Use an index or product ID to find the product and change its quantity. By default, Angular applies the ChangeDetectionStrategy.Default mechanism to the changeDetection attribute. You cannot use both, but at least one is essential. In this chapter, we went into a lot more depth on components, and saw some of the more commonly used attributes when creating components. We can use the code from the previous example where we registered an Input decorator and continue on from there. Take a look at the StockItemComponent code in src/app/stock/stock-item/stock-item.component.ts: We imported the Output decorator as well as the EventEmitter from the Angular library. All other attributes in the Component decorator are optional. This allows you to pull in component-specific CSS, as well as potentially any other common CSS that needs to be applied to it. We will use the base from the input example to build from, so in case you are not coding along, you can take the example from chapter4/component-input to build from there. Unless you export a component, it cannot be accessed or used outside of the direct module where the component is declared. The only reason you might want to split your template code into a separate file is to get nicer IDE features such as syntax completion and the like, which are specific to file extensions. The text was updated successfully, but these errors were encountered: Is there a suggested work around for this? Angular will end up picking one or the other and will lead to unexpected behavior. do you think this is more affordable now with ivy? By the end of the chapter, you should be able to perform most common tasks related to components while understanding what you are doing and why. Say we have a component tree A B C. That is, we have a root component A, which uses a component B in its template, which in turn uses a component C. And lets say component B passes in a composite object compositeObj to component C as input. selector: '[app-stock-item]' would result in the component being used as an attribute on an existing element like in the HTML. This is exactly what we need. Similarly, lets quickly change the main app.component.html file to add another button to trigger the change of the price from the parent component (similar to component B in the earlier hypothetical example): We have added two new buttons to this template: one that will change the reference of the stock object directly, and another that will modify the existing reference of the stock object to change the price from the parent AppComponent. Table4-1 shows the interfaces and functions in the order in which they are called, along with specific details about the step if there is anything to note. Notice again that it is case sensitive and it has to exactly match what member variable we decorated with the Output decorator. Angular has its own HTML parser under the covers that parses the templates for Angular-specific syntax, and does not rely on the DOM API for some of these.

In fact, you can actually tell Angular whether it needs to do this or not, or if the styles can be accessible globally. Clicking Change Price within the StockItemComponent will increase the price of the stock by $5 each time. Uses global CSS, without any encapsulation. As mentioned, the AfterContentInit hook is triggered during component projection cases, and only once during initialization of the component. This can help reduce the build size by compressing your HTML. It also feels like a more organic way of extending components. Lets consider a few examples to see how this might play out. Inheritance of styles and templates not just for javascript classes. If component C makes any changes to any of the attributes of compositeObj, they will also be updated immediately (no change from default behavior). You can of course choose to pass in multiple style strings to the attribute. Your carousel component might also have other features like lazy loading, etc. If there is no projection, this is triggered immediately. For any given component, we can accomplish this by changing the ChangeDetectionStrategy from the default to ChangeDetectionStrategy.OnPush. This is the ideal place to load data from the server and so on, rather than the constructor, both for separation of concerns as well as testability. Projection is an important idea in Angular as it gives us more flexibility when we develop our components and again gives us another tool to make them truly reusable under different contexts. Usually, you wont need it, but if there are certain components where you want to override, or restrict the availability of a class or a service, you can specify an array of providers to a component using the viewProviders attribute. Similarly, we can do the same for the StockItemComponent: We have done exactly the same thing we did on the AppComponent with the StockItemComponent. Note that we are only covering component-specific lifecycle steps, and they are slightly different from a directives lifecycle. The onToggleFavorite now just calls a method on the EventEmitter to emit the entire stock object. This allows us to have the component contain all the information instead of splitting it across HTML and TypeScript code. There are times when the default Angular interpolation markers (the double-curlies {{ and }}) interfere with integrating with other frameworks or technologies. Thus, we will let the parent AppComponent register for the toggleFavorite event and change the state of the stock when the event is triggered. To be able to extend components and keep the template and styles of the extended component. The way I worked around this was to add a static field in the inherited class and add the template to that. in order to avoid this common mistake. For now, it is good to understand the difference between the two change detection strategies that Angular provides. Lets now extend the StockComponent to trigger an event when it is favorited, and move the data manipulation out from the component to its parent. This may not necessarily mean that the views are rendered into the browser, but that Angular has finished updating its internal views to render as soon as possible. DoCheck is Angulars way of giving the component a way to check if there are any bindings or changes that Angular cant or should not detect on its own. Components (and directives) in Angular have their own lifecycle, from creation, rendering, changing, to destruction. Angular will first call the constructor for any component, and then the various steps mentioned earlier in order. One common use case when we start creating components is that we want to separate the content that a component uses from the component itself. A component is truly useful when it is reusable. Projection is useful when we want to build components but set some parts of the UI of the component to not be an innate part of it. Get full access to Angular: Up and Running and 60K+ other titles, with free 10-day trial of O'Reilly.

