forms.md

Forms Back

Form components such as <input>, <textarea> and <option> differ from other native components because of their mutation via use interactions.

Interactive Props

Form components support a few props that can be changed via user interacitons:

• value: <input> or <textarea> components.
• checked: <input> components of type checkbox or radio.
• selected: <option> components.

Form components allow listening for changes by setting a callback to the onChange prop.

Note:

For <input> and <textarea>, onChange supersedes(替代) the DOM's built-in oninput event handler.

Controlled Components

A Controlled <input> has a value prop. Rendering a controlled <input> will reflect the value of the value prop.

var InputComponent = React.creteClass({
    render: function () {
        return <input type="text" value="Hello!" />;
    }
});

User input will have no effect on the rendered element because React has declared the value to be Hello!. To update the value in response to user input, you could use the onChange event:

var InputComponent = React.creteClass({
    getInitialState: function () {
        return { value: 'Hello!' };
    },
    
    handleChanged: function(event) {
        this.setState({ value: event.target.value });
    },
    
    render: function () {
        return (
            <input type="text" value={this.state.value} onChange={this.handleChanged} />
        );
    }
});

Notice that: A Controlled component does not maintain its own internal state; the component renders purely based on props.

Potential Problems with checkbox or radio buttons

Be aware that, in an attempt to normalize change handling for checkbox and radio inputs, React uses a click event in place of a change event. That's because when calling preventDefault in a change handler. preventDefault stops the browser from visually updating the input, even if checked gets toggled. (Solved by removing the call to preventDefault, or putting the toggle of checked in a setTimeout.

Uncontrolled Components

An <input> without a value property is an uncontrolled component:

var InputComponent = React.creteClass({
    render: function () {
        return <input type="text" />;
    }
});

If you want to listen to updates to the value, you can also use onChange like handling controlled components.

Notiec that: An uncontrolled component maintains its own internal state.

Default Value

If you want to initialize the component with a non-empty value, you can supply a defaultValue prop.

var InputComponent = React.creteClass({
    render: function () {
        return <input type="text" defaultValue="Hello!" />;
    }
});

Likewise, <input type="checkbox"> and <input type="radio"> support defaultChecked, and <select> supports defaultValue.

Note:

The defaultValue and defaultChecked props are only used during initial render. If you need to update the value in a subsequent render, you will need to use a controlled component.

Discussions

Why Controlled Components?

In HTML, we suppose that we have a <input> tag:

<input type="text" name="title" value="Untitled" />

When the user updates the input, the node's vaule property will change. However, node.getAtribute('value') should still return the value used at initialization time, Untitled.

var InputComponent = React.creteClass({
    render: function () {
        return <input type="text" name="title" value="Untitled" />;
    }
});

However, in React, the value should always be Untitled, no matter what the user has changed. Therefore, the state of the view has kept in consistent.

Why Textarea Value?

In HTML, the value of <textarea> is usually set using its children:

<!-- antipattern: DO NOT DO THIS! -->
<textarea name="description">This is the description.</textarea>

For HTML, this easily allows developers to supply multiline values. However, since React is JavaScript, we do not have string limitations and can use \n if we want newlines. In a world where we have value and defaultValue, it is ambiguous what role children play. For this reason, you should not use children when setting <textarea> values:

<textarea name="description" value="This is a description." />

Why Select Value?

In HTML, The selected <option> in an HTML <select> is normally specified through that option's selected attribute.

However in React, in order to make components easier to manipulate, the following format is adopted instead:

<select value="B">
    <option value="A">Apple</option>
    <option value="B">Banana</option>
    <option value="C">Cranberry</option>
</select>

To make an uncontrolled component, defaultValue is used instead.

Note:

You can pass an array into the value attribute, allowing you to select multiple options in a select tag: <select multiple={true} value={['B', 'C']}>.

Imperative operations(緊急操作)

If you need to imperatively perform an operation, you have to obtain a reference to the DOM node. For instance, if you want to imperatively submit a form, one approach would be to attach a ref to the form element and manually call form.submit().