从类在Form1上按下捕获键

Question

Here is my class with event handler:

public delegate void MyKeyEventHandler(object sender, KeyEventArgs e);

public class KeyEvent
{
    public event MyKeyEventHandler keyEvent;
    public string key = "";

    protected virtual void KeyPressed(KeyEventArgs e) 
    {
        MyKeyEventHandler handler = keyEvent;
        if (handler != null)
            handler(this, e);
    }

    public KeyEvent()
    {
        keyEvent += new MyKeyEventHandler(keyPressed);
    }

    private void keyPressed(object sender, KeyEventArgs e)
    {
        key = e.KeyCode.ToString();
    }
}

And in my Form1 I have this code: (EDITED)

KeyEvent ke = new KeyEvent();

private void button1_Click(object sender, EventArgs e)
{
    MessageBox.Show(ke.key);
}

When I run form and press key on keyboard and then click on button it pop up empty MessageBox. What I want is everytime I press a key on keyboard a MessageBox to pop up and show me which key I pressed. This code is just for testing, I don't want just to pop up message box, I need the key for other things.

Note: To handle the events on Form1 is not a solution, I need to handle the events in class. I making a libray.

Answer 1

You can do something like this:

static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            Form1 f = new Form1();
            f.KeyPreview = true;
            f.KeyDown += f_KeyDown;
            Application.Run(f);
        }

        static void f_KeyDown(object sender, KeyEventArgs e)
        {
            MessageBox.Show(e.KeyValue.ToString());
        }
    }

If you enable KeyPreview you will receive all keyDowns even of the controls placed on the Form. If you set it to false you only get the KeyDown when the Form has focus.

As you can see in the code you can connect any KeyDownEventHandler method in any class to the Form.

As there where questions left:

 public class FormKeyListener
    {
        private Form formToListen = null;
        public void SetFormToListen(Form form)
        {

            if (formToListen != null)
                formToListen.KeyDown -= formToListen_KeyDown; // Unregister from old form if present

            formToListen = form;
            formToListen.KeyDown += formToListen_KeyDown; // Attach to new form
        }

        void formToListen_KeyDown(object sender, KeyEventArgs e)
        {
            MessageBox.Show(e.KeyValue.ToString());
        }
    }

Which can be call this way somewhere in your code:

FormKeyListener fkl = new FormKeyListener();
Form1 f = new Form1();
f.KeyPreview = true;
fkl.SetFormToListen(f);

Answer 2

It will not be possible for you to create a class that is capable of just automatically listening for events generated by its parent. How would that work? What would happen if you created a new KeyEvent class using something that wasn't a Form and didn't raise events?

You are going to need to give your KeyEvent class a reference to whatever thing that you want to monitor key events for. I realize that your intention is to create some third-party library, so you'd like to keep things as generic as possible in order to maximize its reuse. Since you are dependent on the KeyDown event, using Control is the most generic you can be. This is because it is where the KeyDown event is actually defined.

Let me show you what I mean...

1) Modify the constructor of your KeyEvent class to accept an instance of Control

public class KeyEvent
{
    // You don't need to declare an internal event
    public string key = "";
    private Control _control;

    public KeyEvent(Control control)
    {
        // Here is where we save the reference of Control that was passed to the class.
        // This will enable you to access the instance of the Form or whatever else
        // you want to use, which is helpful for unregistering events during cleanup
        _control = control;

        // Wire up your event handler to this SPECIFIC instance of Control.
        // This will cause your `keyPressed` method to execute whenever
        // the control raises the KeyDown event.  All of the middleman
        // event handling you are doing is unnecessary.
        _control.KeyDown += keyPressed;
    }

    private void keyPressed(object sender, KeyEventArgs e)
    {
        key = e.KeyCode.ToString();
    }

}

2) Modify the parent in which you are creating this KeyEvent to pass itself into the KeyEvent

// Now you pass 'this' into the constructor.  This can be a Form, or any other
// Winforms control that may inherit from Control (which is all of them)
KeyEvent ke = new KeyEvent(this);

private void button1_Click(object sender, EventArgs e)
{
    // At this point, ke.Key will always display the last
    // key pressed in the Form when the button is clicked.
    MessageBox.Show(ke.key);
}