学习laravel所遇问题：Undefined variable: errors

问题：Undefined variable: errors (View: C:\wamp\www\StuLaravel5\resources\views\login.blade.php)

login.blade.php代码：

@section('content')
<div class="container">
    <div class="row">
        <div class="col-md-8 col-md-offset-2">
            <div class="panel panel-default">
                <div class="panel-heading">登陆</div>
                <div class="panel-body">
                    <form class="form-horizontal" role="form" method="POST" action="{{ url('/login') }}">
                        {!! csrf_field() !!}

                        <div class="form-group{{ $errors->has('email') ? ' has-error' : '' }}">
                            <label class="col-md-4 control-label">邮 箱</label>

                            <div class="col-md-6">
                                <input type="email" class="form-control" name="email" value="{{ old('email') }}">

                                @if ($errors->has('email'))
                                    <span class="help-block">
                                        <strong>{{ $errors->first('email') }}</strong>
                                    </span>
                                @endif
                            </div>
                        </div>

                        <div class="form-group{{ $errors->has('password') ? ' has-error' : '' }}">
                            <label class="col-md-4 control-label">密 码</label>

                            <div class="col-md-6">
                                <input type="password" class="form-control" name="password">

                                @if ($errors->has('password'))
                                    <span class="help-block">
                                        <strong>{{ $errors->first('password') }}</strong>
                                    </span>
                                @endif
                            </div>
                        </div>

                        <div class="form-group">
                            <div class="col-md-6 col-md-offset-4">
                                <div class="checkbox">
                                    <label>
                                        <input type="checkbox" name="remember"> 记住我
                                    </label>
                                </div>
                            </div>
                        </div>

                        <div class="form-group">
                            <div class="col-md-6 col-md-offset-4">
                                <button type="submit" class="btn btn-primary">
                                    <i class="fa fa-btn fa-sign-in"></i>登陆
                                </button>

                                <a class="btn btn-link" href="{{ url('/password/reset') }}">忘记密码</a>
                            </div>
                        </div>
                    </form>
                </div>
            </div>
        </div>
    </div>
</div>
@endsection

发现：

如果请求输入参数没有通过给定验证规则怎么办？正如前面所提到的，Laravel将会自动将用户重定向回上一个位置。此外，所有验证错误信息会自动一次性存放到session。
注意我们并没有在GET路由中明确绑定错误信息到视图。这是因为Laravel总是从session数据中检查错误信息，而且如果有的话会自动将其绑定到视图。所以，值得注意的是每次请求的所有视图中总是存在一个$errors变量，从而允许你在视图中方便而又安全地使用。$errors变量是的一个Illuminate\Support\MessageBag实例。想要了解更多关于该对象的信息，查看其文档。

注意：$errors变量会通过web中间件组中的Illuminate\View\Middleware\ShareErrorsFromSession中间件绑定到视图，如果使用了该中间件，那么$errors变量在视图中总是有效，从而方便你随时使用。

解决办法：

在使用$error时需要用到中间件：

\Illuminate\Session\Middleware\StartSession::class,

\Illuminate\View\Middleware\ShareErrorsFromSession::class,

此时在Kernel中注册中间件Kernel.php,使用了laravel定义中间件组web

protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
        ],
        'api' => [
            'throttle:60,1',
        ],
    ];

routes.php

Route::group(['middleware' => 'web'], function () {
    Route::get('login', ['middleware' => 'guest', 'as' => 'login', 'uses' => 'loginController@loginGet']);
Route::post('login', ['middleware' => 'guest', 'uses' => 'loginController@loginPost']);
Route::get('logout', ['middleware' => 'auth', 'as' => 'logout', 'uses' => 'loginController@logout']);
});