[英]Using promises to execute a series of commands in nodejs
我想通過使用Q Promise通過Node.js中的串行線逐步處理一系列命令。
var cmdArr = ['cmd1', 'cmd2','cmd3'];
我不確定如何構建它。 我想到了這樣的事情,但是沒有用:
Q().then(function() {
cmdArr.forEach(command) {
//here to initialize the promise??
}
});
重要的是要保持順序並能夠在每個步驟之間使用Q.delay。
假設要執行的命令是某種異步函數調用:
var Q = require('q');
// This is the function you want to perform. For example purposes, all this
// does is use `setTimeout` to fake an async operation that takes some time.
function asyncOperation(input, cb) {
setTimeout(function() {
cb();
}, 250);
};
function performCommand(command) {
console.log('performing command', command);
// Here the async function is called with an argument (`command`),
// and once it's done, an extra delay is added (this could be optional
// depending on the command that is executed).
return Q.nfcall(asyncOperation, command).delay(1000);
}
// Set up a sequential promise chain, where a command is executed
// only when the previous has finished.
var chain = Q();
[ 'cmd1', 'cmd2', 'cmd3' ].forEach(function(step) {
chain = chain.then(performCommand.bind(null, step));
});
// At this point, all commands have been executed.
chain.then(function() {
console.log('all done!');
});
我對q
不太熟悉,所以可以做得更好。
為了完整bluebird
,這是使用bluebird
的版本:
var Promise = require('bluebird');
...
var asyncOperationAsPromised = Promise.promisify(asyncOperation);
function performCommand(command) {
console.log('performing command', command);
return asyncOperationAsPromised(command).delay(1000);
}
Promise.each(
[ 'cmd1', 'cmd2', 'cmd3' ],
performCommand.bind(null)
).then(function() {
console.log('all done!');
});
對數組上的一系列異步操作進行排序的常見設計模式是使用.reduce()
如下所示:
var cmdArr = ['cmd1', 'cmd2','cmd3'];
cmdArr.reduce(function(p, item) {
return p.delay(1000).then(function() {
// code to process item here
// if this code is asynchronous, it should return a promise here
return someAyncOperation(item);
});
}, Q()).then(function(finalResult) {
// all items done here
});
注意,我還顯示了您可以在其中插入Q的.delay()
。
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