微信小程序利用Canvas绘制图片和竖排文字详解

<view>
<canvas class='share-canvas' style="width:100%;height:{{canvasHeight}}px" canvas-id="share_canvas"></canvas>
</view>

drawHeroImage: function (path) {
var that = this;
// 拿到canvas context
let ctx = wx.createCanvasContext('share_canvas');
// 为了保证图片比例以及绘制的位置，先要拿到图片的大小
wx.getImageInfo({
src: path,
success: function (res) {
// 计算图片占比信息 
let maxWidth = Math.min(res.width, that.data.canvasWidth * 0.65);
let radio = maxWidth / res.width;
let offsetY = (that.data.canvasHeight - res.height * radio) / 2;
console.log('offsetY=' + offsetY);
that.setData({
imageWidth: res.width * radio,
imageHeight: res.height * radio,
offsetY: offsetY,
});
// 绘制canvas背景，不属于绘制图片部分
ctx.setFillStyle('white')
ctx.fillRect(0, 0, that.data.canvasWidth, that.data.canvasHeight);
// 绘制武将图片，path是本地路径，不可以传网络url，如果是网络图片需要先下载
ctx.drawImage(path, 10, offsetY, res.width * radio, res.height * radio)
// 绘制小程序码
that.drawQrCodeImage(ctx);
// 绘制势力汉字：吴
that.drawInfluence(ctx, that.data.hero.HERO.INFLUENCE);
// 绘制武将姓名：陆逊
that.drawName(ctx, that.data.hero.HERO.NAME);
// 绘制武将称号：江陵侯
that.drawHorner(ctx, that.data.hero.HERO.HORNER);
// 最终调用draw函数，生成预览图
// 一个坑点：只能调用一次，否则后面的会覆盖前面的
ctx.draw();
}
});
}

// 绘制武将称号：江陵侯
drawHorner: function (ctx, text) {
// 设置字号
ctx.setFontSize(26);
// 设置字体颜色
ctx.setFillStyle("#000000");
// 计算绘制起点
let x = this.data.offsetX + 35;
let y = this.data.offsetY + 10;
console.log('drawHorner' + text);
console.log(x);
console.log(y);
// 绘制竖排文字，这里是个Util函数，具体实现请继续看
Canvas.drawTextVertical(ctx, text, x, y);
}

CanvasRenderingContext2D.prototype.letterSpacingText = function (text, x, y, letterSpacing)

canvas.js 
/**
* @author zhangxinxu(.com)
* @licence MIT
* @description http://www.zhangxinxu.com/wordpress/?p=7362
*/
function drawTextVertical(context, text, x, y) {
var arrText = text.split('');
var arrWidth = arrText.map(function (letter) {
return 26;
// 这里为了找到那个空格的 bug 做了许多努力，不过似乎是白费力了
// const metrics = context.measureText(letter);
// console.log(metrics);
// const width = metrics.width;
// return 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;
}
module.exports = {
drawTextVertical: drawTextVertical
}

downloadHeroImage: function () {
// 微信不支持非https的图片下载，这里了个替换
let url = this.data.hero.HERO.ICON.replace(/http/, "https");
var that = this;
wx.downloadFile({
url: url,
success: function (res) {
// 下载成功后拿到图片的路径，然后开始绘制
var path = res.tempFilePath;
that.drawHeroImage(path);
}, fail: function (res) {
console.log(res)
}
});
}

saveShareImage: function () {
wx.showLoading({
title: '正在保存图片..',
});
let that = this;
wx.canvasToTempFilePath({
x: 0,
y: 0,
width: that.data.canvasWidth,
height: that.data.canvasHeight,
canvasId: 'share_canvas',
success: function (res) {
wx.saveImageToPhotosAlbum({
filePath: res.tempFilePath,
success(res) {
console.log(res);
wx.showToast({
title: '保存到相册成功',
duration: 1500,
})
},
fail(res) {
console.log(res)
wx.showToast({
title: '保存到相册失败',
icon: 'fail'
})
},
complete(res) {
console.log(res)
}
})
}
})
}