jQuery 1.5带来了新的Deferred对象和附加方法.when、.Deferred和. _deferred。
对于那些以前没有使用过. deferred的人，我已经为它注释了源代码。
这些新方法的可能用途是什么，我们如何将它们纳入模式?
我已经阅读了API和源代码，所以我知道它是做什么的。我的问题是我们如何在日常代码中使用这些新特性?
我有一个简单的缓冲区类示例，它按顺序调用AJAX请求。(上一个结束后下一个开始)。
/* Class: Buffer
* methods: append
*
* Constructor: takes a function which will be the task handler to be called
*
* .append appends a task to the buffer. Buffer will only call a task when the
* previous task has finished
*/
var Buffer = function(handler) {
var tasks = [];
// empty resolved deferred object
var deferred = $.when();
// handle the next object
function handleNextTask() {
// if the current deferred task has resolved and there are more tasks
if (deferred.isResolved() && tasks.length > 0) {
// grab a task
var task = tasks.shift();
// set the deferred to be deferred returned from the handler
deferred = handler(task);
// if its not a deferred object then set it to be an empty deferred object
if (!(deferred && deferred.promise)) {
deferred = $.when();
}
// if we have tasks left then handle the next one when the current one
// is done.
if (tasks.length > 0) {
deferred.done(handleNextTask);
}
}
}
// appends a task.
this.append = function(task) {
// add to the array
tasks.push(task);
// handle the next task
handleNextTask();
};
};
我正在寻找. deferred和.when的演示和可能的用法。
看到. _deferred的例子也很不错。
链接到新的jQuery。Ajax源代码的例子是作弊。
我特别感兴趣的是，当我们抽象出一个操作是同步完成还是异步完成时，可以使用哪些技术。
ehyds的答案将不起作用，因为它缓存了响应数据。它应该缓存jqXHR，这也是一个Promise。
下面是正确的代码:
var cache = {};
function getData( val ){
// return either the cached value or an
// jqXHR object (which contains a promise)
return cache[ val ] || $.ajax('/foo/', {
data: { value: val },
dataType: 'json',
success: function(data, textStatus, jqXHR){
cache[ val ] = jqXHR;
}
});
}
getData('foo').then(function(resp){
// do something with the response, which may
// or may not have been retreived using an
// XHR request.
});
Julian d的答案是正确的，是一个更好的解决方案。
这里是一个与ehynd的答案略有不同的AJAX缓存实现。
正如fortuneRice的后续问题所指出的，如果请求在其中一个返回之前执行，ehynd的实现实际上并不能阻止多个相同的请求。也就是说,
for (var i=0; i<3; i++) {
getData("xxx");
}
如果“xxx”的结果之前没有被缓存过，很可能会导致3个AJAX请求。
这可以通过缓存请求的deferred而不是结果来解决:
var cache = {};
function getData( val ){
// Return a promise from the cache (if available)
// or create a new one (a jqXHR object) and store it in the cache.
var promise = cache[val];
if (!promise) {
promise = $.ajax('/foo/', {
data: { value: val },
dataType: 'json'
});
cache[val] = promise;
}
return promise;
}
$.when(getData('foo')).then(function(resp){
// do something with the response, which may
// or may not have been retreived using an
// XHR request.
});
另一个使用deferred来实现任何类型的计算(通常是一些性能密集型或长时间运行的任务)的缓存的例子:
var ResultsCache = function(computationFunction, cacheKeyGenerator) {
this._cache = {};
this._computationFunction = computationFunction;
if (cacheKeyGenerator)
this._cacheKeyGenerator = cacheKeyGenerator;
};
ResultsCache.prototype.compute = function() {
// try to retrieve computation from cache
var cacheKey = this._cacheKeyGenerator.apply(this, arguments);
var promise = this._cache[cacheKey];
// if not yet cached: start computation and store promise in cache
if (!promise) {
var deferred = $.Deferred();
promise = deferred.promise();
this._cache[cacheKey] = promise;
// perform the computation
var args = Array.prototype.slice.call(arguments);
args.push(deferred.resolve);
this._computationFunction.apply(null, args);
}
return promise;
};
// Default cache key generator (works with Booleans, Strings, Numbers and Dates)
// You will need to create your own key generator if you work with Arrays etc.
ResultsCache.prototype._cacheKeyGenerator = function(args) {
return Array.prototype.slice.call(arguments).join("|");
};
下面是一个使用这个类来执行一些(模拟繁重的)计算的例子:
// The addingMachine will add two numbers
var addingMachine = new ResultsCache(function(a, b, resultHandler) {
console.log("Performing computation: adding " + a + " and " + b);
// simulate rather long calculation time by using a 1s timeout
setTimeout(function() {
var result = a + b;
resultHandler(result);
}, 1000);
});
addingMachine.compute(2, 4).then(function(result) {
console.log("result: " + result);
});
addingMachine.compute(1, 1).then(function(result) {
console.log("result: " + result);
});
// cached result will be used
addingMachine.compute(2, 4).then(function(result) {
console.log("result: " + result);
});
同样的底层缓存也可以用于缓存Ajax请求:
var ajaxCache = new ResultsCache(function(id, resultHandler) {
console.log("Performing Ajax request for id '" + id + "'");
$.getJSON('http://jsfiddle.net/echo/jsonp/?callback=?', {value: id}, function(data) {
resultHandler(data.value);
});
});
ajaxCache.compute("anID").then(function(result) {
console.log("result: " + result);
});
ajaxCache.compute("anotherID").then(function(result) {
console.log("result: " + result);
});
// cached result will be used
ajaxCache.compute("anID").then(function(result) {
console.log("result: " + result);
});
您可以在这个jsFiddle中使用上述代码。
