On a Form I have two controls, A and B. Each one sends an event when its value changes. The Form handles A's ValueChanged event by setting B to some value, and B's ValueChanged by setting A's value.
Do I need to do anything special to prevent an infinite loop, where the user changes A, sending a ValueChanged event which causes B to update, which now sends it ValueChanged event, causing A to update...
I know some GUI toolkits, such as Qt, have logic built into the infrastructure to prevent loops like that. (See the "Enter Your Age" example in Ch. 1 of the Blanchette & Summerfield book on Qt4.) Some older toolkits required the programmer to define and manage a flag to detect recursion. For WinForms, I haven't read a definitive statement anywhere on this.
For a concrete example, suppose A and B are NumericUpDown controls to show temperature in Fahrenheit and Celsius. When the user changes one, the other updates to show the corresponding temperature in the other system.
In my experience, the loop usually ends because the values stop actually changing. Your example falls into this case. Consider the following scenario:
User changes the Fahrenheit to 32
Events update the Celsius to 0
Events set the Fahrenheit to 32
Nothing further happens because Fahrenheit did not change
This is usually implemented in the properties by putting a check at the top of the setter to not raise the changed event when the new value is the same as the current value.
The way in which I've managed to prevent this problem when implementing custom controls, is by raising events only when the value actually does get changed, like this.
public string Text
{
get { return _text; }
set
{
if (_text != value)
{
_text = value;
OnTextChanged();
}
}
}
One could argue that not checking whether the value is actually different before firing the event is bad code.
I don't recall ever experiencing these issues with Windows Forms controls, so the ones I've been using must be doing this check correctly.
The events will not loop in the NumericUpDown example that you have provided. The ValueChanged event of the NumericUpDown will only get fired when the value changes i.e. If the value of a NumericUpDown is 5 and in code you again set it to 5, no event will be fired. This behavior of the event stops it from looping when you have two NumericUpDown controls.
Now suppose you have two NumericUpDown controls A & B.
A was changed by the user, it fires an event
On the event fired by A, you calculate and set the value of B. B detects a value change and fires an event
On the event fired by B, you calculate and set the value of A. However this value would be the same as the original value and Windows will not fire an event of ValueChanged.
So in the case of Windows Form Controls, the Framework manages it for you, if you want to achieve this for your own classes you follow a similar principle. On the setter of a value, check if the new value is different from the old value. Only if it differs fire an event.
I think you should detach B event before set B in A event. So B event never fire when you set B in A event. you should same thing for B event. You can break loop.
Sorry for my English. I hope it help you.
There isn't really any standard. As a user, you should handle controls that raise useless events, and controls that do not. Even for .NET's own controls, documentation is lacking on this aspect so you're left just trying it.
Using a flag to detect recursion is possible, but nicer IMO is to change control.Value = newvalue; to if (control.Value != newvalue) control.Value = newvalue;.
That said, if you write your own controls, please do not raise useless events, and please clearly document that fact.
To answer your question I have built a simple test with the code below.
public partial class Form1 : Form
{
Random r = new Random();
public Form1()
{
InitializeComponent();
}
private void Form1_Shown(object sender, EventArgs e)
{
// This triggers the infinite loop between the two controls
numericUpDown1.Value = 10;
}
private void numericUpDown1_ValueChanged(object sender, EventArgs e)
{
int cur = (int)numericUpDown2.Value;
int r1 = r.Next(1, 100);
while (r1 == cur)
r1 = r.Next(1, 100);
Console.WriteLine("Random in NUM1=" + r1);
numericUpDown2.Value = r1;
}
private void numericUpDown2_ValueChanged(object sender, EventArgs e)
{
int cur = (int)numericUpDown1.Value;
int r1 = r.Next(1, 100);
while (r1 == cur)
r1 = r.Next(1, 100);
Console.WriteLine("Random in NUM2=" + r1);
numericUpDown1.Value = r1;
}
}
The form has only two NumericUpDown initialized with values 1 and 2.
As you could test in Visual Studio 2013 there is nothing to prevent an infinite recursion in the case in which you really manage to generate different numbers at every ValueChanged event.
