在很多小程序都用到了生成海报并保存分享的功能，前面也做了一个这样的功能，用了一段时间，感觉还可以，分享并记录一下。

// 绘制海报
const norPoster = (ops) => {
	let bg = '', bgInfo = null, qr = this.downloadFileImg(ops.qr)
	if (ops.bg) {
		bg = this.downloadFileImg(ops.bg)
		
		// 获取图片信息
		bgInfo = new Promise((resolve) => {
			uni.getImageInfo({
				src: ops.bg,
				success(res) {
					resolve(res)
				},
				fail() {
					uni.showToast({
						title: '请重试一下',
						icon: 'loading'
					})
				}
			})
		})
	}

	return new Promise((resolve) => {
		Promise.all([qr, bg, bgInfo]).then(result => {
			const ctx = uni.createCanvasContext(ops.id, this)
			
			let w = 375, h = 673, imgInfo = result[2]
			
			ctx.setFillStyle('#fff') //设置背景色
			ctx.fillRect(0, 0, w, h)
			
			let dw = w / imgInfo.width, dh = h / imgInfo.height
			
			// 裁剪图片中间部分
			if((imgInfo.width > w && imgInfo.height > h) || (imgInfo.width < w && imgInfo.height < h)) {
				if(dw > dh) {
					ctx.drawImage(result[1], 0, (imgInfo.height - w / dw) / 2, imgInfo.width, w / dw, 0, 0, w, h)
				} else {
					ctx.drawImage(result[1], (imgInfo.height - w / dh) / 2, 0, w / dh, imgInfo.height, 0, 0, w, h)
				}
			} else {
				if(imgInfo.width < w) {
					ctx.drawImage(result[1], 0, (imgInfo.height - w / dw) / 2, imgInfo.width, w / dw, 0, 0, w, h)
				} else {
					ctx.drawImage(result[1], (imgInfo.width - w / dh) / 2, 0, w / dh, imgInfo.height, 0, 0, w, h)
				}
			}

			// 绘制文本信息
			ctx.setTextAlign('left')
			ctx.setFillStyle('#161621')
			ctx.setFontSize(w / 750 * 26)
			ctx.fillText(ops.des, 20, h - 65 + j * 20)
			
			// 绘制二维码
			ctx.drawImage(result[0], w - 80, h - 80, 60, 60) 

			ctx.draw(false, () => {
				// canvas画布转成图片并返回图片地址
				uni.canvasToTempFilePath({
					canvasId: ops.id,
					quality: 1,
					success: res => {
						resolve(res.tempFilePath)
					},
					fail: err => {
						uni.showToast({
							title: '绘制失败'
						})
					}
				}, this)
			})
		})
	})
}

// 获取图片信息
const downloadFileImg = (url) => {
	return new Promise(resolve => {
		uni.getImageInfo({
			src: url,
			success: res => {
				resolve(res.path)
			},
			fail: err => {
				uni.showToast({
					title: '网络错误请重试一下',
					icon: 'loading'
				})
			}
		})
	})
}

// 保存图片到相册
const savePosterImg = (path) => {
	const _this = this
	uni.getSetting({
		success(res) {
			if(res.authSetting['scope.writePhotosAlbum'] == false) {
				uni.showModal({
					content: '是否授权将图片保存到相册？',
					success(res) {
						if(res.confirm) {
							uni.openSetting({
								success(res) {
									setTimeout(() => {
										if(res.authSetting['scope.writePhotosAlbum'] == true) {
											uni.saveImageToPhotosAlbum({
												filePath: path,
												success: () => {
													_this.toast('保存成功，快去分享到朋友圈吧~')
												},
												fail: () => {
													_this.toast('保存失败~')
												}
											})
										}
									}, 500)
								}
							})
						}
					}
				})
			} else {
				uni.saveImageToPhotosAlbum({
					filePath: path,
					success: () => {
						_this.toast('保存成功，快去分享到朋友圈吧~')
					},
					fail: () => {
						_this.toast('保存失败~')
					}
				})
			}
		}
	})
}