser.executeJavaScript("function foo(object) { console.log(object.property1); }");
JSObject object = browser.executeJavaScriptAndReturnValue("new Object()").asObject();
object.setProperty("property1", "value 1");
object.setProperty("property2", 15);
JSFunction function = browser.executeJavaScriptAndReturnValue("window.foo")
.asFunction();
function.invoke(null, object);
hoping this is an easy enough question :)
Some details:
I am using Flash CS5, never touched Flex. Also the SWF that is doing the loading will be a client SWF, so hoping for a solution that could work with a simple couple of lines.
Basically inside the SWF I am working on contains just a simple string:
var theString = "theString";
trace("theString = "+theString);
Now I've been working on a test loader SWF that will load my String SWF and get the variable in the simplest way. Any thoughts? Below is my current broken code:
function loaderComplete(event:Event)
{
trace("... in loaderComplete");
getString = loader.content.toString();
trace("loader.content = "+loader.content);
trace("... getString = "+getString);
}
This is my output window:
theString = theString
... in loaderComplete
loader.content = [object MainTimeline]
... getString = [object MainTimeline]
I've searched on Stack and found similar questions, but none are exactly what I need:
tracking video files - embedding flv to swf
^ Basically what I'm trying to do as well, no answers yet
to pass variable from one swf to another swf in as3
^ sounded just like my problem, but answer was a Flex application example
pass var values from one swf to another swf who is loaded inside the firts one in AS3
^ This was close, but am not sure how to implement the chosen answer, also seems a bit more intricate then I need
Please help!
Let's clarify a bit:
1. The loader swf, we will call the parent.
2. The swf loaded by the parent we will call the child.
The Child contains a string, and you want the parent to be able to read that string>
So...
The Child must define a public variable for the string. (This means you have to use a Class file for it, since you cannot declare a property public on the timeline.)
Finally, the parent will try and get that property. You may want to wrap that in a try/catch to handle cases where the string will not be present.
Here is an example Child Class.
package
{
import flash.display.Sprite;
/**
* ...
* #author Zach Foley
*/
public class Child extends Sprite
{
public var value:String = "This is the child Value";
public function Child()
{
trace("Child Loaded");
}
}
}
And here is the parent loader class:
package
{
import flash.display.Loader;
import flash.display.Sprite;
import flash.events.Event;
import flash.net.URLRequest;
/**
* ...
* #author Zach Foley
*/
public class Parent extends Sprite
{
private var loader:Loader;
public function Parent()
{
trace("PArent Init");
loader = new Loader();
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, onLoaded);
loader.load(new URLRequest("child.swf"));
}
private function onLoaded(e:Event):void
{
trace("Child Loaded");
trace(loader.content['value']);
}
}
}
The Output will be:
PArent Init
Child Loaded
Child Loaded
This is the child Value
you can access variables and code from the loaded swf. First, make sure that both the loader and the loaded content use the same AVM, this means that both project use the same version of language (both as3 or both as2)
Thank in the loader complete handler:
loaderComplete(evt:Event):void{
var mc:MovieClip = loader.content as MovieClip;
if(!mc){
trace("Error loading");
return;
}
trace("Your string: " + mc.theString);
}
I haven't tested this code but it should works