In this article, we’ll take a look a bit closer at Angular 2 components — how they’re defined, and how to get data into them and back out of them.
This is the second part in the Angular 2 series. You can read part one here. We covered the basic idea of components and decorators in an earlier article, and have specifically seen the
@View decorators used to build an Angular application. This article dives in a bit deeper. However, we can’t cover everything about components in a single article, so future articles will take up other aspects of Angular 2 components.
As we’ve covered in an earlier article, Angular 2 components are the key building block for Angular applications. They include a view, defined with HTML and CSS, and an associated controller that implements functionality needed by the view. The controller has three major responsibilities:
- Manage the model, i.e. the application data used by the view
- Implement methods needed by the view for things like submitting data or hiding/showing sections of the UI
- Managing data related to the state of the view, such as which item in a list is currently selected.
Depending on your background, the above list might sound familiar. In fact, the Angular component controller sounds very much like the original definition of a view model as defined by John Gossman in 2005:
The term means “Model of a View”, and can be thought of as abstraction of the view, but it also provides a specialization of the Model that the View can use for data-binding. In this latter role the ViewModel contains data-transformers that convert Model types into View types, and it contains Commands the View can use to interact with the Model. — Source (captured 11/27/2015)
@Component decorator we saw in the first article in this series.
The above describes an individual component, but Angular 2 applications are actually made up of a hierarchy of components – they begin with a root component that contains as descendants all the components used in the application. Angular 2 components are intended to be self-contained, because we want to encapsulate our component functions and we don’t want other code to arbitrarily reach into our components to read or change properties. Also, we don’t want our component to affect another component written by someone else. An obvious example is CSS: if we set CSS for one component, we don’t want our CSS to “bleed out” to another components just as we don’t want other CSS to “bleed in” to our component.
At the same time, components do need to exchange data. Angular 2 components can receive data from their parent as long as the receiving component has specifically said it’s willing to receive data. Similarly, components can send data to their parents by trigger an event the parent listens for. Let’s look at how the component hierarchy behaves. To begin, we can draw it as follows:
Each box is a component and technically this representation is called “graph” — a data structure consisting of nodes and connecting “edges.” The arrows represent the data flow from one component to another, and we can see that data flows in only one direction — from the top downwards to descendants. Also, note there are no paths that allow you to travel from one node, through other nodes and back to the one where you started. The official name for this kind of data structure is a “directed acyclic graph” — that is, it flows in only one direction and has no circular paths in it.
This kind of structure has some important features: it’s predictable, it’s simple to traverse, and it’s easy to see what’s impacted when a change is made. For Angular’s purposes, when data changes in one node, it’s easy to find the downstream nodes that could be affected.
A simple example of how this might be used is a table with rows containing customers and information about them, in which a table component contains multiple individual row components that represent each customer. The table component could manage a record set containing all the customers and pass the data on an individual customer to each of the row components it contains.
This works fine for simply displaying data, but in the real world data will need to flow the other way — back up the hierarchy — such as when a user edits a row. In that case, the row needs to tell the table component that the data for a row has changed so the change can be sent back to the server. The problem is that, as diagrammed above, data only flows down the hierarchy, not back up. To ensure we maintain the simplicity of one-way data flow down the hierarchy, Angular 2 uses a different mechanism for sending data back up the hierarchy: events.
Now, when a child component takes an action that a parent needs to know about, the child fires an event that’s caught by the parent. The parent can take any action it needs which might include updating data that will, through the usual one-way downwards data flow, update downstream components. By separating the downward flow of data from the upwards flow of data, things are kept simpler and data management performs well.
Angular 2 Components: Inputs and Outputs
With that high-level look at Angular 2 components under our belt, let’s look at two properties that can be passed to the
@Component decorator to implement the downward and upward flow of data: “inputs” and “outputs.” These have sometimes been confusing because, in earlier version of the Angular 2 alpha, they were called “properties” (for “inputs”) and “events” (for “outputs”) and some developers were less than enthralled with the name change, though it does seem to make sense: https://github.com/angular/angular/pull/4435.
“Inputs”, as you might guess from the hierarchy discussion above, specifies which properties you can set on a component, whereas “outputs” identifies the events a component can fire to send information up the hierarchy to its parent.
Figure 1: A component that uses “inputs” and “outputs” properties in the
There are several things to notice with regards to inputs and outputs above:
- The “inputs” property passed to the
@Componentsdecorator lists “myname” as a component property that can receive data. We also declare “myname” as a public property within
ParentCompclass. If you don’t declare it, the TypeScript compiler might issue a warning.
- The “outputs” property lists “myevent” as a custom event that
ParentCompcan emit which its parent will be able to receive. Within the ParentComp class, “myevent” is declared as and set to be an
EventEmitteris a built-in class that ships with Angular that gives us methods for managing and firing custom events. Notice that we had to add
EventEmitterto the import statement at the top of the file.
- This component displays the incoming “myname” in the view, but when we try to access it in
ParentCompconstructor it’s not yet defined. That’s because input properties aren’t available until the view has rendered, which happens after the constructor function runs.
- We added a “click” event handler to our template that invokes the myeventEventEmitter’s “
next()” method and passes it the data we want to send with the event. This is the standard pattern for sending data up the component hierarchy — using “EventEmitter” to call the “
Now that we’ve looked at how to define “inputs” and “outputs” in a component, let’s see how to use them. The template for the CompDemo component uses the ParentComp component:
Figure 2: This component uses the input and output defined by ParentComp
The syntax is pretty straightforward for using “ParentComp”:
[myname] = "myFriend": This tells Angular to set the ParentComp input property “myname” to the value of “myFriend” interpolated as a property of CompDemo. Notice we set “myFriend” in the constructor
(myevent) = "handleMyEvent($event)": This tells Angular to invoke the CompDemo “
handleMyEvent($event)” method when ParentComp fires “myevent.” The data that we passed to the “
next()” method in ParentComp is available in CompDemo by passing “
$event” as an argument to the “
In both cases, the left side of the attribute refers to something in ParentComp (an input property or an output event) and the right side refers to something that’s interpreted in the context of CompDemo (an instance property or a method).
If you try to set a property on ParentComp without specifying it as an input property, Angular won’t throw an error, but it also won’t set the property. The above pattern — passing data in through an “input” property and sending data out through an “output” event — is the primary way to share data between Angular 2 components. We’ll see in a future article that we can also share data between components by defining services that can be injected into components, effectively giving us a way to share data or functions between components.
@Input() and @Output()
There is an alternative syntax available to define input properties and output events in a component. In the example above we used the “inputs” and “outputs” properties of the object passed to the
@Component decorator. Angular also lets us use an
@Output decorator to get the same result:
Figure 3: Uses the
In the above version of ParentComp, we dispensed with the “inputs” and “outputs” properties of the
@Component definition object. Instead, we added “Input” and “Output” to the import command on line 2 and used the
@Output decorator in the ParentComp class to declare “myname” and “myevent.”
Whether you use inputs/outputs or
@Output, the result is the same, so choosing which one to use is largely a stylistic decision.
In this article, we’ve looked in more depth at Angular 2 components, how they relate, and how you pass data into them and how to get data back out. We’re still only scratching the surface in terms of components; they’re arguably the major feature of Angular 2 and are involved in every aspect of designing and building Angular 2 applications. In future articles, we’ll continue to explore Angular 2 components by looking in more detail at Angular services as a way to re-use code and encapsulate key functionality.
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