In the previous case, we can either specify the templateUrl as: and it would work. To allow for this, we have replaced the buttons with an ng-content element. Terms of service Privacy policy Editorial independence. We will use these functions to demonstrate the behavior of the change detection in the context of our application.

View providers allow you to define providers that inject classes/services into a component or any of its children. ContentChildren is any child component that gets projected into the view of the component, but is not directly included in the template within the component. We use data binding to pass data in, and we use event binding syntax to register for events. In these cases, we can use the exportAs attribute of the Component decorator. But there are use cases (especially when we start dealing with directives and more complex components) for which we might want to allow the user of the component to call functions on the component from outside. Imagine something like a carousel, where the functionality is encapsulated in the component, but the view, which could be images or pages of a book, comes from the user of the component. The selector takes a string value, which is the CSS selector Angular will use to identify the element. The value that we pass to it is the reference of the object in the AppComponent class, which is stockObj. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. If you click the button in the first stock widget, you will see that the method in the AppComponent gets called and the console.log is triggered. Before we go into the details of the lifecycle of a component, lets quickly sidetrack into how components are linked to modules and what their relation is. The imports attribute allows you to specify modules that you want imported and accessible within your module. Change its template to use NgFor to create a ProductItemComponent for each product. You can read more about this feature in the official documentation. Again, just like templates, Angular will not pull in these styles at runtime, but rather precompile and create a bundle with the necessary styles. Well occasionally send you account related emails. Get Mark Richardss Software Architecture Patterns ebook to better understand how to design componentsand how they should interact. A carousel has a few simple capabilities: it is able to display an item, and allow us to navigate to the next/previous element. The decision between using styles and styleUrls is one of personal preference and has no impact on the final performance of the application. A use case might be that we provide a carousel component, but want to provide functionality to allow the user of the component to control the next/previous functionality. We will also change stock-item.component.html to call the onToggleFavorite method instead of toggleFavorite. All of this can be accomplished using concepts covered in this chapter.

Writing a component library/framework on top of Angular is hard. We use the Output decorator to accomplish this. Either do it inline or do it in the constructor as we did earlier.

There is also one more concept to learn, which we will briefly touch upon in this chapter, and come back to later in more detailthe concept of ViewChildren and ContentChildren. Any update? Modify the app.component.html file as follows: We have added two instances of the app-stock-item component in our HTML. Move the increment/decrement logic from the ProductItem to the ProductListComponent. I'm currently in need of doing the exact same thing. Also, to get access to the value emitted by the component, we use the keyword $event as a parameter to the function. I have a few websites with the same components ("shared components"), but a few need some extra logic, markup or styles. This is because the change originates from component C (no change from default behavior). This is but one use case where we might want to use projection in our components. Note that as a result of moving it to template, we have removed the previous templateUrl attribute. Both of these take an array as an input.

