Add MaxLength limit to cursor position updates

[cat0363]

Description of Change

An exception will occur on Android and iOS if you bind a string that exceeds the value specified in the MaxLength property to the Text property of Entry and Editor. In this PR, we will add consideration of the MaxLength property to cursor position updates.

On iOS, calling the GetTextRange method inside the SetTextRange method of the TextInputExtensions class caused an ArgumentNullException.

[src\Core\src\Platform\iOS\TextInputExtensions.cs]

internal static void SetTextRange(this IUITextInput platformView, int start, int selectedTextLength)
{
    int end = start + selectedTextLength;

    // Let's be sure we have positive positions
    start = Math.Max(start, 0);
    end = Math.Max(end, 0);

    // Switch start and end positions if necessary
    start = Math.Min(start, end);
    end = Math.Max(start, end);

    var startPosition = platformView.GetPosition(platformView.BeginningOfDocument, start);
    var endPosition = platformView.GetPosition(platformView.BeginningOfDocument, end);
    platformView.SelectedTextRange = platformView.GetTextRange(startPosition, endPosition);
}

This is because the startPosition and endPosition passed to the GetTextRange method are null. This is because null is returned if you specify an index that exceeds MaxLength in the offset of the GetPosition method. This offset should be set considering MaxLength.

I fixed it as follows.

[src\Core\src\Platform\iOS\TextInputExtensions.cs]

internal static void SetTextRange(this IUITextInput platformView, int start, int selectedTextLength, int maxLength)
{
    int end = start + selectedTextLength;

    // Let's be sure we have positive positions
    start = Math.Max(start, 0);
    end = Math.Max(end, 0);

    // Switch start and end positions if necessary
    start = Math.Min(start, end);
    end = Math.Max(start, end);

    if (maxLength >= 0)
    {
        start = Math.Min(start, maxLength);
        end = Math.Min(end, maxLength);
    }

    var startPosition = platformView.GetPosition(platformView.BeginningOfDocument, start);
    var endPosition = platformView.GetPosition(platformView.BeginningOfDocument, end);
    platformView.SelectedTextRange = platformView.GetTextRange(startPosition, endPosition);
}

[src\Controls\src\Core\Platform\iOS\Extensions\TextExtensions.cs]

public static void UpdateText(this UITextView textView, InputView inputView)
{
    // Setting the text causes the cursor to be reset to the end of the UITextView.
    // So, let's set back the cursor to the last known position and calculate a new
    // position if needed when the text was modified by a Converter.
    var oldText = textView.Text ?? string.Empty;
    var newText = TextTransformUtilites.GetTransformedText(
        inputView?.Text,
        textView.SecureTextEntry ? TextTransform.Default : inputView.TextTransform
    );

    // Re-calculate the cursor offset position if the text was modified by a Converter.
    // but if the text is being set by code, let's just move the cursor to the end.
    var cursorOffset = newText.Length - oldText.Length;
    var cursorPosition = textView.IsFirstResponder ? textView.GetCursorPosition(cursorOffset) : newText.Length;

    if (oldText != newText)
        textView.Text = newText;

    textView.SetTextRange(cursorPosition, 0, inputView.MaxLength);
}

public static void UpdateText(this UITextField textField, InputView inputView)
{
    // Setting the text causes the cursor to be reset to the end of the UITextView.
    // So, let's set back the cursor to the last known position and calculate a new
    // position if needed when the text was modified by a Converter.
    var oldText = textField.Text ?? string.Empty;
    var newText = TextTransformUtilites.GetTransformedText(
        inputView?.Text,
        textField.SecureTextEntry ? TextTransform.Default : inputView.TextTransform
    );

    // Re-calculate the cursor offset position if the text was modified by a Converter.
    // but if the text is being set by code, let's just move the cursor to the end.
    var cursorOffset = newText.Length - oldText.Length;
    var cursorPosition = textField.IsEditing ? textField.GetCursorPosition(cursorOffset) : newText.Length;

    if (oldText != newText)
        textField.Text = newText;

    textField.SetTextRange(cursorPosition, 0, inputView.MaxLength);
}

On Android, calling the SetSelection method inside the UpdateText method of the EditTextExtensions class caused an IndexOutOfBoundsException.

