Introduction to Angular Signals

What are signals and why are they important in Angular?

Angular, starting from version 16, has introduced a new feature called signals. In this section of the complete Angular course, we will delve into the concept of signals, its uses, and why you might want to consider incorporating signals into your Angular application. We will also explore how to create and use signals, as well as important APIs provided by signals.

Note:

• Signals are currently in developer preview and are expected to be stable in Angular version 17.
• The syntax and feature set around signals are subject to change at this stage.
• Patterns and best practices around signals are still evolving.

Understanding Signals and their Purpose

To begin, let's grasp the concept of signals and why Angular has introduced them.

By definition, a signal is a wrapper around a value that can notify interested consumers when the value changes. Signals are capable of containing any type of value, ranging from simple primitive values to complex data structures such as arrays or objects.

However, if this definition seems unclear at the moment, don't worry. It will become clearer as we proceed to create and use signals in our Angular application.

Now, you may be wondering why Angular introduced the signal feature. Without signals, an Angular application relies on the change detection cycle to detect changes in data and update the user interface accordingly.

To illustrate this, let's consider an example. We have a simple Angular project in which we have a component called SignalsComponent. This component contains an HTML template with two buttons, a counter property, and a message property.

When the plus or minus button is clicked, the respective increment and decrement functions are called, modifying the counter value. The updated value is then displayed in the UI.

Angular achieves this by running a change detection cycle. Each time a button is clicked, Angular checks if any of the properties have changed, and if so, it updates the UI.

However, one drawback of this approach is that the change detection cycle is triggered even when the value doesn't actually change. This can impact the performance of the application. Additionally, Angular relies on the zone.js library to run the change detection cycle, which can affect the size of the bundled application.

Introduction to Signals in Angular

In Angular, change detection is a crucial process for updating the user interface (UI) when the data changes. However, this process is often associated with some drawbacks, such as the reliance on external libraries like Zone.js, increased bundle size, and lack of precision in identifying the specific component or property that has changed.

To address these issues, Angular has introduced signals in version 16. Signals provide an alternative approach to managing and detecting changes in data, resulting in improved performance and a smaller bundle size. In this article, we will explore the benefits of using signals and how to create and use them in an Angular application.

Bundles and Zone.js

Before delving into signals, let's briefly discuss the role of bundles and the Zone.js library in an Angular application. When we run an Angular application, it downloads several bundles and files containing CSS styles, JavaScript code, HTML markup, and third-party libraries. One such external library is Zone.js, which is essential for running the change detection cycle.

However, the reliance on Zone.js comes with some disadvantages. Firstly, it increases the bundle size of the Angular application. Secondly, Zone.js does not provide precise information about which component or property has changed. As a result, Angular has to re-render all components in the component tree, even if the change has occurred only in one component.

Introducing Signals in Angular 16

To overcome the limitations of the change detection mechanism, Angular has introduced signals in version 16. Signals offer a way to improve performance and reduce bundle size by eliminating the need for Zone.js. With signals, we no longer rely on the change detection cycle to detect changes and update the UI.

Instead, developers have more control over when and where the data changes, allowing Angular to selectively update only the relevant parts of the UI. This targeted approach ensures that only the necessary components are rendered, resulting in an enhanced performance.

Benefits and Precautions

While using signals provides better control and improved performance, it is essential to note that signals are still in the developer preview stage and not completely stable yet. Therefore, the benefits discussed here can be fully realized only when signals become stable and are fully integrated into an Angular application.

It is crucial to mention that fully switching to signals in an Angular application is necessary to leverage the advantages they offer. If even a single component relies on the traditional change detection approach, the benefits of signals may not be fully realized.

Creating and Using Signals

To harness the power of signals, you need to have a minimum Angular version 16 or a later version installed. To create a signal, import the signal function from @angular/co and use it as follows:

import{signal}from'@angular/co';classSignalComponent{counter=signal(0);}

In the example above, we create a signal named counter and initialize it with an initial value of 0. Note that the counter variable now holds a signal object instead of a number.

To use the signal in the UI, you need to replace the previous property binding with a signal binding. For instance:

<p>Counter: {{ counter }}</p>

With the signal in place, whenever the value of the counter signal changes, Angular will be notified, allowing it to update the UI with the updated value.

Updating Signal Values with Set, Update, and Mutate Methods

In the previous lecture, we learned how to create and use signals in Angular. Signals are powerful tools for managing and updating values in your application. In this lecture, we will explore different methods for updating signal values: set, update, and mutate.

Set Method

The set method is used to set a new value for a signal. This method is suitable when the new value does not depend on the previous value of the signal. To use the set method, simply call it on the signal object and pass the new value as an argument.

counterSignal.set(0);

For example, let's say we have a counter signal with an initial value of 0. We want to increment the value by 1 every time the plus button is clicked. To achieve this, we can call the set method on the counter signal and pass the new value as 1.

increment(){counterSignal.set(1);}

The advantage of the set method is that it allows you to easily set a new value for a signal without considering its previous value. However, if you need to update the value based on its previous value, you should use the update method instead.

