pas2js AsyncAWait
Overview
Pas2js supports the JS operators async/await to simplify the use of Promise, which itself simplifies writing asynchronous code, like starting a download, waiting for the result, then depending on the result start the next and so forth. Instead of writing a tedious and hard to read mass of sub functions an async procedure can be written in the common serial aka imperative way.
Basics
First of we have the async keyword, which you add as a procedure type modifier to turn it into an async function (or async procedure). An async function knows how to expect the possibility of the await function being used to invoke asynchronous code.
Usually you call an async function via the await function. To explain the inner workings it is useful to see what happens when you call an async function directly. Then you get a TJSPromise:
uses JS;
function Run: word; async;
begin
Result:=3;
end;
var p: TJSPromise;
begin
p:=Run(); // calling directly without await() returns a TJSPromise!
end.
That's because an async function runs till an await and returns. If there is no await it works like a normal function, returning immediately a resolved TJSPromise. If it contains one or more await calls, each await pauses the execution of the async function and continues it asynchronously (single threaded) later. A TJSPromise has a hook for success and one for fail, which usually requires writing some anonymous functions. With async/await you can write much more readable code.
AWait
Pas2js provides a built-in await function, which supports three flavours:
function await(AsyncFunctionOfResultTypeT): T;
function await(aType; p: TJSPromise): aType; // explicit promise requires the resolved type
function await(aType; v: JSValue): aType; // explicit optional promise requires the resolved type
The await function can only be used inside a procedure with the async modifier.
An async procedure can contain multiple await calls.
Giving a promise to await works as if the following code is inside a TJSPromise._then(). If the promise is already resolved, await returns the value immediately. Otherwise the promise is returned as the async function result and the rest of the async function is executed when the promise is resolved.
You can enclose an await function in try..finally or try..except for the case the Promise raises an exception.
You can call await on a jsvalue, any promise will be resolved. For example when a JS function returns either a number or a promise of the number.
You can define your own external class 'Promise'. It does not need to be called TJSPromise.
Example Async try fetch
Example demonstrating waiting for an async function, catching errors with a try..except and no anonymous functions.
// Translated from https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await
program tryfetch;
{$mode objfpc}
uses
browserconsole, JS, Web, SysUtils;
procedure myFetch; async;
var
response: TJSResponse;
myBlob: TJSBlob;
image: TJSHTMLImageElement;
objectURL: string;
begin
try
response := await(window.fetch('test1.png'));
if not response.ok then
raise Exception.Create('HTTP error! status: '+str(response.status))
else begin
myBlob := await(response.blob());
objectURL := TJSURL.createObjectURL(myBlob);
image := TJSHTMLImageElement(document.createElement('img'));
image.src := objectURL;
document.body.appendChild(image);
end;
except
console.log(JSExceptValue);
end;
end;
begin
myFetch;
end.
Example Manually created promise
Example demonstrating using await with a non async function, creating the TJSPromise manually:
Manually created promise Version 1
Program MyModule;
uses JS, Web;
function ResolveAfter2Seconds: TJSPromise;
// returns immediately with a Promise,
// which after 2 seconds gets resolved
begin
Result:=TJSPromise.new(procedure(resolve, reject : TJSPromiseResolver)
begin
window.setTimeout(procedure
begin
resolve('resolved');
end,
2000); // wait 2 seconds
end);
end;
procedure AsyncCall; async;
var s: string;
begin
writeln('calling');
s := await(string,resolveAfter2Seconds());
// the await pauses this procedure returning to the caller
// when the Promise from resolveAfter2Seconds gets resolved
// this procedure is continued
writeln(s); // expected output: 'resolved'
end;
begin
AsyncCall;
// calling AsyncCall returns immediately, while the Promise is waiting
writeln('called');
end.
Expected output:
calling called resolved
Manually created promise Version 2
Alternatively you can use the promise result type (here: string), by passing the TJSPromise to the exit() function:
Program MyModule;
uses JS, Web;
function ResolveAfter2Seconds: string; async;
// returns immediately with a Promise,
// which after 2 seconds gets resolved
var p: TJSPromise;
begin
p:=TJSPromise.new(procedure(resolve, reject : TJSPromiseResolver)
begin
window.setTimeout(procedure
begin
resolve('resolved');
end,
2000); // wait 2 seconds
end);
exit(p); // the exit() function takes either a TJSPromise or a string
end;
procedure AsyncCall; async;
var s: string;
begin
writeln('calling');
s := await(resolveAfter2Seconds()); // no "string" needed
// the await pauses this procedure returning to the caller
// when the Promise from resolveAfter2Seconds gets resolved
// this procedure is continued
writeln(s); // expected output: 'resolved'
end;
begin
AsyncCall;
// calling AsyncCall returns immediately, while the Promise is waiting
writeln('called');
end.
Note: The Result variable is of type string. You cannot assign it a promise.
Example Multi await
Example demonstrating starting multiple async functions and waiting for them while they execute in parallel.
// Translated from https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await
program tryfetchmany;
{$mode objfpc}
uses
browserconsole, JS, Web, SysUtils, Types;
function FetchBlob(url: string): TJSBlob; async;
var
response: TJSResponse;
begin
response := await(window.fetch(url));
if not response.ok then
raise Exception.create('HTTP error! status: '+str(response.status))
else
Result:=await(response.blob());
end;
function FetchText(url: string): string; async;
var
response: TJSResponse;
begin
response := await(window.fetch(url));
if not response.ok then
raise Exception.create('HTTP error! status: '+str(response.status))
else
Result:=await(response.text());
end;
procedure DisplayContent; async;
var
coffee, tea, description: TJSPromise;
objectURL1, objectURL2, descText: String;
values: TJSValueDynArray;
image1, image2: TJSHTMLImageElement;
para: TJSHTMLElement;
begin
try
coffee := FetchBlob('test1.png');
tea := FetchBlob('test1.png');
description := FetchText('description.txt');
values := await(TJSValueDynArray,TJSPromise.all([coffee, tea, description]));
objectURL1 := TJSURL.createObjectURL(values[0]);
objectURL2 := TJSURL.createObjectURL(values[1]);
descText := string(values[2]);
image1 := TJSHTMLImageElement(document.createElement('img'));
image2 := TJSHTMLImageElement(document.createElement('img'));
image1.src := objectURL1;
image2.src := objectURL2;
document.body.appendChild(image1);
document.body.appendChild(image2);
para := TJSHTMLElement(document.createElement('p'));
para.textContent := descText;
document.body.appendChild(para);
except
writeln(JSExceptValue);
end;
end;
begin
DisplayContent;
end.
