我一直试图养成使用Promise的习惯,但是在尝试在Meteor上下文中的服务器端代码中使用它们时遇到了问题.这就是问题:
if (Meteor.isServer) {
Meteor.startup(function () {
// code to run on server at startup
p = function(){
return new Promise(function(res,rej) {
res("asd");
});
};
p().then(function(asd){
console.log("asd is " + asd);
return "zxc"
}).then(Meteor.bindEnvironment(function(zxc){
console.log("zxc is " + zxc);
return "qwe"
})).then(function(qwe){
console.log("qwe is " + qwe);
});
});
}
mvrx:bluebird软件包已安装
代码也可在GitHub获得
预期产量:
asd is asd
zxc is zxc
qwe is qwe
实际输出:
asd is asd
zxc is zxc
qwe is undefined
删除Meteor.bindEnvironment包装器可以解决此问题,但我需要它才能在回调中使用Collections
那我在这里想念什么?只是不可能以这种方式使用Promises Meteor还是有bug?
我实际上想要完成的是并行管道,该管道具有重要的部分结果,但需要同步结束.这样的事情.
if (Meteor.isServer) {
Meteor.startup(function () {
// code to run on server at startup
promises = [];
step1 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 1");
res(input);
});
};
step2 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 2");
res(input);
});
};
step3 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 3");
res(input);
});
};
slowIO = function(input){
var inp = input;
return new Promise( function(res,rej){
setTimeout(function(){
console.log(inp + ":SlowIO");
res(inp);
},Math.random()*20000);
});
};
end = function(input){
return new Promise(function(res,rej){
console.log(input + ": done, commiting to database");
res()
});
};
for (var i = 0; i < 100; ++i) {
promises.push(step1("pipeline-" + i).then(step2).then(slowIO).then(step3).then(end));
};
Promise.all(promises).then(function(){
console.log("All complete")
});
});
}
解决方法:
(更新:I’ve logged an issue on github to see if it can be resolved.)
以此方式使用时,似乎Meteor.bindEnvironment存在问题.
If it’s called from outside a Fiber
it won’t return it’s value.注意Fiber(runWithEnvironment).run()之前缺少的收益
目前,一个简单的解决方案是返回Promise而不是结果:
// when passed as a callback to `Promise#then`
// allows it to resolve asynchronously
var asyncThen = function(fn){
return function(arg){
return new Promise(function(resolve, reject){
fn(arg, resolve, reject);
})
};
};
Promise.resolve("asd").then(function(asd){
console.log("asd is " + asd);
return "zxc"
}).then(
asyncThen(
Meteor.bindEnvironment(function(zxc, resolve, reject){
console.log("zxc is", zxc);
resolve("qwe");
})
)
).then(function(qwe){
console.log("qwe is " + qwe);
});