Update Method

The update method is used to update a signal value based on its previous value. This method receives the previous value of the signal as an argument in the form of a callback function. Inside the callback function, you can perform any logic to calculate the new value and return it.

counterSignal.update((prevValue)=>prevValue+1);

Using the counter signal example, if we want to increment the value based on its previous value, we can call the update method on the counter signal and provide a callback function that takes the previous value as an argument. Inside the callback function, we add 1 to the previous value and return the result.

increment(){counterSignal.update((prevValue)=>prevValue+1);}

By using the update method, the value of the counter signal will be updated correctly each time the plus button is clicked. This method is ideal when the new signal value depends on its previous value.

Mutate Method

The mutate method is another way to update a signal value. Similar to the update method, the mutate method also allows you to update the value based on its previous value. However, instead of using a callback function, the mutate method uses a signal mutation function.

counterSignal.mutate((value)=>value+1);

Using the counter signal example, if we want to increment the value based on its previous value, we can call the mutate method on the counter signal and provide a mutation function. The mutation function takes the previous value as an argument and returns the new value.

increment(){counterSignal.mutate((value)=>value+1);}

The mutate method is similar to the update method in terms of functionality, but it provides a different syntax. You can choose to use either method based on your preference or coding style.

Using the Mutate Method to Update a Signal Value

In the previous lecture, we learned about the set and update methods for updating a signal value. These methods allowed us to modify the value of a signal in our code. However, in this lecture, we are going to focus on another method called mutate, which can also be used to update a signal value.

Overview of the Mutate Method

The mutate method works similarly to the update method with one key difference; it can only be used on mutable values. Mutable values, such as arrays and objects, allow their values to be changed, unlike value types such as numbers, strings, and booleans, which are immutable. When we assign a new value to a mutable variable, a new value is created in memory and the variable points to that new value. However, for immutable types, a new value is created in memory, and the variable points to this new value, while the previous value remains unchanged.

For example, if we create a variable a and assign it a numeric value of 10, the identifier a will be stored in memory and point to the value 10. If we later change the value of a to 20, a new value of 20 will be created in memory, and the variable a will point to this new value. The previous value, 10, remains unchanged. This behavior applies to numbers, strings, and booleans, making them immutable types.

On the other hand, arrays and objects are mutable types. When we modify their values, the changes are made directly to their existing references in memory. For example, if we create an array with three elements, a reference to this array will be stored in memory. If we then add a new element to the array, the original array is modified, and the new value is inserted without creating a new array.

Using the Mutate Method

When using the mutate method, we need to ensure that we have a mutable type, such as an array or an object. We should use the update method when working with immutable types like numbers, strings, or booleans and reserve the mutate method for mutable types like arrays and objects.

To better understand the difference, let's use it practically in our code. In the previous lecture, we used the update method to increment the value of a counter signal by one. Now, let's extend our code to use the mutate method to decrement the counter value by one.

decrement(){this.counter.mutate((prevCounterValue)=>prevCounterValue-1);}

In this code snippet, we call the mutate method on the counter signal and pass a callback function as an argument. This callback function receives the previous value of the counter signal, referred to as prevCounterValue, and decrements it by one. The mutate method modifies the value of the counter signal directly without creating a new signal or value.

Updating a Signal with the Mutate Method

Let's apply the mutate method to update another property, message, which stores an array. Previously, we assigned a value to the message property directly, but now we want to change it to a signal.

message=signal<string[]>([]);

Here, we use the signal function to create a new signal for the message property and initialize it with an empty array. Note that we specify the type of the signal as string[] to indicate that it is an array of strings.

Next, instead of directly assigning a value to the message property, we need to use the signal function to access the value inside the signal.

this.message(()=>{// Accessing the value inside the signal});

To update the message signal, we can use the mutate method as follows:

this.message.mutate((prevMessage)=>[...prevMessage,"New element"]);

The mutate method receives a callback function that takes the previous value of the message signal, referred to as prevMessage, and returns a new array that includes the previous elements (...prevMessage) and an additional element ("New element").

By using the mutate method, we can now add or remove elements from the message array without creating a new array each time.

The Difference Between Update and Mutate Methods

One important aspect to note in Angular is the difference between the update and mutate methods when working with signals.

When we use the update method, clicking the plus button creates a new array and extracts the elements from the previous array before adding the new element. However, this approach is not ideal because we don't want to create a new array every time the button is clicked. Instead, we can use the mutate method to avoid this behavior. By calling the mutate method on the signal, we pass a callback function that receives the previous value of the signal. In this case, we call it prevMessage. To achieve the desired behavior, we simply push the new element to the previous array.

this.message.mutate((prevMessage)=>{prevMessage.push("string value");});

By using the mutate method, we are able to push a new element to the existing array and update the UI without creating a new array every time the button is clicked.

It's worth noting that the update method, although applicable to mutable types like arrays, does not actually mutate the value. Instead, it creates a new value by overriding the existing one. Hence, when dealing with mutable types, it's important to use the mutate method for proper value mutation.

