javascript - react 条件渲染

Question

我有一个从 React 构建的简单表单。提交后，如果错误再次出现，我希望呈现一个额外的 div 以在表单上显示错误。

现在我可以使用它了，但我不喜欢这个解决方案。我的解决方案基于这样的知识，即如果渲染函数内的状态发生变化(在本例中为 this.state.errorMessages)，组件只会重新渲染。所以我不得不像这样显式地将 if 条件放在渲染函数中

renderError() {
    var errArr = [];
    for (var key in this.state.errorMessages) {
        errArr = [...errArr, ...this.state.errorMessages[key]];
    }
    return (
        <div className="alert alert-danger">
            {errArr.map((err) => {
                return <p>{err}</p>
            })}
        </div>
    )
}

renderForm() {
    return (
        <form onSubmit={this.handleRegisterFormSubmit}>
            <div className="form-group">
                <label>First Name</label>
                <input type="text" className="form-control" name="name" placeholder="Name" value={this.state.firstName} onChange={this.handleFirstNameChange} required/>
            </div>
            <div className="form-group">
                <label>Last Name</label>
                <input type="text" className="form-control" name="lastName" placeholder="Last Name" value={this.state.lastName} onChange={this.handleLastNameChange} required/>
            </div>
            <div className="form-group">
                <label>Email address</label>
                <input type="email" className="form-control" id="exampleInputEmail1" aria-describedby="emailHelp" placeholder="Enter email" value={this.state.email} onChange={this.handleEmailChange} />
                <small id="emailHelp" className="form-text text-muted">We'll never share your email with anyone else.</small>
            </div>
            <div className="form-group">
                <label>Password</label>
                <input type="password" className="form-control" name="password" placeholder="Password" value={this.state.password} onChange={this.handlePasswordChange}/>
            </div>
            <div className="form-group">
                <label>Password Confirmation</label>
                <input type="password" className="form-control" name="password_confirmation" placeholder="Password Confirmation" value={this.state.passwordConfirmation} onChange={this.handlePasswordConfirmationChange}/>
            </div>
            <div>
                <button type="submit" className="btn btn-primary">Submit</button>
                <button type="button" className="btn btn-danger" onClick={this.handleCancelClick}>Cancel</button>
            </div>
        </form>
    )
}

render () {
    if (!this.state.errorMessages) {
        return (
            <div>
                {this.renderForm()}
            </div>
        )
    } else {
        return (
            <div>
                {this.renderForm()}
                {this.renderError()}
            </div>
        )
    }
}

我不太喜欢这种方法，因为如果我有更多条件需要重新渲染，这可能会变得很糟糕。我希望有一个解决方案，即在实际渲染函数中没有太多逻辑并将其提取出来。例如……

renderError() {
    if (!this.state.errorMessages) {
        return;
    }
    var errArr = [];
    for (var key in this.state.errorMessages) {
        errArr = [...errArr, ...this.state.errorMessages[key]];
    }
    return (
        <div className="alert alert-danger">
            {errArr.map((err) => {
                return <p>{err}</p>
            })}
        </div>
    )
}

render () {
    <form onSubmit={this.handleRegisterFormSubmit}>
        <div className="form-group">
            <label>First Name</label>
            <input type="text" className="form-control" name="name" placeholder="Name" value={this.state.firstName} onChange={this.handleFirstNameChange} required/>
        </div>
        <div className="form-group">
            <label>Last Name</label>
            <input type="text" className="form-control" name="lastName" placeholder="Last Name" value={this.state.lastName} onChange={this.handleLastNameChange} required/>
        </div>
        <div className="form-group">
            <label>Email address</label>
            <input type="email" className="form-control" id="exampleInputEmail1" aria-describedby="emailHelp" placeholder="Enter email" value={this.state.email} onChange={this.handleEmailChange} />
            <small id="emailHelp" className="form-text text-muted">We'll never share your email with anyone else.</small>
        </div>
        <div className="form-group">
            <label>Password</label>
            <input type="password" className="form-control" name="password" placeholder="Password" value={this.state.password} onChange={this.handlePasswordChange}/>
        </div>
        <div className="form-group">
            <label>Password Confirmation</label>
            <input type="password" className="form-control" name="password_confirmation" placeholder="Password Confirmation" value={this.state.passwordConfirmation} onChange={this.handlePasswordConfirmationChange}/>
        </div>
        <div>
            <button type="submit" className="btn btn-primary">Submit</button>
            <button type="button" className="btn btn-danger" onClick={this.handleCancelClick}>Cancel</button>
        </div>
        {this.renderError}
    </form>
}

这会抛出一个错误，因为它提示 this.renderError 应该作为一个函数来调用。但是当我把它作为 这个.renderError()该错误永远不会呈现，因为它不会在错误返回时自动调用。

------------更新------------

或者，为什么我不能做类似下面的事情

render () {
    <div>
        <form onSubmit={this.handleRegisterFormSubmit}>
            ...
        </form>
        {if (this.state.errorMessages) {this.renderError()}}
    </div>
}

这会抛出控制台错误

Uncaught Error: Module build failed: SyntaxError: Unexpected token (117:13)

------------更新2------------

本质上，我正在寻找一种解决方案，在渲染函数内部，当状态发生变化时，我可以轻松地显示整个代码块。在 Vue 中，我可以做类似的事情

<form>
    <input type="text">
</form>
<div v-if="hasError">
    <div class="alert alert-danger">{{something}}</div>
</div>

Can I do something as easy as this in React?

Answer 1

您可以只使用 map 来从您的对象中提取错误消息。

下面是 React 中表单验证和错误的最小示例。很高兴了解它是如何工作的，但就我而言，我使用 Formik这简化了这个过程。

class Test extends React.Component {
    constructor(props) {
        super(props);
        this.state = { errorMessages: {} };
    }

    handleRegisterFormSubmit = e => {
        e.preventDefault(); // don't submit the form until we run what's below
        let errorMessages = {};

        if (!this.state.lastName) errorMessages.lastName = 'You need to enter your last name';
        // And so on for each field validation

        // Do we have errors ?
        if (Object.keys(errorMessages).length > 0) {
            this.setState(errorMessages);
        } else {
            // Submit to server
        }
    };

    handleChange = e => {
        this.setState({
            [e.target.name]: e.target.value,
            errorMessages: {
                ...this.state.errorMessages,
                [e.target.name]: null // remove the error of this field if it's being edited
            }
        });
    };

    render() {
        const errArr = Object.keys(this.state.errorMessages).map(key => this.state.errorMessages[key]);
        return (
            <form onSubmit={this.handleRegisterFormSubmit}>
                <div className="form-group">
                    <label>Last Name</label>
                    <input type="text" className="form-control" name="lastName" placeholder="Last Name" value={this.state.lastName} onChange={this.handleChange} />
                </div>

                {/* ... your dom */}

                <div>
                    <button type="submit" className="btn btn-primary">
                        Submit
                    </button>
                    <button type="button" className="btn btn-danger" onClick={this.handleCancelClick}>
                        Cancel
                    </button>
                </div>

                {errArr.length > 0 && (
                    <div className="alert alert-danger">
                      {errArr.map(err => {
                        return <p>{err}</p>;
                      })}
                    </div>
                )}
            </form>
        );
    }
}

另一种不显示警报 div 的方法是为您的类名使用三元运算符并使用 Bootstrap 的 d-none

<div className={errArr.length ? "alert alert-danger" : "d-none"}>
    {errArr.map(err => {
        return <p>{err}</p>;
    })}
</div>