Nested jQuery $.when

问题

Essentially I am tring to write this:

var async1 = $.when( a1() ).then(function(){ a2() });
var async2 = $.when( a3() ).then(function(){ a4() });

$.when(async1, async2).then(function(){ 
    console.log("complete");
}); 

But at the moment when a1 and a3 have executed the function considers itself resolved.

I put together the same example in a fiddle: http://jsfiddle.net/Z7fzR/

回答1:

You never actually return the promise objects created by a2() and a4() from the callback; this effectively returns null, which apparently counts as a completion for $.when purposes:

http://jsfiddle.net/Z7fzR/1/

回答2:

You are throwing away the promise objects that a2 and a4 return, essentially passing undefined back to the original when, which causes it to resolve immediately:

If a single argument is passed to jQuery.when and it is not a Deferred, it will be treated as a resolved Deferred and any doneCallbacks attached will be executed immediately.

Add some returns and it works fine.

var async1 = $.when(a1()).then(function(){ return a2(); });
var async2 = $.when(a3()).then(function(){ return a4(); });

$.when(async1, async2).then(function(){
    console.log("complete");
});  

http://jsfiddle.net/Z7fzR/2/

来源：https://stackoverflow.com/questions/13843286/nested-jquery-when