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FinRL 入门指南

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 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 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 LoginControl .

It will render either or depending on its current state. It FinRL 入门指南 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 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 && expression always evaluates to expression , and false && expression always evaluates to false .

Therefore, if the condition is true , the element right after && will appear in the output. If it is false , React will ignore FinRL 入门指南 and skip it.

Inline If-Else with Conditional Operator

Another method for conditionally rendering elements inline is to use the FinRL 入门指南 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 and your team consider more readable. Also remember that whenever conditions become too complex, it might FinRL 入门指南 be a good time to extract a component.

Preventing Component from Rendering

In rare cases you might want a FinRL 入门指南 FinRL 入门指南 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

Viaggio fotografico (1): Arrivo a Reykjavik

Viaggio fotografico (1): Arrivo a Reykjavik

App Eachine TEC: risolvere il crash col drone e520s

App Eachine TEC: risolvere il crash col drone e520s

Scegliere il primo drone fotografico

Scegliere il primo drone fotografico

Fotografare stelle e costellazioni puntiformi

Fotografare stelle e costellazioni puntiformi

Quando comincia la notte per l

FinRL 入门指南 Quando comincia la notte per l'astrofotografo?

Luna: stacking con Autostakkert! 3

Luna: stacking con Autostakkert! 3

L

L'uso dei filtri LRGB in astrofotografia

2020: Benvenuto Clicks from Sicily

2020: Benvenuto Clicks from Sicily

Filtro Hutech IDAS LPS-D1, prime impressioni

Filtro Hutech IDAS LPS-D1, prime impressioni

About

Mi chiamo Sebastiano, vivo in Sicilia orientale e sono una guida turistica di professione con una profonda predilezione per le escursioni archeologiche in siti minori. Da tanti anni, quando e come posso mi occupo di fotografia. La mia passione per la fotografia nasce all' "epoca della pellicola", affiancandosi a quella per l'astronomia e portandomi giovanissimo a sperimentare le prime riprese della volta celeste con le pellicole più sensibili che riuscissi a trovare nei negozi e usando la motorizzazione del mio piccolo telescopio. In seguito l'interesse per la fotografia si è ampliando, sconfinando dalla nicchia specialistica dell'astrofotografia. Attualmente amo molto anche la fotografia di paesaggio, la travel photography e ho accomunato gli interessi professionali per i monumenti e i siti archeologici siciliani all'amore per la fotografia, cominciando un progetto - senza data di scadenza - di documentazione di luoghi, siti e monumenti dell'isola. In quasi quindici anni di attività e innumerevoli escursioni esplorative è facile immaginare che l'archivio fotografico sia cresciuto a dismisura e, nei ritagli di tempo, cerco di condividere la documentazione dei luoghi più belli della Sicilia andando ad aggiungere nuove gallerie fotografiche a questo sito. Dimenticavo! Anche il sito internet esiste ormai da numerosi anni ma ha recentemente cambiato nome. Per tanti anni il mio sito internet era chiamato "Sicilystockphoto" ma maturando obiettivi e interessi diversi nel 2020 ho deciso di dargli una rinfrescata nella graffica e nel nome. Come spesso capita gli interessi fotografici vanno cambiando nel tempo. Attualmente sono di nuovo molto impegnato nell'astrofotografia e per tale motivo sia tra gli articoli nel blog che tra le foto, è possibile trovare ampio spazio per questo particolare e specialissimo settore della fotografia. Potete trovare piccoli FinRL 入门指南 articoli da me scritti su quanto ho appreso e sui miei progetti fotografici nella sezione blog.

Per maggiori informazioni sull'attrezzatura utilizzata per realizzare le fotografie pubblicate su questo sito potete dare un'occhiata alla pagina:

Per quanto riguarda l'astrofotografia trovate dettagli sul mio attuale setup alla pagina

Se invece doveste essere interessati ai miei servizi professionali come guida turistica, potete visitare il sito:

Composition vs Inheritance

React has a powerful composition model, 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 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 passed into the FancyBorder component as a children prop. Since FancyBorder renders inside a , the passed elements appear in the final output.

While this FinRL 入门指南 is less common, sometimes you might need multiple “holes” in a component. In such cases you may come FinRL 入门指南 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 FinRL 入门指南 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 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 FinRL 入门指南 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.

FinRL 入门指南

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Photo credits
Water meter: deepblue4you / istockphoto.com
Dumpster: teptong / thinkstockphotos.de
Fork lifter: Bet_Noire / thinkstockphotos.de