[英]Q.js, promises, classes and “this”, what is the context?
I'm completely confused about the context inside a Q promise. 我对Q承诺中的上下文感到困惑。 I don't think that's Q specific, but general with all promises. 我不认为这是Q的具体,但一般都有所有的承诺。 What the hell is the context of this
inside a class? 到底是什么的情况下this
一个类里面呢?
This code uses TypeScript, everything is static now, because I basically failed to do anything non-static. 这段代码使用TypeScript,现在一切都是静态的,因为我基本上没有做任何非静态的事情。 This code works fine. 这段代码工作正常。
I tried to add a private _config;
我试图添加一个private _config;
instance variable and use the _getConfig
method to set the _config
in the constructor. 实例变量并使用_getConfig
方法在构造函数中设置_config
。 But when I used this._config
inside the method checkMongodbConnection
, well, it wasn't the same object as what was returned by the _getConfig()
method. 但是当我在方法checkMongodbConnection
使用this._config
时,它与_getConfig()
方法返回的对象不同。 (I watched variable states in debug mode) (我在调试模式下观察了变量状态)
So I guess that this
, inside a class, because I call the code from a Q promise, don't have the class instance context. 所以我猜this
,在一个类中,因为我从一个Q promise中调用代码,没有类实例上下文。
I'm wondering if using promises is a good idea after all, if I run into context issues the code will just be a lot more difficult to understand and to debug. 我想知道,如果使用的承诺是一个好主意,毕竟,如果我碰上的语境问题的代码将只是很多更难理解和调试。 I would appreciate to understand why and made a choice in consequence. 我很感激理解为什么并做出选择。 I don't want to lost the class instance context, that's way too much tricky. 我不想丢失类实例上下文,这太麻烦了。 I'm afraid to use a technology that will actually makes things more complicated, I prefer callback hell to that. 我害怕使用一种让事情变得更复杂的技术,我更喜欢回调地狱 。
///<reference path='./def/defLoader.d.ts'/>
export class App {
/**
* Constructor.
* Load the config.
* @return {}
*/
private static _getConfig(){
if(typeof __config !== "undefined"){
return __config;
}else{
require('./../../shared/lib/globals/services');
return configHelper.load('_serverConfig', require('./../../shared/config/_serverConfig.json').path.config, __dirname + '/../../');
}
}
/**
* Check that the mongoose connection open correctly, meaning that the mongod process is running on the host.
* @return {Q.Promise<T>|Function}
*/
public static checkMongodbConnection(){
var config = App._getConfig();
// Building promise
var deferred: any = Q.defer();
if(config.game.checkMongodb){
// Retrieves the mongoose configuration file, the env doesn't matter here.
var mongodbConfig = require('./../../shared/config/mongodb.json')['development'];
// Try mongoose connexion
mongoose.connect('mongodb://' + mongodbConfig.host + '/' + mongodbConfig.database);
// Bind connexion
var db: mongoose.Connection = mongoose.connection;
// Get errors
db.on('error', function(err) {
deferred.reject('Mongodb is not running, please run the mongod process: \n' + err)
});
// If the connexion seems to be open
db.once('open', function callback () {
// Close it
db.db.close();
// Resolve promise
deferred.resolve();
});
}else{
deferred.resolve();
}
// Get back promise
return deferred.promise;
}
/**
* Check that the redis connection is open, meaning that the redis-server process is running on the host.
* @return {Q.Promise<T>|Function}
*/
public static checkRedisConnection(){
var config = App._getConfig();
// Building promise
var deferred: any = Q.defer();
if(config.game.checkRedis) {
// Create the redis client to test to connexion on server
var redisClient:any = redis.createClient();
// Get client errors
redisClient.on("error", function (err) {
deferred.reject(err);
});
// Try applying a key
redisClient.set("keyTest", true);
// Now key is applied, try getting it
redisClient.get("keyTest", function (err, reply) {
if (err) {
deferred.reject("Redis is not running, please make sure to run redis before to start the server. \n" + err);
} else {
deferred.resolve();
}
});
}else{
deferred.resolve();
}
// Get back promise
return deferred.promise;
}
}
Code that calls the class: 调用类的代码:
Q.fcall(App.checkRedisConnection)
.then(App.checkMongodbConnection)
.then(function(result) {
// run server
}, console.error);
The promises/A+ specification dictates explicitly that the value of this
inside a promise chain is always undefined (strict mode) or the global object via: promises / A +规范明确规定promise链中的this
值总是未定义(严格模式)或全局对象via:
2.2.5 onFulfilled and onRejected must be called as functions (ie with no this value). 2.2.5 onFulfilled和onRejected必须作为函数调用(即没有此值)。
If you're not using a promise library like Bluebird that allows setting this
explicitly (via .bind
), you can still utilize TypeScript's fat arrows (also in ES6) to call something with lexical this
. 如果您使用的不是一个承诺库像蓝鸟,允许设置this
(通过明确.bind
),你仍然可以利用打字稿的脂肪箭(也ES6)来调用与词汇的东西this
。
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