Is there a way to access the watchExpression inside the listener function? My goal is to extract parameters from the watchExpression.
$watch(watchExpression, [listener], [objectEquality]);
Is there a way to access the watchExpression inside the listener function? My goal is to extract parameters from the watchExpression.
$watch(watchExpression, [listener], [objectEquality]);
Imagine you are interested in monitoring a model on scope that is defined as:
$scope.model = {firstName: 'John', lastName: 'Doe'};
There are 3 different ways to write the watcher below:
1. Watching Values
$scope.$watch('model', function(value) {
// Listens for any changes to each item
// The watcher will fire whenever firstName or lastName changes
console.log(value.firstName);
console.log(value.lastName);
});
2. Monitoring New and Old Values
$scope.$watch('model', function(newVal, oldVal) {
// Retrieves references of the new and old values of model
// Normally checks for object reference equality
// if (newVal === oldVal)
console.log(newVal.firstName);
console.log(oldVal.firstName);
});
3. Directly Accessing from Scope
$scope.$watch('model', function() {
// Listens for any changes to each item
// Directly accesses from scope
// It triggers only when a change occurs in the model
console.log($scope.model.firstName);
});
You can also deep watch an object if necessary, like so:
$scope.$watch('model', function(newVal, oldVal) {
// Watches for changes deeply
}, true);
Deep watching/object equality tracks changes from the object graph to the root node. For instance, for a model such as:
$scope.model = { firstName: 'John', lastName: 'Doe', address: { zip: 1234 } };
The watch would also trigger for changes to zip in address.
You can watch not just an object but also specify a function whose return value can be monitored. The function takes the scope as a parameter by default:
$scope.$watch(function(scope) {
return scope.model.firstName;
}, function(newVal, oldVal) {
// Listens only to changes in the first name
console.log(newVal.firstName);
console.log(oldVal.firstName);
});
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