guarantees optimal functionality exclusively when paired with the then function

Question

guarantees optimal functionality exclusively when paired with the then function

I have a button that triggers a function which uses a promise to fetch and display data from Firebase in an HTML environment (I'm incorporating AngularJS, Ionic, and Firebase).

The issue is this: if I do not include a .then(function(){}) right after it, the promise runs asynchronously, requiring me to click the button again for the data to show up on the webpage.

I aim to place the data into the scope following the promise that retrieves data from Firebase. Strangely enough, it only seems to work when I add a .then function afterwards.

However, the data appears normally in the console but does not reflect in the HTML (which makes me suspect that the function isn't properly attached to the scope).

Below is the code snippet:

$scope.displayChat = function () {
    var userId = firebase.auth().currentUser.uid; // Get ID
    var deferred = $q.defer()

    var db = firebase.database();
    var ref = db.ref("12346787");

  ref.on("value", function(snapshot) {
           console.log(snapshot.val());
           $scope.chatArray = snapshot.val();
           deferred.resolve()

       }, function (errorObject) {
           console.log("The read failed: " + errorObject.code);
       })

   return deferred.promise.then(function(){
        // Removing this empty .then(function(){}) function
        // will result in asynchronousity.
        // just "return deferred.promise;" doesn't work.
    })
}

Any suggestions or solutions? I am relatively new to promises so I haven't found any related resources. Thanks.

javascript angularjs firebase ionic-framework firebase-realtime-database

Answer 1

Answer №1

Why do we use promises? They are essential for handling asynchronous methods, but it seems like some people still struggle to grasp their importance...

In the displayChat function, the code should be executed first and then the callback function. By returning a promise, you can ensure that the callback only runs once the necessary asynchronous tasks have been completed.

Answer 2

Why do we use promises? They are essential for handling asynchronous methods, but it seems like some people still struggle to grasp their importance...

In the displayChat function, the code should be executed first and then the callback function. By returning a promise, you can ensure that the callback only runs once the necessary asynchronous tasks have been completed.

Answer 3

Answer №2

When external events trigger changes to the scope in AngularJS, the framework must perform an $apply to start a digest cycle and update the DOM.

https://i.sstatic.net/TXJiX.png
_{(source: angularjs.org)}

The .then method of a $q service promise automatically triggers the necessary digest cycle. When the promise returned by the displayChat function is not utilized, no digest cycle is initiated until a subsequent button click occurs.

In future scenarios, it may be beneficial to chain from the promise generated by the displayChat function. I suggest enhancing the function's versatility by returning a proper promise and relocating any scope modifications within a .then method.

$scope.displayChat = function () {
    var userId = firebase.auth().currentUser.uid; // Get ID
    var deferred = $q.defer()

    var db = firebase.database();
    var ref = db.ref("12346787");

    //ref.on("value", function(snapshot) {
    //USE once
    ref.once("value", function(snapshot) {
           console.log(snapshot.val());
           //$scope.chatArray = snapshot.val();
           deferred.resolve(snapshot.val());

       }, function (errorObject) {
           console.log("The read failed: " + errorObject.code);
           //ADD rejection case
           deferred.reject(errorObject);
       })

   return deferred.promise.then(function(chatArray){
       //Move scope changes here
       $scope.chatArray = chatArray;
       //RETURN to chain data
       return chatArray;

       // Removing this empty .then(function(){}) function
       // will result in asynchronousity.
       // just "return deferred.promise;" doesn't work.
   })
}

To prevent memory leaks, opt for ref.once over ref.on and ensure to reject the promise in error scenarios.

Answer 4

When external events trigger changes to the scope in AngularJS, the framework must perform an $apply to start a digest cycle and update the DOM.

https://i.sstatic.net/TXJiX.png
_{(source: angularjs.org)}

The .then method of a $q service promise automatically triggers the necessary digest cycle. When the promise returned by the displayChat function is not utilized, no digest cycle is initiated until a subsequent button click occurs.

