How can I write this mutation observer code, using async/await?
I want to return true after console.log("Button is appearing...");. Could someone show me the best way to write this code?
I also need to clarify, this code is watching for a button, which appears and then disappears. And the reappears again, multiple times.
So the mutationObserver, is watching for the button to appear multiple times. Not just once.
var target = document.querySelector('[search-model="SearchPodModel"]')
var observer = new MutationObserver(mutate);
function mutate(mutations) {
for (let i = 0; i < mutations.length; i++) {
if (mutations[i].oldValue === "ej-button rounded-corners arrow-button search-submit holiday-search ng-hide") {
console.log("Button is appearing...");
return true;
};
};
};
var config = { childList: true, attributes: true, characterData: true, subtree: true, attributeOldValue: true };
observer.observe(target, config);
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Answer
Preface: I would strongly recommend not relying on a mutation observer to watch for a button’s class attribute to change. It’s very much a last resort thing to do. Look for anything else you can hook into that happens which is what makes the button appear/disappear and hook into that instead.
But getting to your question:
Since you want repeated notifications, promises (and thus async/await) is not the right model for this. A promise is only settled once.
There’s no JavaScript built-in for it, but what you want is often called an observable and it has (typically) subscribe and unsubscribe methods. Here’s a really basic, naive implementation of an observable (using modern JavaScript; run it through Babel or similar if you need to support older environments), but you may want to go looking for a library (such as Rx.js — not an endorsement, I haven’t used it, just an example I happen to know about) with something more feature-rich and, you know, tested:
class Observable {
// Constructs the observable
constructor(setup) {
// Call the observable executor function, give it the function to call with
// notifications.
setup((spent, value) => {
// Do the notifications
this.#notifyObservers(spent, value);
if (spent) {
// Got a notification that the observable thing is completely done and
// won't be providing any more updates. Release the observers.
this.#observers = null;
}
});
}
// The observers
#observers = new Set();
// Notify observers
#notifyObservers(spent, value) {
// Grab the current list to notify
const observers = new Set(this.#observers);
for (const observer of observers) {
try { observer(spent, value); } catch { }
}
}
// Add an observer. Returns a true if the subscription was successful, false otherwise.
// You can't subscribe to a spent observable, and you can't subscribe twice.
subscribe(observer) {
if (typeof observer !== "function") {
throw new Error("The observer must be a function");
}
if (this.#observers.has(observer) || !this.#observers) {
return false;
}
this.#observers.add(observer);
return true;
}
// Remove an observer. Returns true if the unsubscription was successful, false otherwise.
unsubscribe(observer) {
return this.#observers ? this.#observers.delete(observer) : false;
}
}
Then you might create an observable for this mutation:
// Create an observable for the button
const buttonAppearedObservable = new Observable(notify => {
const target = document.querySelector('[search-model="SearchPodModel"]');
const observer = new MutationObserver(mutate);
function mutate(mutations) {
for (const mutation of mutations) {
if (mutation.oldValue === "ej-button rounded-corners arrow-button search-submit holiday-search ng-hide") {
// Notify observers. The first argument is `false` because this observable isn't "spent" (it may still
// send more notifications). If you wanted to pass a value, you'd pass a second argument.
notify(
false, // This observable isn't "spent"
mutation.target // Pass along the mutation target element (presumably the button?)
);
};
};
};
// Set up the observer
const config = { childList: true, attributes: true, characterData: true, subtree: true, attributeOldValue: true };
observer.observe(target, config);
});
Once you’d set that observable up, you could subscribe to it:
buttonAppearedObservable.subscribe((spent, button) => {
if (spent) {
// This is a notification that the button appeared event will never happen again
}
if (button) {
// The button appeared!
console.log(`Button "${button.value}" appeared!`);
}
});
Live Exmaple:
class Observable {
// Constructs the observable
constructor(setup) {
// Call the observable executor function, give it the function to call with
// notifications.
setup((spent, value) => {
// Do the notifications
this.#notifyObservers(spent, value);
if (spent) {
// Got a notification that the observable thing is completely done and
// won't be providing any more updates. Release the observers.
this.#observers = null;
}
});
}
// The observers
#observers = new Set();
// Notify observers
#notifyObservers(spent, value) {
// Grab the current list to notify
const observers = new Set(this.#observers);
for (const observer of observers) {
try { observer(spent, value); } catch { }
}
}
// Add an observer. Returns a true if the subscription was successful, false otherwise.
// You can't subscribe to a spent observable, and you can't subscribe twice.
subscribe(observer) {
if (typeof observer !== "function") {
throw new Error("The observer must be a function");
}
if (this.#observers.has(observer) || !this.#observers) {
return false;
}
this.#observers.add(observer);
return true;
}
// Remove an observer. Returns true if the unsubscription was successful, false otherwise.
unsubscribe(observer) {
return this.#observers ? this.#observers.delete(observer) : false;
}
}
// Create an observable for the button
const buttonAppearedObservable = new Observable(notify => {
const target = document.querySelector('[search-model="SearchPodModel"]');
const observer = new MutationObserver(mutate);
function mutate(mutations) {
for (const mutation of mutations) {
if (mutation.oldValue === "ej-button rounded-corners arrow-button search-submit holiday-search ng-hide") {
// Notify observers. The first argument is `false` because this observable isn't "spent" (it may still
// send more notifications). If you wanted to pass a value, you'd pass a second argument.
notify(
false, // This observable isn't "spent"
mutation.target // Pass along the mutation target element (presumably the button?)
);
};
};
};
// Set up the observer
const config = { childList: true, attributes: true, characterData: true, subtree: true, attributeOldValue: true };
observer.observe(target, config);
});
buttonAppearedObservable.subscribe((spent, button) => {
if (spent) {
// This is a notification that the button appeared event will never happen again
}
if (button) {
// The button appeared!
console.log(`Button "${button.value}" appeared!`);
}
});
// Stand-in code to make a button appear/disappear every second
let counter = 0;
let button = document.querySelector(`[search-model="SearchPodModel"] input[type=button]`);
let timer = setInterval(() => {
if (button.classList.contains("ng-hide")) {
++counter;
} else if (counter >= 10) {
console.log("Stopping the timer");
clearInterval(timer);
timer = 0;
return;
}
button.value = `Button ${counter}`;
button.classList.toggle("ng-hide");
}, 500);.ng-hide {
display: none;
}<!-- NOTE: `search-model` isnt' a valid attribute for any DOM element. Use the data-* prefix for custom attributes -->
<div search-model="SearchPodModel">
<input type="button" class="ej-button rounded-corners arrow-button search-submit holiday-search ng-hide" value="The Button">
</div>All of that is very off-the-cuff. Again, you might look for robust libraries, etc.