Flutter textfield that auto expands when text is e

问题:

I've tried many configurations of the Flutter TextField but can't figure out how to build this one.

I'm looking for a textfield that is a single line initially and it auto expands as the text is entered into it and then at some point begins scrolling itself.

This can be achieved partially by using the maxLines: null attribute. But then when a lot of text is entered the Text in the textfield itself overflows.

And if the maxLines is set to a value then the whole textfield itself gets expanded to those many lines to start off with rather than begining with a single line.

Is there a way to limit the height of textfield at some point like done in many chat apps like WhatsApp and telegram.

Thank you.

回答1:

Container(
    child: new ConstrainedBox(
        constraints: BoxConstraints(
            maxHeight: 300.0,
        ),
        child: new Scrollbar(
            child: new SingleChildScrollView(
                scrollDirection: Axis.vertical,
                reverse: true,
                child: new TextField(
                    maxLines: null,
                ),
            ),
        ),
    ),
)

回答2:

The accepted answer by Gunter is good enough if you don't have any style for the TextField. But if you have at least an underline / bottom border for the TextField, it will disappear when scroll up.

My recommendation is to calculating the lines with TextPainter, then apply the calculated number of lines to TextField. Here's the code, replace your current TextField with LayoutBuilder :

LayoutBuilder(
    builder: (context, size){
      TextSpan text = new TextSpan(
        text: yourTextController.text,
        style: yourTextStyle,
      );

      TextPainter tp = new TextPainter(
          text: text,
          textDirection: TextDirection.ltr,
          textAlign: TextAlign.left,
      );
      tp.layout(maxWidth: size.maxWidth);

      int lines = (tp.size.height / tp.preferredLineHeight).ceil();
      int maxLines = 10;

      return TextField(
        controller: yourTextController,
        maxLines: lines < maxLines ? null : maxLines,
        style: yourTextStyle,
      );
    }
  )