In future scenarios, it may be beneficial to chain from the promise generated by the displayChat function. I suggest enhancing the function's versatility by returning a proper promise and relocating any scope modifications within a .then method.

$scope.displayChat = function () {
    var userId = firebase.auth().currentUser.uid; // Get ID
    var deferred = $q.defer()

    var db = firebase.database();
    var ref = db.ref("12346787");

    //ref.on("value", function(snapshot) {
    //USE once
    ref.once("value", function(snapshot) {
           console.log(snapshot.val());
           //$scope.chatArray = snapshot.val();
           deferred.resolve(snapshot.val());

       }, function (errorObject) {
           console.log("The read failed: " + errorObject.code);
           //ADD rejection case
           deferred.reject(errorObject);
       })

   return deferred.promise.then(function(chatArray){
       //Move scope changes here
       $scope.chatArray = chatArray;
       //RETURN to chain data
       return chatArray;

       // Removing this empty .then(function(){}) function
       // will result in asynchronousity.
       // just "return deferred.promise;" doesn't work.
   })
}

To prevent memory leaks, opt for ref.once over ref.on and ensure to reject the promise in error scenarios.

Answer 5

Answer №3

Deferred execution of logic using promises is a common practice, allowing the code to wait until certain conditions are met, such as receiving data from a database. In this case, the promise code seems redundant as the console already displays the results once received. Updating data in the scope may require triggering a digest cycle, which Angular usually handles automatically. However, you can force a digest cycle by wrapping scope-related logic in a timeout function.

$scope.displayChat = function () {
var userId = firebase.auth().currentUser.uid; // Get ID

var db = firebase.database();
var ref = db.ref("12346787");

ref.on("value", function(snapshot) {
       console.log(snapshot.val());
       $timeout(function () {
            $scope.chatArray = snapshot.val();
       });

   }, function (errorObject) {
       console.log("The read failed: " + errorObject.code);
   })

Your use of the promise's .then method triggers a digest cycle, but doesn't add much value in this scenario.

If you intend to pass the snapshot back to the caller, a promise can be useful:

$scope.displayChat = function () {
    var userId = firebase.auth().currentUser.uid; // Get ID
    var deferred = $q.defer()

    var db = firebase.database();
    var ref = db.ref("12346787");

    ref.on("value", function(snapshot) {
            deferred.resolve(snapshot)

       }, function (errorObject) {
           console.log("The read failed: " + errorObject.code);
       })

   return deferred.promise;
};

$scope.callDisplayChat = function () {
    $scope.displayChat().then(function (result) {
        $scope.chatArray = result.val();
    });
};

Answer 6

Deferred execution of logic using promises is a common practice, allowing the code to wait until certain conditions are met, such as receiving data from a database. In this case, the promise code seems redundant as the console already displays the results once received. Updating data in the scope may require triggering a digest cycle, which Angular usually handles automatically. However, you can force a digest cycle by wrapping scope-related logic in a timeout function.

$scope.displayChat = function () {
var userId = firebase.auth().currentUser.uid; // Get ID

var db = firebase.database();
var ref = db.ref("12346787");

ref.on("value", function(snapshot) {
       console.log(snapshot.val());
       $timeout(function () {
            $scope.chatArray = snapshot.val();
       });

   }, function (errorObject) {
       console.log("The read failed: " + errorObject.code);
   })

Your use of the promise's .then method triggers a digest cycle, but doesn't add much value in this scenario.

If you intend to pass the snapshot back to the caller, a promise can be useful:

$scope.displayChat = function () {
    var userId = firebase.auth().currentUser.uid; // Get ID
    var deferred = $q.defer()

    var db = firebase.database();
    var ref = db.ref("12346787");

    ref.on("value", function(snapshot) {
            deferred.resolve(snapshot)

       }, function (errorObject) {
           console.log("The read failed: " + errorObject.code);
       })

   return deferred.promise;
};

$scope.callDisplayChat = function () {
    $scope.displayChat().then(function (result) {
        $scope.chatArray = result.val();
    });
};

guarantees optimal functionality exclusively when paired with the then function

Answer №1

Answer №2

Answer №3

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