Each component in turn has some behavior and a template that gets rendered. We will add all of these hooks to both our AppComponent and the StockItemComponent, with a simple console.log to just see when and how these functions are executed. Finally, lets tie it all together by subscribing to the new output from our StockComponent in the app-component.html file: We just added an event binding using Angulars event-binding syntax to the output declared in the stock-item component. In case it is not a component that you wrote, make sure that you have imported the module that provides/exports the component. This is because the styles applied on the AppComponent are not restricted to just the component but are now taking the global namespace. We have been working so far by using the component classs functions within the context of the template. You should see, in order of execution: First, the AppComponent gets created. To do this, we use the changeDetection attribute, where we can override the default value of ChangeDetectionStrategy.Default to ChangeDetectionStrategy.OnPush. There are times when these lifecycle events are useful to us in developing our application, so Angular provides hooks into this lifecycle so that we can observe and react as necessary. Now if we refresh our application, you will see that the name of the stock has been blown up to 50px. Note also that the name of the function is not relevant, and you could name it whatever you want. Now, I copy styles and this approach is not very good related to keeping my project up to date. But one thing that is not the purview of the carousel component is the content it displays. For a very large application, you will have lots of bindings on a given page.

Angular will not check the components bindings automatically.

The EventEmitter can be typed for additional type safety.

@totyl not exact duplicate. Lets talk about the attributes of the decorator in a bit more detail. Angular provides ways for us to signal when to check the bindings from within the component as well, to have absolute control on Angulars data binding. You can set this by using the encapsulation attribute on the Component decorator. In this chapter, we will go a bit deeper into components, those elements we have been creating to render the UI and let users interact with the applications we build. To accomplish this, it provides its own DSL, which allows Angular to animate on state changes within the element. Without it, the function would still get triggered, but you would not get any arguments with it. Similarly, a CSS rule given to a button in one corner of the page might end up impacting another totally unrelated button. For our third exercise, we can build on top of the previous exercise (chapter3/exercise) by including concepts from this chapter: Create a ProductListComponent. You can set this feature (which is set to false by default) by using the preserveWhitespaces attribute on the component. First, modify the stock-item.component.ts file to change the ChangeDetectionStrategy in the child component: In addition to changing the ChangeDetectionStrategy, we also added another function to changeStockPrice(). The final finished example is available in chapter4/component-projection. HTML, CSS, and JavaScript have a default tendency to be global in the context of the current page. We leverage this usually when we define inline templates. We have simply removed the buttons we previously had, and are going to let the user of the component decide what buttons are to be shown. We register an EventEmitter as an output from any component.

I have the same problem and same request! Bump for visibility. We have added two new methods: changeStockObject(), which creates a new instance of the stock object in the AppComponent, and changeStockPrice(), which modifies the prices of the stock object in the AppComponent. Now say we marked component Cs changeDetection attribute as ChangeDetectionStrategy.OnPush.

Lets see how our stock-item component might look with an inline template: ECMAScript 2015 (and TypeScript) allows us to define multiline templates using the ` (backtick) symbol, instead of doing string concatenation across multiple lines using the + (plus) operator. Maybe something like: That is, inputToC is the input variable marked with the Input decorator in component C, and is passed the object compositeObj from component B. This means that after the initial render, it will be up to us to let Angular know when the value changes. View all OReilly videos, Superstream events, and Meet the Expert sessions on your home TV.

