写法一：
//点击保存图片
save() {
let that = this
//若二维码未加载完毕，加个动画提高用户体验
wx.showToast({
icon: 'loading',
title: '正在保存图片',
duration: 1000
})
//判断用户是否授权"保存到相册"
wx.getSetting({
success(res) {
//没有权限，发起授权
if (!res.authSetting['scope.writePhotosAlbum']) {
wx.authorize({
scope: 'scope.writePhotosAlbum',
success() { //用户允许授权，保存图片到相册
that.savePhoto();
},
fail() { //用户点击拒绝授权，跳转到设置页，引导用户授权
wx.openSetting({
success() {
wx.authorize({
scope: 'scope.writePhotosAlbum',
success() {
that.savePhoto();
}
})
}
})
}
})
} else { //用户已授权，保存到相册
that.savePhoto()
}
}
})
},
//下载图片地址并保存到相册，提示保存成功
savePhoto() {
let that = this
wx.downloadFile({
url: that.data.img,
success: function(res) {
wx.saveImageToPhotosAlbum({
filePath: res.tempFilePath,
success(res) {
wx.showToast({
title: '保存成功',
icon: "success",
duration: 1000
})
}
})
}
})
}