Composition vs Inheritance
React has a powerful composition model, and we recommend using composition instead of inheritance to reuse FinRL 入门指南 code between components.
In this section, we will consider a FinRL 入门指南 few problems where developers new to React often reach for FinRL 入门指南 inheritance, and show how we can solve them with composition.
Some components don’t know their children ahead of time. This FinRL 入门指南 is especially common for components like Sidebar or Dialog that represent generic “boxes”.
We recommend that such components use the special children prop to pass children elements directly into their output:
This lets other components pass arbitrary children to them by nesting the JSX:
Anything inside the JSX tag gets FinRL 入门指南 passed into the FancyBorder component as a children prop. Since FinRL 入门指南 FancyBorder renders inside a , the passed elements appear in the FinRL 入门指南 final output.
While this is less common, sometimes you might need multiple “holes” in a component. In such cases you may come up with your own convention instead of using children :
React elements like and are just objects, so you can pass them as props like any other data. This approach may remind you of “slots” in other libraries but there are no limitations on what you can pass as props in React.
Sometimes we think about components as being “special cases” of other components. For example, we might say that a WelcomeDialog is a special case of Dialog .
In React, this is also achieved by composition, where a FinRL 入门指南 more “specific” component renders a more “generic” one and configures it with props:
Composition works equally well for components defined as classes:
So What About Inheritance?
At Facebook, we use React in thousands of components, and we haven’t found any use cases where we would recommend creating component inheritance hierarchies.
Props and composition give you all the flexibility you need to customize a component’s look and behavior in an explicit and safe way. Remember that components may accept arbitrary props, including primitive values, React elements, or functions.
If you want to reuse non-UI functionality between components, we suggest extracting it into a separate JavaScript module. The components may import it and use that function, object, or a class, without extending it.
Conditional Rendering
In React, you can create distinct components that encapsulate behavior you need. Then, you can render only some FinRL 入门指南 of them, depending on the state of your application.
Conditional FinRL 入门指南 rendering in React works the same way conditions work in JavaScript. Use JavaScript operators like if or the conditional operator to create elements representing the current state, and let FinRL 入门指南 React update the UI to match them.
Consider these two components:
We’ll create a Greeting component that displays either of these components depending on whether a user is logged in:
This example renders a different greeting depending on the value of isLoggedIn prop.
You can use variables to store elements. This can help you conditionally render a part of the component while the rest of the output doesn’t change.
Consider these two new components representing Logout and Login buttons:
In the example below, we will create a stateful component called FinRL 入门指南 LoginControl .
It will render either or depending on its current state. It will also render a from the previous example:
While declaring a variable and using an if statement is a fine way to conditionally render a component, sometimes you might want to use a shorter syntax. There are a few ways to inline conditions in JSX, explained below.
Inline FinRL 入门指南 If with Logical && Operator
You may embed any expressions in FinRL 入门指南 JSX by wrapping them in curly braces. This includes the JavaScript logical && operator. It can be handy for conditionally including an element:
It works because in JavaScript, true FinRL 入门指南 && expression always evaluates to expression , and false && expression FinRL 入门指南 always evaluates to false .
Therefore, if the condition is true , the element right after && will appear in the output. If FinRL 入门指南 it is false , React will ignore and skip it.
Inline If-Else with Conditional Operator
Another method for conditionally rendering elements inline is to use the JavaScript conditional operator condition ? true : false .
In the example below, we use it to conditionally render a small block of text.
It can also be FinRL 入门指南 FinRL 入门指南 used for larger expressions although it is less obvious what’s going on:
Just like in JavaScript, it is up to you to choose an appropriate style based on what you and your team consider more readable. Also remember that whenever conditions become too complex, it might be a good time to extract a component.
Preventing Component from Rendering
In rare cases you might want a component to hide itself even though FinRL 入门指南 it was rendered by another component. To do this return null instead of its render output.
In the example below, FinRL 入门指南 the is rendered depending on the value of the prop called warn . If the value of the prop is false , then the component does not render:
Returning null from a component’s render method does not affect the firing of the component’s lifecycle methods. For instance componentDidUpdate will still be called.
FinRL 入门指南
1NCE ist Teil der IOTHINGS Milan, eines der führenden IoT-Events in Italien, das vom 3. bis 4. April im MIND (Milano Innovation District) stattfindet. Treffen Sie das italienische 1NCE-Team am Stand Nr. 23 und nehmen Sie an unseren FinRL 入门指南 FinRL 入门指南 Terminen für beide Vorträge teil.
- 3. April um 11.FinRL 入门指南 20 Uhr: Connigività nativa IoT per modelli di business cost-effective von Luigi Capobianco, kürzlich zum Sales Manager Italy von FinRL 入门指南 1NCE ernannt
- 4. April um 11.30 Uhr: Embedded Connectivity: FinRL 入门指南 Sicurezza, Controlled Efficacia per Applicazioni IoT von Giuseppe PITTARI, Vice President Business Development Italy bei 1NCE
IOTHINGS findet zweimal im Jahr statt: die Frühjahrsausgabe in Mailand und die Herbstausgabe in Rom, FinRL 入门指南 um unterschiedliche Ziele zu erreichen. Die aktuellen Themen von Mailand FinRL 入门指南 2019 werden sein: Cybersecurity / DSDPR und Data Monetization for IoT. Mit einem kompletten Konferenzprogramm veranstaltet IOTHINGS Milan 4 Veranstaltungen FinRL 入门指南 am selben Ort: ITALIA 5G, AI + BOTS World, BLOCKCHAIN JETZT und IOMOBILITY. Die Veranstaltungen schaffen einen exklusiven Aktualisierungspunkt für die disruptivsten Technologien, um die Entwicklung des IoT-Ökosystems zu fördern.