[Flutter] 支持描边效果的Text

新版的flutter已经自带这个功能了。TextSyle 中一个shadow 。

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目前flutter中没找到很好的办法给Text增加描边。自己扩展了一个TextEx，可以实现简单的描边效果，能满足大部分情况下的需求。

class TextEx extends LeafRenderObjectWidget {
    TextEx(this.data, {
        Key key,
        this.style,
        this.textAlign = TextAlign.left,
        this.textDirection,
        this.locale,
        this.softWrap = true,
        this.overflow = TextOverflow.clip,
        this.textScaleFactor = 1.0,
        this.maxLines,
        this.semanticsLabel,
        this.shadow = Colors.black12
    }) : assert(data != null),
                super(key: key);

    final String data;
    TextSpan textSpan;
    final TextStyle style;
    final TextAlign textAlign;
    final TextDirection textDirection;
    final Locale locale;
    final bool softWrap;
    final TextOverflow overflow;
    final double textScaleFactor;
    final int maxLines;
    final String semanticsLabel;
    final Color shadow;

    @override
    RenderParagraph createRenderObject(BuildContext context) {
        assert(textDirection != null || debugCheckHasDirectionality(context));
        final textSpan = new TextSpan(
            style: style,
            text: data,
            children: null,
        );
        return new RenderParagraphEx(textSpan,
            textAlign: textAlign,
            textDirection: textDirection ?? Directionality.of(context),
            softWrap: softWrap,
            overflow: overflow,
            textScaleFactor: textScaleFactor,
            maxLines: maxLines,
            shadow: shadow,
            locale: locale ?? Localizations.localeOf(context, nullOk: true),
        );
    }

    @override
    void updateRenderObject(BuildContext context, RenderParagraph renderObject) {
        assert(textDirection != null || debugCheckHasDirectionality(context));
        final textSpan = new TextSpan(
            style: style,
            text: data,
            children: null,
        );
        renderObject
            ..text = textSpan
            ..textAlign = textAlign
            ..textDirection = textDirection ?? Directionality.of(context)
            ..softWrap = softWrap
            ..overflow = overflow
            ..textScaleFactor = textScaleFactor
            ..maxLines = maxLines
            ..locale = locale ?? Localizations.localeOf(context, nullOk: true);
    }

}

const String _kEllipsis = 'u2026';

class RenderParagraphEx extends RenderParagraph {
    RenderParagraphEx(TextSpan text, {
        TextAlign textAlign = TextAlign.start,
        @required TextDirection textDirection,
        bool softWrap = true,
        TextOverflow overflow = TextOverflow.clip,
        double textScaleFactor = 1.0,
        int maxLines,
        Locale locale,
        this.shadow = Colors.black87,
    }): super(text, textAlign: textAlign, textDirection: textDirection, softWrap: softWrap, overflow: overflow, textScaleFactor: textScaleFactor, maxLines: maxLines, locale: locale) {
        initTextPainter();
    }

    TextPainter _textPainter;

    final Color shadow;

    void initTextPainter() {
        if (_textPainter == null || _textPainter.text.text != text.text) {
            _textPainter = new TextPainter(text: TextSpan(style: TextStyle(
                inherit: text.style.inherit,
                color: shadow,
                fontSize: text.style.fontSize,
                fontWeight: text.style.fontWeight,
                fontFamily: text.style.fontFamily,
                fontStyle: text.style.fontStyle,
                wordSpacing: text.style.wordSpacing,
                textBaseline: text.style.textBaseline,
                letterSpacing: text.style.letterSpacing,
                height: text.style.height,
                locale: text.style.locale,
            ), text: text.text, children: text.children, recognizer: text.recognizer
            ),
                textAlign: textAlign,
                textDirection: textDirection,
                textScaleFactor: textScaleFactor,
                maxLines: maxLines,
                ellipsis: overflow == TextOverflow.ellipsis ? _kEllipsis : null,
                locale: locale,
            );
        }
    }

    void _layoutText({ double minWidth = 0.0, double maxWidth = double.infinity }) {
        final bool widthMatters = softWrap || overflow == TextOverflow.ellipsis;
        _textPainter.layout(minWidth: minWidth, maxWidth: widthMatters ? maxWidth : double.infinity);
    }

    void _layoutTextWithConstraints(BoxConstraints constraints) {
        _layoutText(minWidth: constraints.minWidth, maxWidth: constraints.maxWidth);
    }

    @override
    void paint(PaintingContext context, Offset offset) {
        final Canvas canvas = context.canvas;
        initTextPainter();
        _layoutTextWithConstraints(constraints);
        _textPainter.paint(canvas, offset.translate(1.0, 1.0));
        _textPainter.paint(canvas, offset.translate(-1.0, -1.0));
        _textPainter.paint(canvas, offset.translate(-1.0, 1.0));
        _textPainter.paint(canvas, offset.translate(1.0, -1.0));
        _textPainter.paint(canvas, offset.translate(1.0, 0.0));
        _textPainter.paint(canvas, offset.translate(0.0, 1.0));
        _textPainter.paint(canvas, offset.translate(1.5, 1.5));
        canvas.scale(1.0, 1.0);
        super.paint(context, offset);
    }
}