Angular Directive - introducing a fresh approach to two-way binding and enable "pass-by-value" functionality

In a previous question, I inquired about the possibility of incorporating an attribute on a directive to allow for values to be passed in various formats, such as:

<my-directive att>                //Evaluates to true
<my-directive att="true">        
<my-directive att="false">
<my-directive att="51">
<my-directive att="51.234">
<my-directive att="'john smith'">  

Alternatively, is it possible to use two-way binding with a variable on the controller's scope like this:

<my-directive att="SomeVariableOnControllerScope"> 

Standard "=" two-way binding doesn't work in this scenario as attempts to change something within the directive will result in an error when trying to write back to the variable. However, there has been mention of creating a new type of binding that can intelligently interpret the value being passed and determine whether it is a boolean, integer, float, or string enclosed in single quotes. This new binding would essentially "switch off" two-way binding if the value fits these criteria, ensuring nothing gets written back. If the value does not match any of these types, then standard two-way binding would be implemented.

I am unsure of how to approach this, but with some guidance, I believe I could figure it out. The main benefit of this concept is that we often need to output HTML from the server (for SEO purposes) without the necessity of binding to a controller. However, there are instances where two-way binding is required.

In essence, I am seeking a hybrid between "@" and "=" bindings that can dynamically ascertain whether a value or variable name is being supplied.

Any suggestions?

Answer №1

1/ The $parse function within AngularJS can analyze a value and determine if it is a constant.

https://docs.angularjs.org/api/ng/service/$parse

2/ This code is utilized for bidirectional data binding on isolated scopes:

        var dataBind = function(parentScopeName, scopeName, childScope, parentScope) {
        if(parentScope == null) {
            parentScope = childScope.$parent;
        }
        var parentGet = $parse(parentScopeName);
        var compare = parentGet.literal ? angular.equals : function(a,b) { return a === b; };
        var lastValue;

        var parentSet = parentGet.assign || function() {
            // reset the change, or we will throw this exception on every $digest
            lastValue = childScope[scopeName] = parentGet(parentScope);
            throw "didnt understand this exception";
        };
        lastValue = childScope[scopeName] = parentGet(parentScope);
        return childScope.$watch(function parentValueWatch() {
            var parentValue = parentGet(parentScope);
            if (!compare(parentValue, childScope[scopeName])) {
                // we are out of sync and need to copy
                if (!compare(parentValue, lastValue)) {
                    // parent changed and it has precedence
                    childScope[scopeName] = parentValue;
                } else {
                    // if the parent can be assigned then do so
                    parentSet(parentScope, parentValue = childScope[scopeName]);
                }
            }
            return (lastValue = parentValue);
        }, null, parentGet.literal);
    }

By combining this approach with the $parse service, you have flexibility in your application (excluding the use of isolated scope '=' or '@'):

var parsed = $parse($attrs.myAttribute);
if(parsed.constant) {
     $scope.whereIWantMyConstantInChildScope = parsed();
} else {
     dataBind($attrs.myAttribute, "whereIWantMyConstantInChildScope", $scope); // 4rth param is optional, will fallback to parent scope.
}

This technical solution is effective. However, I believe a better practice would involve utilizing separate attributes for constants versus bindings to enhance clarity and avoid unnecessary complexity in your isolated-scope directive linking logic.

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