javascript - 如何 promise Cordova 文件插件？

Question

我想在 Apache Cordova 文件插件中 promise 一种方法，但我对“this”有疑问。

最初，我使用箭头函数，但当“this”问题开始出现时删除了这些函数。

    /**
     * @param {Function} funcToPromisify -
     * @param {*} firstArgForFuncToPromisify -
     * @returns {Promise<any>} -
     */
    const promisifyFunctionOne = function(funcToPromisify, firstArgForFuncToPromisify) {
        return new Promise(function (resolve, reject) {
            funcToPromisify(firstArgForFuncToPromisify, resolve, reject);
        });
    };


    /**
     * @param {Function} funcToPromisify -
     * @returns {Promise<any>} -
     */
    const promisifyFunctionTwo = function(funcToPromisify) {
        return new Promise(function(resolve, reject) {
                funcToPromisify(resolve, reject);
            }
        );
    };


    /**
     * @param {string} pathToFile -
     * @param {Function} urlResolverFunc -
     * @param {object} stateObj -
     * @returns {void} -
     */ 
    const readFile = async function(pathToFile, urlResolverFunc) {
        const fileEntry = await promisifyFunctionOne(urlResolverFunc, pathToFile);  
        console.log(fileEntry); 
        try {
            const fileObj = await promisifyFunctionTwo(fileEntry.file);            
            let reader = new FileReader();        
            reader.onloadend = function() {
                console.log('Successful file read: ' + this.result);            
            };        
            reader.readAsText(fileObj);                    
        }
        catch(error) {        
            console.log(error);
        }
    };

    // Example of how readFile() is being called.
    readFile(pathToFile, window.resolveLocalFileSystemURL);

我希望能够 promise 函数 FileEntry.file()，但我尝试此操作时出现此错误: TypeError: this.toInternalURL is not a function at FileEntry.file (FileEntry.js:82)

可以在此处查看该代码:https://github.com/apache/cordova-plugin-file/blob/74a46467a081a87bb69b3f2518cbb1db5375028f/www/FileEntry.js#L80

Answer 1

定义promisify时可以使用箭头函数。这是一个不依赖于上下文的基本示例 (this) -

const promisify = (f) =>
  (...a) => new Promise ((res, rej) => f (...a, res, rej))

const basic = (a, b, c, success, failure) =>
  c === 0
    ? failure (Error("c cannot be zero"))
    : success (a + b + c)

promisify(basic)(1, 2, 3).then(console.log, console.error)
// 6

promisify(basic)(1, 2, 0).then(console.log, console.error)
// Error: "c cannot be zero"

现在对于需要上下文的函数，请观看我们如何使用 .bind 来保留上下文 -

const promisify = (f) =>
  (...a) => new Promise ((res, rej) => f (...a, res, rej))

const account =
  { balance: 100
  , withdraw: function (amount, success, failure)
    { if (amount > this.balance)
        failure(Error("insufficient funds"))
      else
        (this.balance -= amount, success(this))
    }
  }
  
const withdraw = 
  promisify(account.withdraw.bind(account))
 
withdraw(35).then(console.log, console.error)
// { balance: 65, withdraw: fn... }

withdraw(9999).then(console.log, console.error)
// Error: "insufficient funds"

<小时/>

请注意，这与节点样式的 promisify 不同，其中异步函数仅接受一个 (1) 回调，并且错误始终作为回调的第一个参数传递 -

const promisify = f =>
  (...a) => new Promise ((res, rej) =>
    f (...a, (e, x) =>
      e ? rej(e) : res(x)
    )
  )
  
const divide = (a, b, nodeCallback) =>
  b === 0
    ? nodeCallback(Error("cannot divide by zero"))
    : nodeCallback(null, a/b)
    
const pdivide =
  promisify(divide)

pdivide(10,2).then(console.log, console.error)
// 5

pdivide(10,0).then(console.log, console.error)
// Error: "cannot divide by zero"

注意，最后一个代码段中定义的 promisify 包含在 Node 中，作为 util.promisify