To prevent the infinite loop you could use the usual technique. Declare a global form variable
bool changing = false;
Inside both events add this code
try
{
if (changing) return;
changing = true;
.... event code ....
}
finally
{
changing = false;
}
Related
I would like to use a numericupdown control on my application. I'm well aware that I could use a plain textbox instead, but I rather like the way this particular control's UI fits with what I'm doing in my application.
It also needs to have 0's at the left, per desired text output. If I'm not mistaken, this is not supported by standard numericupdown controls. It should never exceed 4 digits in length. However, if I input more, it must show favor to new keystrokes and drop left-most digits instead. The up and down arrows should increment/decrement the value per usual behavior. Even after keying in values.
It should never be allowed to run negative. It should only accept whole integers. This is easily handled by the stock functionality though.
Partly posting an answer for others who may follow. Partly looking for assurance that I'm not being an idiot.
Note that this hack depends on firing Sync() before extracting the final value. The timer will fire pretty quickly, but does not guarantee that things will happen in the correct order. It may not hurt to manually trigger Sync() immediately before extracting values.
public class UpDownWith0 : System.Windows.Forms.NumericUpDown
{
private System.Windows.Forms.Timer addzeros = new System.Windows.Forms.Timer();
public UpDownWith0()
{
this.addzeros.Interval = 500; //Set delay to allow multiple keystrokes before we start doing things
this.addzeros.Stop();
this.addzeros.Tick += new System.EventHandler(this.Sync);
}
protected override void OnTextBoxTextChanged(object source, System.EventArgs e)
{
this.addzeros.Stop(); //reset the elapsed time every time the event fires, handles multiple quick proximity changes as if they were one
this.addzeros.Start();
}
public void Sync(object sender, System.EventArgs e)
{
int val;
this.addzeros.Stop();
if (this.Text.Length > 4)
{
//I never want to allow input over 4 digits in length. Chop off leftmost values accordingly
this.Text = this.Text.Remove(0, this.Text.Length - 4);
}
int.TryParse(this.Text, out val); //could use Value = int.Parse() here if you preferred to catch the exceptions. I don't.
if (val > this.Maximum) { val = (int)this.Maximum; }
else if (val < this.Minimum) { val = (int)this.Minimum; }
this.Value = val; //Now we can update the value so that up/down buttons work right if we go back to using those instead of keying in input
this.Text = val.ToString().PadLeft(4, '0'); //IE: display will show 0014 instead of 14
this.Select(4, 0); //put cursor at end of string, otherwise it moves to the front. Typing more values after the timer fires causes them to insert at the wrong place
}
}
Background:
In my winforms form, I have a Checked ListView and a "master" checkbox called checkBoxAll.
The behaviour of the master is as follows:
If the master is checked or unchecked, all ListViewItems must change accordingly.
If the user unchecks a ListViewItem, the master must change accordingly.
If the user checks a ListViewItem, and all other ListViewItems are checked aswell, the master must change accordingly.
I have written the following code to mimic this behaviour:
private bool byProgram = false; //Flag to determine the caller of the code. True for program, false for user.
private void checkBoxAll_CheckedChanged(object sender, EventArgs e)
{
//Check if the user raised this event.
if (!byProgram)
{
//Event was raised by user!
//If checkBoxAll is checked, all listviewitems must be checked too and vice versa.
//Check if there are any items to (un)check.
if (myListView.Items.Count > 0)
{
byProgram = true; //Raise flag.
//(Un)check every item.
foreach (ListViewItem lvi in myListView.Items)
{
lvi.Checked = checkBoxAll.Checked;
}
byProgram = false; //Lower flag.
}
}
}
private void myListView_ItemChecked(object sender, ItemCheckedEventArgs e)
{
//Get the appropiate ListView that raised this event
var listView = sender as ListView;
//Check if the user raised this event.
if (!byProgram)
{
//Event was raised by user!
//If all items are checked, set checkBoxAll checked, else: uncheck him!
bool allChecked = true; //This boolean will be used to set the value of checkBoxAll
//This event was raised by an ListViewItem so we don't have to check if any exist.
