Fig 1. SVG text editor with support for selection, copy, paste. Works on PC and mobile.

Demo | GitHub

The article describes how to make a text editor. Source code is attached.

Multiline text in SVG

SVG does not have a line break character. For multiline text SVG uses <tspan>.

Fig 2. Multiline text, third line is empty

<text x="0" y="0">
    <tspan x="0" y="0">Line 1</tspan>
    <tspan x="0" y="20px">Line 2</tspan>
    <!-- Line 3 is empty
    <tspan x="0" y="40px"></tspan> -->
    <tspan x="0" y="60px">Line 4</tspan>
</text>

Listing 1. Multiline text in SVG. The third line is empty. The line height is 20px.

The position of the <tspan> elements is relative to the top edge of the <text>. The value of the 'y' attribute must be calculated.

The 'y' attribute calculations can be avoided. Listing 2 gives the same result. The 'dy' attribute is used with a fixed value. 'dy' indicates the position relative to the previous element.

<text x="0" y="0">
    <tspan x="0" dy="0">Line 1</tspan>
    <tspan x="0" dy="20px">Line 2</tspan>
    <tspan x="0" dy="20px" visibility="hidden">.</tspan>
    <tspan x="0" dy="20px">Line 4</tspan>
</text>

Listing 2. Multiline text in SVG. The third line is empty. The line height is 20px. The indent is set relative to the previous element.

Forming multi-line markup with JavaScript

The function below renders markup with a fixed 'dy' attribute. The markup is obtained as in Listing 2.

/**
 * create multiline tspan markup
 * @param {string} str
 * @param {number} lineHeight
 * @returns {string}
 */
function svgStrToTspan(str, lineHeight) {
    return str.split('\n').map((t, i) => {
        return `<tspan
            x="0"
            dy="${i === 0 ? '0' : `${lineHeight}px`}"
            ${t.length === 0 ? 'visibility="hidden"' : ''}>

                ${t.length === 0
                    ? '.'
                    : escapeHtml(t).replaceAll(' ', '&nbsp;')}

            </tspan>`;
    }).join('');
}

Listing 3. Function makes multi-line markup

In Figure 1, when you add a line, the text moves up. Thus the text is always in the center of the circle. Listing 4 shows how this is implemented:

/**
 * @param {SVGTextElement} textEl target text element
 * @param {string} str
 * @param {{lineHeight:number, verticalMiddle?:number}} param
 * @returns {void}
 */
export function svgTextDraw(textEl, str, param) {
    textEl.innerHTML = svgStrToTspan(str, param.lineHeight);
    if (param.verticalMiddle != null) {
        textEl.y.baseVal[0].value =
            param.verticalMiddle - textEl.getBBox().height / 2;
    }
}

Listing 4. The function inserts text into SVG. When specifying verticalMiddle , the text is centered vertically.

Text editor

The editor must support all standard features:

• text navigation, selection, insertion, copying;
• autocorrect, spell check;
• work on PC and mobile

For standard features, there is a standard <textarea>.

The algorithm of the editor:

• A transparent <textarea> is positioned above the text. The <textarea> font is also transparent;
• On input, the svgTextDraw from Listing 4 is called;
• The dimensions and position of the <textarea> are recalculated.

The algorithm is implemented in the textareaCreate function. Function code in a separate file on GitHub.

An editor can be attached to any <text> element:

const textEditor = textareaCreate(
    // {SVGTextElement}
    textEl,
    // text params
    { lineHeight: 20, verticalMiddle: 10 },
    // init value
    'init text',
    // onchange
    val => {...},
    // onblur
    val => {...});
…
// delete textarea
textEditor.remove();

Listing 5. Creating a text editor for <text>

Other articles about dgrm.net

• JavaScript SVG diagram editor 3.9 KB (open source library)
• JavaScript diagram editor that renders diagrams from PNG images (open source)
• JavaScript text editor for SVG
• Flowchart editor UI
• JavaScript: Zoom like in maps for SVG/HTML

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