using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace Nuclex.Windows.Forms {

  /// <summary>
  ///   Proxy stand-in to delay checking for the main window until it has been created
  /// </summary>
  /// <remarks>
  ///   <para>
  ///     The issue: when the view model for the main window is created, the main window
  ///     may only exist as a .NET object, without the underlying operating system window
  ///     (done in <see cref="System.Windows.Forms.Control.CreateControl" /> checkable via
  ///     <see cref="System.Windows.Forms.Control.IsHandleCreated" />). Not only will things
  ///     like <see cref="System.Windows.Forms.Control.Invoke(Delegate)" /> fail, we can't
  ///     even locate the main window at that stage.
  ///   </para>
  ///   <para>
  ///     Thus, if the main window cannot be found at the time a view model is created,
  ///     this late-checking synchronizer will jump into its place and re-check for
  ///     the main window only when something needs to be executed in the UI thread.
  ///   </para>
  /// </remarks>
  class LateCheckedSynchronizer : ISynchronizeInvoke {

    /// <summary>Initializes a new late-checked main window synchronizer</summary>
    /// <param name="uiContextFoundCallback"></param>
    public LateCheckedSynchronizer(Action<ISynchronizeInvoke> uiContextFoundCallback) {
      this.uiContextFoundCallback = uiContextFoundCallback;
    }

    /// <summary>Finds the application's main window</summary>
    /// <returns>Main window of the application or null if none has been created</returns>
    /// <remarks>
    ///   The application's main window, if it has been created yet
    /// </remarks>
    public static Form GetMainWindow() {
      IntPtr mainWindowHandle = System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle;

      // We can get two things: a list of all open windows and the handle of
      // the window that the process has registered as main window. Use the latter
      // to pick the correct window from the former.
      FormCollection openForms = Application.OpenForms;
      int openFormCount = openForms.Count;
      for(int index = 0; index < openFormCount; ++index) {
        Form form = openForms[index];

        IntPtr handle;
        if(form.InvokeRequired) {
          handle = (IntPtr)form.Invoke(new Func<Form, IntPtr>(getWindowHandle), form);
        } else {
          handle = getWindowHandle(form);
        }
        if(handle != IntPtr.Zero) {
          if(handle == mainWindowHandle) {
            return form;
          }
        }
      }

      // No matching main window found: use the first one in good faith or fail.
      if(openFormCount > 0) {
        return openForms[0];
      } else {
        return null;
      }
    }

    /// <summary>Checks whether the calling thread needs to use Invoke()</summary>
    public bool InvokeRequired {
      get { return getMainWindowOrFail().InvokeRequired; }
    }

    /// <summary>Schedules a method to be run by the main UI thread</summary>
    /// <param name="method">Method that will be scheduled to run</param>
    /// <param name="args">Arguments that will be passed to the method</param>
    /// <returns>An asynchronous result handle that can be used to track the call</returns>
    public IAsyncResult BeginInvoke(Delegate method, object[] args) {
      return getMainWindowOrFail().BeginInvoke(method, args);
    }

    /// <summary>Waits for a call scheduled on the main UI thread to complete</summary>
    /// <param name="result">Asynchronous result handle returned by BeginInvoke()</param>
    /// <returns>The value returned by the method ran in the main UI thread</returns>
    public object EndInvoke(IAsyncResult result) {
      return getMainWindowOrFail().EndInvoke(result);
    }

    /// <summary>Executes a method on the main UI thread and waits for it to complete</summary>
    /// <param name="method">Method that will be run by the main UI thread</param>
    /// <param name="arguments">Arguments that will be passed to the method</param>
    /// <returns>The value returned by the method</returns>
    public object Invoke(Delegate method, object[] arguments) {
      return getMainWindowOrFail().Invoke(method, arguments);
    }

    /// <summary>Retrieves the application's current main window</summary>
    /// <returns>The application's current main window</returns>
    /// <remarks>
    ///   If there is no main window, an exception will be thrown
    /// </remarks>
    private Form getMainWindowOrFail() {
      Form mainWindow = GetMainWindow();
      if(mainWindow == null) {
        throw new InvalidOperationException(
          "Could not schedule work for the UI thread because no WinForms UI main window " +
          "was found. Create a main window first or specify the UI synchronization context " +
          "explicitly to the view model."
        );
      }

      if(this.uiContextFoundCallback != null) {
        this.uiContextFoundCallback(mainWindow);
        this.uiContextFoundCallback = null;
      }

      return mainWindow;
    }

    /// <summary>Returns a Form's window handle without forcing its creation</summary>
    /// <param name="form">Form whose window handle will be returned</param>
    /// <returns>The form's window handle of IntPtr.Zero if it has none</returns>
    private static IntPtr getWindowHandle(Form form) {
      if(form.IsHandleCreated) {
        return form.Handle;
      } else {
        return IntPtr.Zero;
      }
    }

    /// <summary>Called when the late-checked synchronizer finds the main window</summary>
    private Action<ISynchronizeInvoke> uiContextFoundCallback;

  }

} // namespace Nuclex.Windows.Forms