App.razor

@using NCrontab
@using Microsoft.Extensions.Logging
@using Microsoft.AspNetCore.WebUtilities
@inject ILogger<Index> Logger
@inject NavigationManager NavigationManager
@inject IJSRuntime JSRuntime
<label for="crontab-expression">NCrontab Expression:</label>
<div class="input-group mb-3">
    <input @onkeyup="SetExpression"
           @bind="cronExpression"
           @bind:event="oninput"
           placeholder="Cron Expression"
           class="form-control"
           id="crontab-expression" />
    <div class="input-group-append">
        <button 
            @onclick="PrintOccurrences" 
            type="button" 
            class="btn btn-primary" 
            disabled="@(!isExpressionValid)">
            Print More
        </button>
        <button @onclick="CopyLink" type="button" class="btn btn-light">Copy Link</button>
    </div>
</div>
<label>Occurrences Output:</label>
@if (!string.IsNullOrEmpty(errorMessage))
{
    <div class="alert alert-danger" role="alert">
        @errorMessage
    </div>
}
@if (!isExpression6PartFormat)
{
    <div class="alert alert-warning" role="alert">
        You are using the classic five-part CRON format. Services such as
        Azure Functions and Azure WebJobs require the six-part format.
    </div>
}
<ul class="occurrences-list">
    @foreach (var occurence in occurrences)
    {
        <li>
            @occurence.ToString("yyyy-MM-dd HH:mm:ss")
        </li>
    }
</ul>

@code {
    private const string DefaultCronExpression = "0 0 12 * */2 Mon";
    private DateTime startDate = DateTime.Today;
    private string cronExpression = DefaultCronExpression;
    private CrontabSchedule cronSchedule = CrontabSchedule.Parse(DefaultCronExpression, new CrontabSchedule.ParseOptions { IncludingSeconds = true });
    private List<DateTime> occurrences = new List<DateTime>();
    private string errorMessage = string.Empty;
    private List<Action> actionsToRunAfterRender = new List<Action>();
    private bool isExpression6PartFormat = true;
    private bool isExpressionValid = true;

    protected override Task OnInitializedAsync()
    {
        var uri = new Uri(NavigationManager.Uri);
        var queryPairs = QueryHelpers.ParseQuery(uri.Query);
        if (queryPairs.ContainsKey("expression"))
        {
            cronExpression = ReadableQueryToCron(queryPairs["expression"].First());
        }

        SetExpression();

        return Task.CompletedTask;
    }

    protected override Task OnAfterRenderAsync(bool firstRender)
    {
        foreach (var actionToRun in actionsToRunAfterRender)
        {
            actionToRun();
        }
        actionsToRunAfterRender.Clear();
        return base.OnAfterRenderAsync(firstRender);
    }

    private void PrintOccurrences()
    {
        var mostRecentDate = occurrences.Count == 0 ? startDate : occurrences.Last();
        for (int i = 0; i < 10; i++)
        {
            mostRecentDate = cronSchedule.GetNextOccurrence(mostRecentDate);
            occurrences.Add(mostRecentDate);
        }

        actionsToRunAfterRender.Add(() => JSRuntime.InvokeVoidAsync("scrollToOccurrence", occurrences.Count - 1));
    }

    private void SetExpression()
    {
        occurrences.Clear();
        isExpression6PartFormat = cronExpression
            .Trim()
            .Replace(@"\t", " ")
            .Split(' ', StringSplitOptions.RemoveEmptyEntries)
            .Count() == 6;

        try
        {
            cronSchedule = CrontabSchedule.Parse(cronExpression, new CrontabSchedule.ParseOptions { IncludingSeconds = isExpression6PartFormat });
            errorMessage = string.Empty;
            isExpressionValid = true;
            PrintOccurrences();
        }
        catch (CrontabException exception)
        {
            Logger.LogError(exception, exception.Message);
            errorMessage = exception.Message;
            isExpression6PartFormat = true;
            isExpressionValid = false;
        }
    }

    private void CopyLink()
    {
        var uri = new Uri(NavigationManager.Uri);
        var readableCron = CronToReadableQuery(cronExpression);
        var uriToCopy = $"{uri.GetComponents(UriComponents.SchemeAndServer | UriComponents.Path, UriFormat.UriEscaped)}?expression={readableCron}";
        JSRuntime.InvokeVoidAsync("copyText", uriToCopy);
    }

    private string CronToReadableQuery(string query) => query.Replace(' ', '+');
    private string ReadableQueryToCron(string query) => query.Replace('+', ' ');
}