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Add onFirstPause utility method #2

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Add onFirstPause utility method #2

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5 changes: 5 additions & 0 deletions README.md
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Expand Up @@ -194,3 +194,8 @@ const loadFiles = gensync(function* () {
]);
});
```

### gensync.onFirstPause(generator, callback)

This method returns a gensync generator which calls `callback` when `generator`
yields an async operation.
18 changes: 17 additions & 1 deletion index.js
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Expand Up @@ -71,6 +71,16 @@ module.exports = Object.assign(
}
},
}),
onFirstPause: buildOperation({
name: "onFirstPause",
arity: 2,
sync: function([item]) {
return evaluateSync(item);
},
async: function([item, cb], resolve, reject) {
evaluateAsync(item, resolve, reject, cb);
},
}),
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I think you can do what you want with something like this?

  async: function([item, cb], resolve, reject) {
    let completed = false;

    evaluateAsync(item, value => {
      completed = true;
      resolve(value);
    }, err => {
      completed = true;
      reject(value);
    });

    if (!completed) {
      cb();
    }
  },

or alternatively using the existing public API:

const runGenerator = gensync(function* (item) {
  return yield* item;
});

const onFirstPause = gensync({
  name: "onFirstPause",
  arity: 2,
  sync: function([item]) {
    return runGenerator.sync(item);
  },
  errback: function([item, firstPause], cb) {
    let completed = false;

    // Gensync calls `errback` callbacks synchronously if it can, even in the async case, if I remember right
    runGenerator.errback(item, (err, value) => {
      completed = true;
      cb(value);
    });

    if (!completed) {
      firstPause();
    }
  },
})

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Ok, I will try the second approach 👍

}
);

Expand Down Expand Up @@ -247,7 +257,8 @@ function evaluateSync(gen) {
return value;
}

function evaluateAsync(gen, resolve, reject) {
function evaluateAsync(gen, resolve, reject, onFirstPause) {
let didFirstPause = false;
(function step() {
try {
let value;
Expand All @@ -271,6 +282,11 @@ function evaluateAsync(gen, resolve, reject) {
assertSuspend(out, gen);

if (!didSyncResume) {
if (!didFirstPause && onFirstPause) {
didFirstPause = true;
onFirstPause();
}

// Callback wasn't called synchronously, so break out of the loop
// and let it call 'step' later.
return;
Expand Down
1 change: 1 addition & 0 deletions index.js.flow
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Expand Up @@ -29,4 +29,5 @@ declare module.exports: {

all<Return>(Array<Handler<Return>>): Handler<Return[]>,
race<Return>(Array<Handler<Return>>): Handler<Return>,
onFirstPause<Return>(Handler<Return>, cb: Function): Handler<Return>,
};