[src\Controls\src\Core\Platform\Android\Extensions\EditTextExtensions.cs]

public static void UpdateText(this EditText editText, InputView inputView)
{
    (var oldText, var newText) = GetTexts(editText, inputView);

    if (oldText != newText)
    {
        editText.Text = newText;

        // When updating from xplat->plat, we set the selection (cursor) to the end of the text
        editText.SetSelection(newText.Length);
    }
}

This is because an index exceeding the MaxLength property was specified as an argument when the SetSelection method was called.

I fixed it as follows.

[src\Controls\src\Core\Platform\Android\Extensions\EditTextExtensions.cs]

public static void UpdateText(this EditText editText, InputView inputView)
{
    (var oldText, var newText) = GetTexts(editText, inputView);

    if (oldText != newText)
    {
        editText.Text = newText;

        // When updating from xplat->plat, we set the selection (cursor) to the end of the text
        int selectionLength = newText.Length;
        if (inputView.MaxLength >= 0)
        {
            selectionLength = Math.Min(selectionLength, inputView.MaxLength);
        }
        editText.SetSelection(selectionLength);
    }
}

On Windows, the exception is not thrown, but updating the cursor position requires a similar fix.

[src\Controls\src\Core\Platform\Windows\Extensions\TextBoxExtensions.cs]

public static void UpdateText(this TextBox platformControl, InputView inputView)
{
    var hasFocus = platformControl.FocusState != UI.Xaml.FocusState.Unfocused;
    var passwordBox = platformControl as MauiPasswordTextBox;
    var isPassword = passwordBox?.IsPassword ?? false;
    var textTransform = inputView?.TextTransform ?? TextTransform.None;

    // Setting the text causes the cursor to be reset to position zero.
    // So, let's retain the current cursor position and calculate a new cursor
    // position if the text was modified by a Converter.
    var oldText = platformControl.Text ?? string.Empty;
    var newText = TextTransformUtilites.GetTransformedText(
        inputView?.Text,
        isPassword ? TextTransform.None : textTransform
    );

    // Re-calculate the cursor offset position if the text was modified by a Converter.
    // but if the text is being set by code, let's just move the cursor to the end.
    var cursorOffset = newText.Length - oldText.Length;
    int cursorPosition = hasFocus ? platformControl.GetCursorPosition(cursorOffset) : newText.Length;

    if (inputView?.MaxLength >= 0)
    {
        cursorPosition = Math.Min(cursorPosition, inputView.MaxLength);
    }

    if (oldText != newText && passwordBox is not null)
        passwordBox.Password = newText;
    else if (oldText != newText)
        platformControl.Text = newText;

    platformControl.Select(cursorPosition, 0);
}

However, there is no problem with the Windows TextBox Select method even if you specify an Index that exceeds MaxLength, so there is no problem even if you do not include a guard condition.

In addition to the solution in PR #7371, the MaxLength property should be considered.
This PR targets iOS, Android, and Windows, which were targeted for fixes in PR #7371.

Issues Fixed

Fixes #15937

[ghost]

Hey there @cat0363! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[juniorsaraviao]

who should review this PR?

[waguilar]

I have the same issue :/
Any chance someone can review this PR, pretty please?

[rodrigojuarez]

+1

[jfversluis]

/rebase

[jfversluis]

Hey @cat0363 I was just looking into this one, it seems like some work was done in this area. Could you confirm that this fix is still needed? And if you think it is, could you rebase it? Thanks!

[ghost]

Hi @cat0363. We have added the "s/pr-needs-author-input" label to this issue, which indicates that we have an open question/action for you before we can take further action. This PRwill be closed automatically in 14 days if we do not hear back from you by then - please feel free to re-open it if you come back to this PR after that time.

[cat0363]

@jfversluis , Thank you for the good news.
I tested the latest version of main and confirmed that the issue was resolved on Android, iOS, and Windows platforms.
I was busy with my day job, so my reply was delayed.

[jfversluis]

Hehehe no worries my friend, I think we should be apologising to you for the delay on this PR 😉

Thanks for your efforts here and confirming this is fixed! Looking forward to your future work on this project!