如何推广Cordova File插件？

Question

我想在Apache Cordova File插件中提供一个方法，但是“ this”存在问题。

最初，我使用的是箭头功能，但是当出现“ this”问题时，我将其删除了。

    /**
     * @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);

我希望能够对函数FileEntry.file（）进行赋值，但是在尝试执行此操作时遇到此错误：TypeError：this.toInternalURL不是FileEntry.file的函数（FileEntry.js：82）

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

Answer 1

定义promisify时可以使用箭头功能。 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，请观察我们如何使用.bind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" 

---

请注意，这与节点风格的promisify不同， promisify异步函数仅接受一 （1）个回调，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， promisify一个代码段中定义的promisify随util.promisify包含在Node中