Finally, we covered projection in components and how we can make some truly powerful components that allow the user of the component to decide parts of the UI. First, we can modify the src/app/app.component.ts file and add the hooks as follows: You can see that we have implemented the interfaces for OnInit, OnChanges, OnDestroy, DoCheck, AfterContentChecked, AfterContentInit, AfterViewChecked, AfterViewInit on the AppComponent class, and then went ahead and implemented the respective functions. In one, we have a button that triggers the testMethod we added in the AppComponent, and the other simply has text content. This means that all listeners of the toggleFavorite event will get the current stock object as a parameter. There are three specific attributes on the NgModule that directly impact components and their usage, which are important to know. (This is why we added the counter!). In Chapter2, we saw how any time we created a new component, we had to include it in a module. The code for the stock-item.component.ts file should look like the following: We have removed all instantiation logic from the app-stock-item component, and marked the stock variable as an input. This will only work on browsers and platforms that natively support it. When a user takes any action, you as a developer might know for sure that most of the page will not change. If you want to use a component from other modules, make sure you import the relevant modules into the module you have declared and where the component exists. In the preceding section, we talked about how Angular encapsulates the styles to ensure that it doesnt contaminate any of your other components. There is no code change in our component class; we only need to modify the src/app/stock/stock-item/stock-item.component.html file as follows: The new ng-content element for projection. Check, in order, the following: Whether the component is added as a declaration in the module. Generally, you might want to keep your templates separate if they are over three or four lines or have any complexity. We took a detailed look at the Component decorator, talking about attributes like template versus templateUrl, styles, and also covered at a high level how Angulars change detection works and how we can override it. 2022, OReilly Media, Inc. All trademarks and registered trademarks appearing on oreilly.com are the property of their respective owners. I have a angular library (ng generate library shared) with a BaseComponent in, I want to be able to extend that component in another project and keep the styles of both the BaseComponent and the new component (AnotherComponent). What this means is that an ID given to an element can easily clash with another element somewhere else on the page. Thus, any element that adds the name class to itself will get this font-size applied to it. A given component can have multiple styles attached to it. Lets see how we can extend our stock-item component from the previous chapter to allow us to pass in the stock object, rather than hardcoding it within the component itself. In such cases, you can actually give a hint to the Angular change detector to check or not check certain components as you see fit. Thus, users of the component now have the capability to change part of the UI of the component as they see fit. Have a question about this project? It is a good place to do all cleanup, like unsubscribing any listeners you may have initialized and the like. You can make the selector as simple or complex as you want, but as a rule of thumb, try to stick to simple element selectors unless you have a very strong reason not to. We mentioned changeDetection as an attribute on the Component decorator. This ensures that the component is visible to all other components within the module. One of the ways we can make a component reusable (rather than having default, hardcoded values inside it) is by passing in different inputs depending on the use case.

This shows that the model is getting updated, but Angular is not updating the view. It is called before the ngOnInit method. Then the following hooks are triggered on the AppComponent: The preceding two immediately execute because we dont have any content projection in our application so far. Instead, Angular precompiles a build and ensures that the template is inlined as part of the build process. If you want to code along and dont have the previous code, you can use the chapter3/ng-if codebase as the starter to code along from. Each of these lifecycle steps comes with an interface that should be implemented when a component cares about that particular lifecycle, and each interface provides a function starting with ng that needs to be implemented. First, we will modify our StockItemComponent to allow for content projection. Lets see how we might use content projection in our Angular application.

Change the ProductItemComponent to take the product as an input.

You might wonder how this is even possible. AfterViewChecked is triggered each time after all the child components have been checked and updated. While the official Angular documentation is spare on this topic, there is a great article that can give you more insight on content projection. With this, if you run the application (remember, ng serve), you should see the fully functional app, and when you click the Add to Favorite button, it should trigger the method in the AppComponent. The attribute name is case sensitive, so make sure it matches exactly with the input variable name. If you created a new component that needs to be used in other components, make sure that you export the component in its module so that any application including the module will get access to your newly created component. I don't want to open up my encapsulation. When you first start out building Angular applications, you might easily forget to add your newly created components to the declarations attribute, so keep track of that (if you are not using the Angular CLI, that is!) It provides the ability to have scoped styling to a component (thus preventing the styles from leaking out and affecting the rest of the application) and also the ability to isolate and make the DOM self-contained. So in our case, app-stock-item would be a ViewChild of the AppComponent. The Angular CLI will automatically add your component or directive to the module when you create a component through it. With this in place, now lets see how we can use our updated StockItemComponent and use the power of projection. Move the ProductItemComponent to be optimal and move from the default ChangeDetectionStrategy to an OnPush ChangeDetectionStrategy.