//Check all items untill one is not checked.
foreach (ListViewItem lvi in listView.Items)
{
allChecked = lvi.Checked;
if (!allChecked) break;
}
byProgram = true; //Raise flag.
//Set the checkBoxAll according to the value determined for allChecked.
checkBoxAll.Checked = allChecked;
byProgram = false; //Lower flag.
}
}
In this example, I use a flag (byProgram) to make sure an event was caused by the user or not, thereby preventing an infinite loop (one event can fire another, which can fire the first one again etc. etc.). IMHO, this is a hacky solution.
I searched around but I couldn't find a MSDN documented method to determine if an User Control Event was directly fired thanks to the user. Which strikes me as odd (again, IMHO).
I know that the FormClosingEventArgs has a field which we can use to determine if the user is closing the form or not. But as far as I know, that is the only EventArg that provides this kind of functionality...
So in summary:
Is there a way (other than my example) to determine if an event was fired directly by the user?
Please note: I don't mean the sender of an event! It won't matter if I code someCheckBox.Checked = true; or manually set someCheckBox, the sender of the event will always be someCheckBox. I want to find out if it is possible to determine whether it was through the user (click) or by the program (.Checked = true).
Aaand also: 30% of the time it took to write this question was to formulate the question and the title correctly. Still not sure if it is a 100% clear so please edit if you think you can do better :)
No, there's no practical way to determine whether the change came from GUI or was done by program (in fact, you could analyze the callstack - but that's not recommended because it's very slow and error-prone).
BTW, there's one other thing you could do instead of setting byProgram. You could remove and add the event handler prior or after, respectively, change your controls:
checkBoxAll.CheckedChanged -= checkBoxAll_CheckedChanged;
// do something
checkBoxAll.CheckedChanged += checkBoxAll_CheckedChanged;
Instead of using the changed event, you could use the clicked event to cascade the change through to the relevant controls. This would be in response to a user click, and not the value being changed programatically.
This is something I come across quite a lot and what I tend to try do is not split it between user interaction vs program interaction - I use more generic code i.e. the UI is being updated and doesn't require any events to be handled. I usually package this up through BeginUpdate/EndUpdate methods e.g.
private int updates = 0;
public bool Updating { get { return updates > 0; } }
public void BeginUpdate()
{
updates++;
}
public void EndUpdate()
{
updates--;
}
public void IndividualCheckBoxChanged(...)
{
if (!Updating)
{
// run code
}
}
public void CheckAllChanged(...)
{
BeginUpdate();
try
{
// run code
}
finally
{
EndUpdate();
}
}
I am working with a DataGridView, and I use the CellValueChanged event.
I dont want this event to be triggered when I change a cell value by the code. However, I want it to be triggered when the user edits it.
That's why I enclose my cell value change operations with the following code :
void changeCellOperation()
{
dgv.CellValueChanged -= new DataGridViewCellEventHandler(dgv_CellValueChanged);
...
cell.Value = myNewCellValue
...
dgv.CellValueChanged += new DataGridViewCellEventHandler(dgv_CellValueChanged);
}
I ended to have several differents functions where my DataGridView cells are updated this way.
Because these functions are called from different places and can be nested, I cannot afford to keep this code as is to avoid event unwanted event reactivation.
So I ended up this way :
int valueChangedEventMask = 0;
void changeCellOperation()
{
valueChangedEventMask++;
...
cell.Value = myNewCellValue
...
valueChangedEventMask--;
}
void dgv_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
if (valueChangedEventMask > 0)
return
...
}
This works fine. Also when the calls are nested, including inside the event itself.
But the CellValueChanged event is now fired too many times for no reasons.
Because I often have to cope with this pattern, I am looking for a solution that can be applicable generally for Events in UIs, not only the DataGridView.
So my question is:
What is the best tip to mask UI Events correctly and avoid unnecessary Events fires ?
