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FinRL 入门指南

Composition vs Inheritance

React has a powerful composition model, FinRL 入门指南 and we recommend using composition instead of inheritance to reuse code between components.

In this section, we will consider a few problems where developers new to React often reach for 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 FinRL 入门指南 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 FinRL 入门指南 FinRL 入门指南 by nesting the JSX:

Anything inside the JSX tag gets passed into the FancyBorder component as a children prop. Since FancyBorder renders inside a , the passed elements appear in the 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 .FinRL 入门指南

In React, this is also achieved by composition, where a 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.FinRL 入门指南

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 FinRL 入门指南 FinRL 入门指南 it.

Conditional Rendering

In React, you can create distinct components that encapsulate behavior you need. Then, you can render only some of them, depending on the state of your application.

Conditional rendering in React works the same way conditions work in FinRL 入门指南 JavaScript. Use JavaScript operators like if or the conditional operator to create elements representing the current state, and let 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 FinRL 入门指南 FinRL 入门指南 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 FinRL 入门指南 these two new components representing Logout and Login buttons:

In the example below, we will create a stateful component called 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 FinRL 入门指南 few ways to inline conditions in JSX, explained below.

Inline If with Logical && Operator

You may embed any expressions in 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 always evaluates to false .

Therefore, if the condition is true , FinRL 入门指南 the element right after && will appear in the output. If 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 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 FinRL 入门指南 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 it was rendered by another component. To do this return null instead of its render output.

In the example below, 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

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