canvas 绘制文本自动换行

2018/11/26

canvas 绘制文本自动换行

CanvasRenderingContext2D.prototype.wrapText = function (text, x, y, maxWidth, lineHeight) {
    if (typeof text != 'string' || typeof x != 'number' || typeof y != 'number') {
        return;
    }
    
    var context = this;
    var canvas = context.canvas;
    
    if (typeof maxWidth == 'undefined') {
        maxWidth = (canvas && canvas.width) || 300;
    }
    if (typeof lineHeight == 'undefined') {
        lineHeight = (canvas && parseInt(window.getComputedStyle(canvas).lineHeight)) || parseInt(window.getComputedStyle(document.body).lineHeight);
    }
    
    // 字符分隔为数组
    var arrText = text.split('');
    var line = '';
    
    for (var n = 0; n < arrText.length; n++) {
        var testLine = line + arrText[n];
        var metrics = context.measureText(testLine);
        var testWidth = metrics.width;
        if (testWidth > maxWidth && n > 0) {
            context.fillText(line, x, y);
            line = arrText[n];
            y += lineHeight;
        } else {
            line = testLine;
        }
    }
    context.fillText(line, x, y);
};
  1. 在链接页面还带着一些特效

借助 SVG 直接把 CSS 效果绘制上去

var canvas = document.querySelector('canvas');
var context = canvas.getContext('2d');
context.font = '16px sans-serif';
var width = canvas.width;
var height = canvas.height;
var tempImg = new Image();
tempImg.width = width;
tempImg.height = height;
tempImg.onload = function () {
    // 把img绘制在canvas画布上
    context.drawImage(this, 0, 0, width, height);
};
tempImg.src = 'data:image/svg+xml;charset=utf-8,<svg xmlns="http://www.w3.org/2000/svg"><foreignObject width="'+ width +'" height="'+ height +'"><body xmlns="http://www.w3.org/1999/xhtml" style="margin:0;font:'+ context.font +';">我是一段需要换行的文字啦啦啦</body></foreignObject></svg>';

canvas借助SVG foreignObject实现文本自动换行demo

  1. canvas 支持字符间距

    使用 css 的 letter-spacing 属性

var canvas = document.querySelector('canvas');
var context = canvas.getContext('2d');
var range = document.querySelector('input[type=range]');
// 绘制方法
var draw = function () {
    // 清除之前的绘制
    context.clearRect(0, 0, canvas.width, canvas.height);
    // 字符间距设置
    canvas.style.letterSpacing = range.value + 'px';
    // 并绘制文本,font属性值设置一定要在这里
    context.font = '32px sans-serif';
    context.fillText('我是一段文本', 0, 50);
};
// 改变字符间距后重绘
range.addEventListener('change', draw);
// 一进来根据默认值绘制
draw();

使用 canvas 计算逐字绘制

CanvasRenderingContext2D.prototype.letterSpacingText = function (text, x, y, letterSpacing) {
    var context = this;
    var canvas = context.canvas;
    
    if (!letterSpacing && canvas) {
        letterSpacing = parseFloat(window.getComputedStyle(canvas).letterSpacing);
    }
    if (!letterSpacing) {
        return this.fillText(text, x, y);
    }
    
    var arrText = text.split('');
    var align = context.textAlign || 'left';
    
    // 这里仅考虑水平排列
    var originWidth = context.measureText(text).width;
    // 应用letterSpacing占据宽度
    var actualWidth = originWidth + letterSpacing * (arrText.length - 1);
    // 根据水平对齐方式确定第一个字符的坐标
    if (align == 'center') {
        x = x - actualWidth / 2;
    } else if (align == 'right') {
        x = x - actualWidth;
    }
    
    // 临时修改为文本左对齐
    context.textAlign = 'left';
    // 开始逐字绘制
    arrText.forEach(function (letter) {
        var letterWidth = context.measureText(letter).width;
        context.fillText(letter, x, y);
        // 确定下一个字符的横坐标
        x = x + letterWidth + letterSpacing;
    });
    // 对齐方式还原
    context.textAlign = align;
};

3.canvas 支持竖直排列

CanvasRenderingContext2D.prototype.fillTextVertical = function (text, x, y) {
    var context = this;
    var canvas = context.canvas;
    
    var arrText = text.split('');
    var arrWidth = arrText.map(function (letter) {
        return context.measureText(letter).width;
    });
    
    var align = context.textAlign;
    var baseline = context.textBaseline;
    
    if (align == 'left') {
        x = x + Math.max.apply(null, arrWidth) / 2;
    } else if (align == 'right') {
        x = x - Math.max.apply(null, arrWidth) / 2;
    }
    if (baseline == 'bottom' || baseline == 'alphabetic' || baseline == 'ideographic') {
        y = y - arrWidth[0] / 2;
    } else if (baseline == 'top' || baseline == 'hanging') {
        y = y + arrWidth[0] / 2;
    }
    
    context.textAlign = 'center';
    context.textBaseline = 'middle';
    
    // 开始逐字绘制
    arrText.forEach(function (letter, index) {
        // 确定下一个字符的纵坐标位置
        var letterWidth = arrWidth[index];
        // 是否需要旋转判断
        var code = letter.charCodeAt(0);
        if (code <= 256) {
            context.translate(x, y);
            // 英文字符,旋转90°
            context.rotate(90 * Math.PI / 180);
            context.translate(-x, -y);
        } else if (index > 0 && text.charCodeAt(index - 1) < 256) {
            // y修正
            y = y + arrWidth[index - 1] / 2;
        }
        context.fillText(letter, x, y);
        // 旋转坐标系还原成初始态
        context.setTransform(1, 0, 0, 1, 0, 0);
        // 确定下一个字符的纵坐标位置
        var letterWidth = arrWidth[index];
        y = y + letterWidth;
    });
    // 水平垂直对齐方式还原
    context.textAlign = align;
    context.textBaseline = baseline;
};