CellValueChanged is not an UI event, but a property changed event. That means you can not use it to distinguish user input from programmatic change. You can always use subscriber/unsucscribe or flag+/- or BeginEdit/EndEdit-similar technique, but maybe you have to find another (better) approach. To example, in case of checkbox you can use Click event instead of Changed, because (surprise!) it will tell you when the user click it and otherwise safely change value of Checked programmatically.
In case of DataGridView easiest would be to use Changed with some flag (which will be set when edit begins and reset when ends - see, CellBeginEdit/CellEndEdit ).
You could use CellEndEdit instead of CellValueChange. I don't know what your method dgv_CellValueChanged does, just be careful that CellEndEdit is fired every time you exit the edit mode for the cell, even if its value has not been changed. This means that you have to keep trace of the current values of your cells if you don't want the method to be executed when the value doesn't change.
I would avoid events related with the mouse such as CellClick because your users could use just the keyboard.
Anyway I usually avoid this kind of problems by separating the logic from the user interface, i.e. I write a separate class which is bound to the form. Take a look at MVVM (you can implement your own version in WinForms if you want) or the good old MVC.
I ended up mixing both solutions in a very simple one. I use a counter and I only hook/unhook the events I want to mask.
EventMask valueChangedEventMask;
// In the class constructor
valueChangedEventMask = new EventMask(
() => { dgv.CellValueChanged += new DataGridViewCellEventHandler(dgv_CellValueChanged); },
() => { dgv.CellValueChanged -= new DataGridViewCellEventHandler(dgv_CellValueChanged); }
);
// The value change operation I want to hide from the event
void changeCellOperation()
{
valueChangedEventMask.Push();
...
cell.Value = myNewCellValue
...
valueChangedEventMask.Pop();
}
// The class
public class EventMask
{
Action hook;
Action unHook;
int count = 0;
public EventMask(Action hook, Action unHook)
{
this.hook = hook;
this.unHook = unHook;
}
public void Push()
{
count++;
if (count == 1)
unHook();
}
public void Pop()
{
count--;
if (count == 0)
hook();
}
}
I need to add a textbox in my application that starts showing with the value 0.00 just like a ATM, as you type the numbers then it keeps the two decimal point until satisfied with the value for example the sequence to end up of a value of 1023.00 would be (as I type)
0.01
0.10
1.02
10.23
102.30
1023.00
Is this possible to do in a windows forms application?. I am just not sure how to go about it.
Thank you.
In this kind of scenario I would not use a textbox, but a label or a read-only textbox. To get the user input just use the key-press event on your form (you have to enable KeyPreview on the form too) - then just remember the keys/numbers pressed and output the format you are targeting - should be a rather easy algorithm (Char.IsNumber, and String.Format might come in handy):
private int _inputNumber = 0;
private void Form_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if (!Char.IsNumber(e.KeyChar)) return;
_inputNumber = 10*_inputNumber + Int32.Parse(e.KeyChar.ToString());
ReformatOutput();
}
private void ReformatOutput()
{
myOutput.Text = String.Format("{0:0.00}", (double)_inputNumber / 100.0);
}
Note: Why not use a textbox - because it's internal logic with select/replace/remove/type is so overcomplicated that the cases you would have to check are just to much to handle gracefully - so instead of trying to change a cannon into a sling you should start with the sling.
I'm making a simple Guess-The-Number game with a GUI. I need to wait on a loop waiting for the user to input a number in a text box and press "OK". How do I wait for an event inside a loop?
Note: I don't want message boxes. This is done in the main window, hence the need to wait for input.
EDIT: I should have explained myself better. I know that there's a loop inside the GUI. What I want is another loop inside a method. Maybe there's a better way to do this. I could code stuff inside the button's event handler, now that I think about it. Although I'd need global variables. Whataver, I'll think about it, but I hope my question is clearer now.
EDIT 2: Sorry that my question wasn't clear and the edit didn't do much help. First of all, the code is too big to be posted here. I'd probably have to post a screenshot of the GUI, so it wouldn't be of much use. Basically, I have two fields, "Max number" and "Number of allowed guesses". The user enters these two and clicks "Play". A new panel becomes available, with a text box and a "Guess" button. The user enters a guess, and the program checks to see if it's correct.
