Forcing a checkbox bound to a DataSource to update when it has not been viewed yet

问题

Here is a test framework to show what I am doing:

create a new project
add a tabbed control
on tab 1 put a button
on tab 2 put a check box
paste this code for its code

(use default names for controls)

public partial class Form1 : Form
{
    private List<bool> boolList = new List<bool>();
    BindingSource bs = new BindingSource();
    public Form1()
    {
        InitializeComponent();
        boolList.Add(false);
        bs.DataSource = boolList;
        checkBox1.DataBindings.Add("Checked", bs, "");
        this.button1.Click += new System.EventHandler(this.button1_Click);
        this.checkBox1.CheckedChanged += new System.EventHandler(this.checkBox1_CheckedChanged);

    }
    bool updating = false;
    private void button1_Click(object sender, EventArgs e)
    {
        updating = true;
        boolList[0] = true;
        bs.ResetBindings(false);
        Application.DoEvents();
        updating = false;
    }

    private void checkBox1_CheckedChanged(object sender, EventArgs e)
    {
        if (!updating)
            MessageBox.Show("CheckChanged fired outside of updating");
    }
}

The issue is if you run the program and look at tab 2 then press the button on tab 1 the program works as expected, however if you press the button on tab 1 then look at tab 2 the event for the checkbox will not fire untill you look at tab 2.

The reason for this is the controll on tab 2 is not in the "created" state, so its binding to change the checkbox from unchecked to checked does not happen until after the control has been "Created".

checkbox1.CreateControl() does not do anything because according to MSDN

CreateControl does not create a control handle if the control's Visible property is false. You can either call the CreateHandle method or access the Handle property to create the control's handle regardless of the control's visibility, but in this case, no window handles are created for the control's children.

I tried getting the value of Handle(there is no public CreateHandle() for CheckBox) but still the same result.

Any suggestions other than have the program quickly flash all of my tabs that have data-bound check boxes when it first loads?

EDIT-- per Jaxidian's suggestion I created a new class

public class newcheckbox : CheckBox
{
    public new void CreateHandle()
    {
        base.CreateHandle();
    }
}

I call CreateHandle() right after updating = true same results as before.

回答1:

I think I have a solution. The problem is not that you cannot create a Handle. You can do that by simply accessing the Handle get accessor on the Control. The problem is that WinForms does not create the control because it is not visible. As it turns out, behind the scenes, a System.Windows.Forms.Control has two overloads for CreateControl. The first, which is public, takes no parameters and it calls the second which is internal which takes a single boolean parameter: ignoreVisible which as the name implies allows the calling code to create the control even if it is not visible. The CreateControl method with no arguments passes false to this internal method which means that if the control is not visible, it is not created. So, the trick is to use Reflection to call the internal method. First, I created two methods for creating controls:

private static void CreateControls( Control control )
{
    CreateControl( control );
    foreach ( Control subcontrol in control.Controls )
    {
        CreateControl( subcontrol );
    }
}
private static void CreateControl( Control control )
{
    var method = control.GetType().GetMethod( "CreateControl", BindingFlags.Instance | BindingFlags.NonPublic );
    var parameters = method.GetParameters();
    Debug.Assert( parameters.Length == 1, "Looking only for the method with a single parameter" );
    Debug.Assert( parameters[0].ParameterType == typeof ( bool ), "Single parameter is not of type boolean" );

    method.Invoke( control, new object[] { true } );
}

Now, we add a call to CreateControls for the second tab:

public Form1()
{
    InitializeComponent();
    boolList.Add( false );
    bs.DataSource = boolList;
    checkBox1.DataBindings.Add( "Checked", bs, "" );
    this.button1.Click += this.button1_Click;
    this.checkBox1.CheckedChanged += this.checkBox1_CheckedChanged;

    CreateControls( this.tabPage2 );
}

In addition, I added some debugging messages so I could see if the event fired:

private void button1_Click( object sender, EventArgs e )
{
    Debug.WriteLine( "button1_Click" );
    updating = true;
    boolList[0] = true;
    bs.ResetBindings( false );
    Application.DoEvents();
    updating = false;
}

private void checkBox1_CheckedChanged( object sender, EventArgs e )
{
    Debug.WriteLine( "checkBox1_CheckedChanged" );
    if ( !updating )
    {
        Debug.WriteLine( "!updating" );
        MessageBox.Show( "CheckChanged fired outside of updating" );
    }
}

Now, whether you navigate to the second tab or not, clicking on the button on the first tab will fire the checkbox1_Changed event procedure. Given the design you provided, if you click on the button, it will not show the MessageBox because updating will be true. However, the Debug.WriteLine will show that it fired in the Output window.

来源：https://stackoverflow.com/questions/2523778/forcing-a-checkbox-bound-to-a-datasource-to-update-when-it-has-not-been-viewed-y

标签

winforms

controls

deferred-loading