Removing Elements with the Decrement Method

To illustrate the difference between the update and mutate methods further, let's apply the same logic to the decrement method. Instead of pushing a new element, we want to remove the last element from the array. Using the mutate method, we can achieve this by removing the line that pushes an element and instead remove the last element from the array.

this.message.mutate((prevMessage)=>{prevMessage.pop();});

By removing the last element from the array, the array gets updated, and Angular recognizes that the message signal has changed. As a result, the UI is updated accordingly.

Computed Functions for Calculations

Another feature worth exploring is the computed function. By importing it from angular/Co, we can use it to compute a value from a signal.

Suppose we create a new property called doubleCounter and assign the computed function to it:

import{computed}from"angular/Co";...this.doubleCounter=computed(()=>{returnthis.counter()*2;});

In this example, the computed function receives a callback function that computes a new value from the counter signal. The resulting value is stored in the doubleCounter signal. Whenever the counter signal changes, the doubleCounter value is re-evaluated.

To demonstrate this in the view template, we can replace the display of the counter signal with the doubleCounter value:

<p>The value of doubleCounter signal is: {{ doubleCounter() }}</p>

Now, whenever the counter signal changes, the doubleCounter value is updated and displayed in the UI.

Executing Code on Signal Changes with the Effect Function

The effect function is used when we want to execute code whenever the value of a signal changes. It is handy for performing additional logic after a signal has been updated.

For example, let's say we want to execute some code whenever the counter signal changes. We can achieve this by using the effect function in the constructor or any lifecycle hook:

import{effect}from"angular/Co";...constructor(){effect(()=>{console.log("New counter value is "+this.counter());});}

In this example, the effect function receives a callback function that logs the new value of the counter signal whenever it changes. Hence, each time the counter signal is updated, the console will display the corresponding message.

Using the effect function allows us to execute additional logic based on signal changes without modifying the value of the signal itself.

Signals in Angular: A New Way to Handle Change Detection

In this section, we will explore the concept of signals in Angular and how they provide a new way to handle change detection. Signals offer several advantages over the traditional change detection mechanism, including improved performance and greater control over when changes are detected.

What are Signals?

Signals in Angular are a powerful tool for managing change detection. They allow developers to define a code that depends on a signal, and Angular will automatically re-execute the callback function associated with the effect whenever the value of the signal changes.

Using Signals in Code

Let's take a closer look at how signals are used in code. When a signal changes, the effect function that is passed to it as a callback will be called. For example, if we have a counter signal and we want to log a message whenever its value changes, we can define an effect function that logs the new value of the counter whenever it changes.

constcounterSignal=createSignal(0);// Create a new signal with an initial value of 0consteffect=createEffect(()=>{console.log(`New counter value is ${counterSignal.value}`);});// Whenever the counter signal changes, the effect function will be called

Benefits of Using Signals

Using signals in Angular provides several benefits. Firstly, signals improve performance by allowing more control over when changes should be detected. Instead of triggering change detection for every little change in the application, signals enable developers to selectively detect changes only when necessary.

Additionally, signals offer greater flexibility in handling change detection. Developers can define specific code to be executed when a signal changes, instead of relying on Angular's default change detection behavior.

Future Enhancements

While signals in Angular are still in development and not yet final and stable, there are exciting future enhancements to look forward to. One such enhancement is the introduction of signal-based components. This new feature, currently in the RFC process, will allow the creation of components based on signals.

To create a signal-based component, the component decorator will include a signal property set to true. Although this syntax is not yet functional in Angular 16, it is likely to be added in future versions.

If you are interested in learning more about the development of signals in Angular, you can explore the Angular Signal RFCs on GitHub. These Request for Comments documents provide insights into the ongoing development and future features related to signals.

Understanding signals and their purpose in angular is crucial for maximizing the performance of your application. in the next part of this series, we will explore how to create and use signals in your angular application. stay tuned!

check out the video below for a visual explanation of angular signals:

Signals offer an alternative approach to managing and detecting changes in data in angular applications. by utilizing signals, developers can achieve better performance and reduce the bundle size of their application. though still in the developer preview stage, signals hold great promise for improving the overall efficiency of angular applications.
In this part of the article, we delved into the different methods for updating signal values in angular: set, update, and mutate. these methods provide flexibility in managing and updating signal values based on various requirements. by understanding how to use each method effectively, you can harness the power of signals in your angular applications.
In this lecture, we explored the mutate method, which allows us to update signal values for mutable types such as arrays and objects. we learned that using the update method on mutable types can lead to errors since it attempts to create a new value instead of modifying the existing one. by using the mutate method, we avoided this issue and efficiently updated the value of the counter and message signals.

note: in the next lecture, we will dive deeper into the concept of signals and explore advanced techniques for working with them.
Understanding the concepts of the update, mutate, computed, and effect functions in angular provides us with powerful tools to manage and react to changes in signals effectively. by utilizing these functions appropriately, we can ensure proper signal mutation, compute values from signals, and execute code on signal changes.

In conclusion, signals in Angular offer a new way to handle change detection, providing improved performance and greater control. While still in development, signals show promise for offering a more efficient and flexible approach to change detection in Angular applications.

Stay tuned for more updates and new features related to signals, as Angular continues to refine and enhance this exciting feature. If you have any questions or feedback, please feel free to reach out. Thank you for reading and have a great day!