// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace Microsoft.AspNetCore.Components.Forms
{
    /// <summary>
    /// Holds metadata related to a data editing process, such as flags to indicate which
    /// fields have been modified and the current set of validation messages.
    /// </summary>
    public sealed class EditContext
    {
        // Note that EditContext tracks state for any FieldIdentifier you give to it, plus
        // the underlying storage is sparse. As such, none of the APIs have a "field not found"
        // error state. If you give us an unrecognized FieldIdentifier, that just means we
        // didn't yet track any state for it, so we behave as if it's in the default state
        // (valid and unmodified).
        private readonly Dictionary<FieldIdentifier, FieldState> _fieldStates = new Dictionary<FieldIdentifier, FieldState>();

        /// <summary>
        /// Constructs an instance of <see cref="EditContext"/>.
        /// </summary>
        /// <param name="model">The model object for the <see cref="EditContext"/>. This object should hold the data being edited, for example as a set of properties.</param>
        public EditContext(object model)
        {
            // The only reason we disallow null is because you'd almost always want one, and if you
            // really don't, you can pass an empty object then ignore it. Ensuring it's nonnull
            // simplifies things for all consumers of EditContext.
            Model = model ?? throw new ArgumentNullException(nameof(model));
        }

        /// <summary>
        /// An event that is raised when a field value changes.
        /// </summary>
        public event EventHandler<FieldChangedEventArgs> OnFieldChanged;

        /// <summary>
        /// An event that is raised when validation is requested.
        /// </summary>
        public event EventHandler<ValidationRequestedEventArgs> OnValidationRequested;

        /// <summary>
        /// An event that is raised when validation state has changed.
        /// </summary>
        public event EventHandler<ValidationStateChangedEventArgs> OnValidationStateChanged;

        /// <summary>
        /// Supplies a <see cref="FieldIdentifier"/> corresponding to a specified field name
        /// on this <see cref="EditContext"/>'s <see cref="Model"/>.
        /// </summary>
        /// <param name="fieldName">The name of the editable field.</param>
        /// <returns>A <see cref="FieldIdentifier"/> corresponding to a specified field name on this <see cref="EditContext"/>'s <see cref="Model"/>.</returns>
        public FieldIdentifier Field(string fieldName)
            => new FieldIdentifier(Model, fieldName);

        /// <summary>
        /// Gets the model object for this <see cref="EditContext"/>.
        /// </summary>
        public object Model { get; }

        /// <summary>
        /// Signals that the value for the specified field has changed.
        /// </summary>
        /// <param name="fieldIdentifier">Identifies the field whose value has been changed.</param>
        public void NotifyFieldChanged(in FieldIdentifier fieldIdentifier)
        {
            GetFieldState(fieldIdentifier, ensureExists: true).IsModified = true;
            OnFieldChanged?.Invoke(this, new FieldChangedEventArgs(fieldIdentifier));
        }

        /// <summary>
        /// Signals that some aspect of validation state has changed.
        /// </summary>
        public void NotifyValidationStateChanged()
        {
            OnValidationStateChanged?.Invoke(this, ValidationStateChangedEventArgs.Empty);
        }

        /// <summary>
        /// Clears any modification flag that may be tracked for the specified field.
        /// </summary>
        /// <param name="fieldIdentifier">Identifies the field whose modification flag (if any) should be cleared.</param>
        public void MarkAsUnmodified(in FieldIdentifier fieldIdentifier)
        {
            if (_fieldStates.TryGetValue(fieldIdentifier, out var state))
            {
                state.IsModified = false;
            }
        }

        /// <summary>
        /// Clears all modification flags within this <see cref="EditContext"/>.
        /// </summary>
        public void MarkAsUnmodified()
        {
            foreach (var state in _fieldStates)
            {
                state.Value.IsModified = false;
            }
        }

        /// <summary>
        /// Determines whether any of the fields in this <see cref="EditContext"/> have been modified.
        /// </summary>
        /// <returns>True if any of the fields in this <see cref="EditContext"/> have been modified; otherwise false.</returns>
        public bool IsModified()
        {
            // If necessary, we could consider caching the overall "is modified" state and only recomputing
            // when there's a call to NotifyFieldModified/NotifyFieldUnmodified
            foreach (var state in _fieldStates)
            {
                if (state.Value.IsModified)
                {
                    return true;
                }
            }

            return false;
        }

        /// <summary>
        /// Gets the current validation messages across all fields.
        ///
        /// This method does not perform validation itself. It only returns messages determined by previous validation actions.
        /// </summary>
        /// <returns>The current validation messages.</returns>
        public IEnumerable<string> GetValidationMessages()
        {
            // Since we're only enumerating the fields for which we have a non-null state, the cost of this grows
            // based on how many fields have been modified or have associated validation messages
            foreach (var state in _fieldStates)
            {
                foreach (var message in state.Value.GetValidationMessages())
                {
                    yield return message;
                }
            }
        }

        /// <summary>
        /// Gets the current validation messages for the specified field.
        ///
        /// This method does not perform validation itself. It only returns messages determined by previous validation actions.
        /// </summary>
        /// <param name="fieldIdentifier">Identifies the field whose current validation messages should be returned.</param>
        /// <returns>The current validation messages for the specified field.</returns>
        public IEnumerable<string> GetValidationMessages(FieldIdentifier fieldIdentifier)
        {
            if (_fieldStates.TryGetValue(fieldIdentifier, out var state))
            {
                foreach (var message in state.GetValidationMessages())
                {
                    yield return message;
                }
            }
        }

        /// <summary>
        /// Gets the current validation messages for the specified field.
        ///
        /// This method does not perform validation itself. It only returns messages determined by previous validation actions.
        /// </summary>
        /// <param name="accessor">Identifies the field whose current validation messages should be returned.</param>
        /// <returns>The current validation messages for the specified field.</returns>
        public IEnumerable<string> GetValidationMessages(Expression<Func<object>> accessor)
            => GetValidationMessages(FieldIdentifier.Create(accessor));

        /// <summary>
        /// Determines whether the specified fields in this <see cref="EditContext"/> has been modified.
        /// </summary>
        /// <returns>True if the field has been modified; otherwise false.</returns>
        public bool IsModified(in FieldIdentifier fieldIdentifier)
            => _fieldStates.TryGetValue(fieldIdentifier, out var state)
            ? state.IsModified
            : false;

        /// <summary>
        /// Determines whether the specified fields in this <see cref="EditContext"/> has been modified.
        /// </summary>
        /// <param name="accessor">Identifies the field whose current validation messages should be returned.</param>
        /// <returns>True if the field has been modified; otherwise false.</returns>
        public bool IsModified(Expression<Func<object>> accessor)
            => IsModified(FieldIdentifier.Create(accessor));

        /// <summary>
        /// Validates this <see cref="EditContext"/>.
        /// </summary>
        /// <returns>True if there are no validation messages after validation; otherwise false.</returns>
        public bool Validate()
        {
            OnValidationRequested?.Invoke(this, ValidationRequestedEventArgs.Empty);
            return !GetValidationMessages().Any();
        }

        internal FieldState GetFieldState(in FieldIdentifier fieldIdentifier, bool ensureExists)
        {
            if (!_fieldStates.TryGetValue(fieldIdentifier, out var state) && ensureExists)
            {
                state = new FieldState(fieldIdentifier);
                _fieldStates.Add(fieldIdentifier, state);
            }

            return state;
        }
    }
}
