如何鏈接數組的承諾

Question

問題：使用Node 4.3和圖形魔術創建AWS Lambda函數來調整上傳的S3圖片的大小。

我的問題：我想不出如何將圖片對象數組集成到承諾流中。我可以找到的所有堆棧示例都顯示了在已知所有數據時已將Promise的對象數組顯示出來。但我需要先讀取圖像數據，然後將其傳遞給調整大小功能。

下面的函數在resizePicture解析前完成。我知道這個問題沒有返回承諾並按正確的順序解決。我很難過。

var pictureSizes = [ 
    {width: 100, size: 'thumbnail', suffix: '_t'}, 
    {width: 300, size: 'small', suffix: '_s'}, 
    {width: 600, size: 'medium', suffix: '_m'}, 
    {width: 1000, size: 'large', suffix: '_l'} 
]; 

exports.handler = function(event, context) { 

    var srcBucket = event.Records[0].s3.bucket.name;. 
    var srcKey = decodeURIComponent(event.Records[0].s3.object.key.replace(/\+/g, " ")); 
    var dstBucket = srcBucket + "pictures"; 
    var typeMatch = srcKey.match(/\.([^.]*)$/); 
    var fileName = path.basename(srcKey); 

    downloadImage({ Bucket: srcBucket, Key: srcKey}) 
    .then(result => pictureSizes.map(picture => resizePicture(result, picture))) 
    .then(result => saveImage(result, dstBucket, fileName)) 
    .catch(err => console.log('error', err)) 
} 

function downloadImage(params) { 
    console.log('download Image', params); 
    return new Promise(function(resolve, reject) { 
    s3.getObject(params, function(err, data) { 
     if (err) { 
     reject(err); 
     } 
     resolve(data); 
    }) 
    }); 
} 

function resizePicture(result, picture) { 
    return new Promise(function(resolve, reject) { 
    const extension = result.ContentType.split('/')[1]; 
    gm(result.Body) 
     .resize(picture.width) 
     .toBuffer(extension, function(err, buffer) { 
     if (err) { 
      reject(err); 
     } else { 
      resolve({ 
      key: `${picture.suffix}.${extension}`, 
      binary: buffer, 
      ContentType: result.contentType 
      }) 
     } 
     }); 
    }) 
} 

function saveImage(result, dstBucket, fileName) { 
    return new Promise(function(resolve, reject) { 
    s3.putObject({ 
     Bucket: dstBucket, 
     Key: `${fileName}${result.key}`, 
     Body: result.binary, 
     ContentType: result.ContentType 
    }, function(err, data) { 
     if (err) { 
     reject(err); 
     } 
     resolve(data); 
    }) 
    }) 
} 

Answer 1

啊，你已經差不多了！您需要爲您在此處生成的一系列承諾創建一個觀察器：pictureSizes.map(picture => resizePicture(result, picture))。

嘗試Bluebird's Promise.map，和你的線路可以閱讀：

.then(result => Promise.map(pictureSizes, picture => resizePicture(result, picture))) 

如果你想沒有任何嵌套函數，#bind是你想要什麼：

.then(result => Promise.map(pictureSizes, resizePicture.bind(null, result)))