The purpose of the second infinite loop is to avoid global variables. See, each time the user clicks "Play", the game has to generate a new random number as the correct guess. If everything is done inside a method, no problem. But if the "Guess" button's event handler is called multiple times, the number has to be stored as an instance variable of the Form. Sure, it's not big deal, but I think the number should be a property of the method directing the current game, not of the Form.
I'd also have to keep track of the remaining number of guesses outside of the method. Again, it's no big deal. I just want to avoid globals if I can.
Again, I'm sorry that my question wasn't too clear. I'm kind of tired, and I didn't feel like writing too much. If this still isn't clear, then don't bother. I'll think of something.
C# automatically loops infinitely waiting for events until your form is closed. You just need to respond to the button click event.
Jason Down's suggestion is wise, create a new GuessingGame class and add it to your project. I know you're worried about "global variables" (which everyone is taught in school never to use unless you absolutely have to), but think about your design specifications for a minute.
But if the "Guess" button's event handler is called multiple times, the number has to be stored as an instance variable of the Form. Sure, it's not big deal, but I think the number should be a property of the method directing the current game, not of the Form.
As an alternative, store an instance of your GuessingGame class in the form. This is not a global variable! You said so yourself, the point of the game is keep track of the guesses and generate new numbers to guess every time "Play" is clicked. If you store an instance of the game in the form then open another form (e.g. a Help or About box), then the game's instance would not be available (thus, not global).
The GuessingGame object is going to look something like:
public class GuessingGame
{
private static Random _RNG = new Random();
private bool _GameRunning;
private bool _GameWon;
private int _Number;
private int _GuessesRemaining;
public int GuessesRemaining
{
get { return _GuessesRemaining; }
}
public bool GameEnded
{
get { return !_GameRunning; }
}
public bool GameWon
{
get { return _GameWon; }
}
public GuessingGame()
{
_GameRunning = false;
_GameWon = false;
}
public void StartNewGame(int numberOfGuesses, int max)
{
if (max <= 0)
throw new ArgumentOutOfRangeException("max", "Must be > 0");
if (max == int.MaxValue)
_Number = _RNG.Next();
else
_Number = _RNG.Next(0, max + 1);
_GuessesRemaining = numberOfGuesses;
_GameRunning = true;
}
public bool MakeGuess(int guess)
{
if (_GameRunning)
{
_GuessesRemaining--;
if (_GuessesRemaining <= 0)
{
_GameRunning = false;
_GameWon = false;
return false;
}
if (guess == _Number)
{
_GameWon = true;
return true;
}
else
{
return false;
}
}
else
{
throw new Exception("The game is not running. Call StartNewGame() before making a guess.");
}
}
}
This way, all the data related to the game is encapsulated within the class. Hooking up the events is easy in the codebehind of the form:
GuessingGame game = new GuessingGame();
private void btnPlay_Click(object sender, EventArgs e)
{
int numberOfGuesses = Convert.ToInt32(txtNumberOfGuesses.Text);
int max = Convert.ToInt32(txtMax.Text);
game.StartNewGame(numberOfGuesses, max);
}
private void btnGuess_Click(object sender, EventArgs e)
{
int guess = Convert.ToInt32(txtGuess.Text);
bool correct = game.MakeGuess(guess);
if (correct)
lblWin.Visible = true;
if (game.GameEnded)
{
// disable guess button, show loss label
}
}
You should probably look for a book to actually learn windows programming.
The very basics:
1) There is already an infinite loop deep down in the windows code somewhere. Any windows program is constantly looping and scanning for input.
2) Once input is found, this loop fires off an Event.
3) Your mission, should you choose to accept it, is to write event handlers to handle those events.
you are most likely doing it wrong as it has already been pointed out, but you can use this
Application.DoEvents();
to process events when you are on an actual loop
to do it the right way
- don't use a loop
- use an edit box for the input, then a button
- implement the button onclick event
Yes, and What if I am waiting for Speech events, it could happen anytime event when a function is running, I need to handle that without recursively call a function