Growing TextArea in Blazor

Question

I need a text area that grows as the number of lines increases. As soon as lines are deleted, the TextArea should shrink again. In the best case with a maximum height.

How it works with Javascript I could already read here: textarea-to-resize-based-on-content-length

But in Blazor I have unfortunately, as far as I know, no "scrollHeight" available for the TextArea.

(My problem relates to the Blazor framework, which allows browser frontend development using C# (+HTML/CSS) and not a desktop UI like WPF/WinForms.)

Answer 1

As long as you know how many lines are in your text, you can just use the "rows" attribute on the TextView, like this

<textarea rows="@Rows"
          @bind-value="MyText"
          @bind-value:event="oninput" />

And in your code, you can determine the value for Rows

Note, I use Math.Max(Rows,2) to keep a minimum of two rows.

private void CalculateSize(string value)
{
  Rows = Math.Max(value.Split('\n').Length, value.Split('\r').Length);
  Rows = Math.Max(Rows, 2);
}

I call CalculateSize from code that handles changes to "MyText" - either a custom setter like this or another method

string _myText;
protected string MyText
{
  get => _myText;
  set
  {
    _myText = value; 
    CalculateSize(value);
  }
}

The maximum height can easily be set either through CSS for a design approach or by adding another constraint to the CalculateSize method.

private void CalculateSize(string value)
{
  Rows = Math.Max(value.Split('\n').Length, value.Split('\r').Length);
  Rows = Math.Max(Rows, MIN_ROWS);
  Rows = Math.Min(Rows, MAX_ROWS);
}

---

Option 2

If you want simplicity and don't mind a bit of inline JS (if you do, then it's time to crack open the JSInterop....)

<textarea 
      rows="2" 
      placeholder="Sample text."
      style="resize:both;"
      oninput="this.style.height = 'auto'; this.style.height = (this.scrollHeight) + 'px';">
</textarea>

---

Option 3

If you did want to use JSInterop, you could do something like this or place your code in a JS file and include it in the page.

<textarea id="MyTextArea"
      rows="2" 
      placeholder="Sample text."
      @oninput="Resize"></textarea>

<label>This area is @(MyHeight)px</label>
@code
{
[Inject] IJSRuntime JSRuntime { get; set; }
double MyHeight=0;
async Task Resize()
{
    var result = await JSRuntime.InvokeAsync<object>("eval",@"(function() {
            MyTextArea.style.height='auto';
            MyTextArea.style.height=(MyTextArea.scrollHeight)+'px';
            return MyTextArea.scrollHeight;
        })()");
    Double.TryParse(result.ToString(), out MyHeight);
}
}