[英]How to promisify Cordova File plugin?
I'd like to promisify a method within the Apache Cordova File plugin, but I'm having an issue with 'this'. 我想在Apache Cordova File插件中提供一个方法,但是“ this”存在问题。
Initially, I was using arrow functions, but removed those when the issue with 'this' started occurring. 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);
I'd like to be able to promisify the function FileEntry.file(), but I'm getting this error when attempting this: TypeError: this.toInternalURL is not a function at FileEntry.file (FileEntry.js:82) 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)
That code can be viewed here: https://github.com/apache/cordova-plugin-file/blob/74a46467a081a87bb69b3f2518cbb1db5375028f/www/FileEntry.js#L80 可以在这里查看该代码: https : //github.com/apache/cordova-plugin-file/blob/74a46467a081a87bb69b3f2518cbb1db5375028f/www/FileEntry.js#L80
It's okay for your to use arrow functions when defining promisify
. 定义promisify
时可以使用箭头功能。 Here's a basic exmample which doesn't depend on context ( this
) - 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"
Now for a function that needs context, watch how we use .bind
to preserve context - 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"
Note this differs from the node-style promisify
where async functions only take one (1) callback and errors are always passed as the first argument to the callback - 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"
NB, promisify
as defined in this last snippet is included with Node as util.promisify
注意, promisify
一个代码段中定义的promisify
随util.promisify
包含在Node中
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