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POChaining serialized Deferred for posting data
 

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    <p>I am using Firebase and attempting to use jQuery Deferred when I post data to a series of locations. I want a mechanism where if any of the posts fail, I want to recover and possibly undo previous posts.</p> <p><strong>The syntax for doing a Firebase post is</strong></p> <pre><code>var fbRef = new Firebase("https://my.firebaseIO.com/path/to/data"); fbRef.set(data,function (error){ if (error){ // do something about it } else { // all went well // next post would go here... } } </code></pre> <p>Naturally writing a series of posts this way - where each depends on the success of the previous will result in the pyramiding of code AKA callback hell.</p> <p><strong>For my concrete example, I have data about 'words' that need to be put at various locations</strong></p> <pre><code>/words/upCase/&lt;word&gt;/id =&lt;word&gt; /words/upCase/&lt;word&gt;/type = &lt;type&gt; /words/&lt;type&gt;/&lt;word&gt;/id = &lt;word&gt; </code></pre> <p>Each of these will have its own Firebase <code>set(data,cb)</code> call, and the latter two should not be conducted should any of their precedents fail. This is a simplified example, so the solution should be geared to many more data points.</p> <p>Also, in this example there is no flow of data from one operation to the next, all data is known at the start, so I am also interested in data flow examples.</p> <p><strong>My latest attempt involves genericizing the callback</strong></p> <pre><code>// function factory. function will set firebase reference value and handle the deferred if exists var makeFn=function (fbr,val,task){ return function (){ fbr.set(val,function (err){ if (err) { console.log(fbr.toString()+' Error in Firebase:'+err.toSource()); if (task) { task.reject(err); } } if (task) { task.resolve(fbr); } }); if (task) { return task.promise(); } } } // factory that returns a function that returns a promise about setting data var make=function (fbr,val){ return function (){ var task=new $.Deferred(); return makeFn(fbr,val,task)(); } }; </code></pre> <p><strong>Then I call like this</strong></p> <pre><code>var fbr = new Firebase("https://my.firebaseIO.com/words/upCase/" + word + "/id"); make(fbr,word)() // set id promise .done(makeFn(fbr.parent().child('type'),type)) // HERE, not right .done(makeFn(fbr.parent().parent().parent().child(type).child(word).child('id'),word)) // set third data, still not right .done(function(){ //inform all went well } ); </code></pre> <p>My attempt is not chaining dependencies as I intend. What I want to say is...</p> <pre><code>make(fbr,word).done(&lt;new promise returning fn&gt;).&lt;get the done of that new promise&gt;(...) </code></pre> <p>... without pyramiding like:</p> <pre><code>make(fbr,word).done( &lt;new promise returning fn&gt;.done( ... ) ) </code></pre> <p>The one thing I am sure of is that I am doing something wrong. How do I chain dependencies without pyramiding my code? Treat me as if I am new to promises, because I am. =)</p> <p><strong>UPDATE</strong></p> <p>Here is a walk-through of what I've come up with. This just deals with setters.</p> <p>First extend <code>Firebase</code> to return what I call a promisy function when we want to set, I am just using <code>.s()</code> here...</p> <pre><code>Firebase.prototype.s=function(v){ var fn=makeSet(this,v); return fn; }; </code></pre> <p>Here is the maker, it has some commonalities...</p> <pre><code>var makeSet=function (fbr, val){ // factory that returns a function that returns a promise about setting data var task=Q.defer(); var fn=setFn(fbr,val,task); makeCOMMON(task,fn,fbr); return fn; }; </code></pre> <p>In the commonalities, we link functions and promises, and also create a new kind of <code>.then()</code> that returns the promise of the fn fed into it, its called <code>.ok()</code>.</p> <pre><code>var makeCOMMON=function (task, fn, fbr){ var p=task.promise; fn.task=task; fn.fbr=fbr.toString(); fn.p=p; p.fbr=fbr.toString(); p.fn=fn; p.ok=function (fnn){ this.then(fnn); return fnn.p; }; } </code></pre> <p>That part is probably redundant, and maybe iffy, it exposes my ignorance. Back to <code>setFn()</code>:</p> <pre><code>var setFn=function (fbr, val, task){ // function factory. function will set firebase reference value and handle the deferred if exists return function (){ task.notify('Setting '+fbr.toString()+' to '+val.toSource()); fbr.set(val,function (err){ if (err) { task.notify('Rejecting Update due to error '+fbr.toString()); task.reject({ error:err, FbPath:fbr.toString(), FbTask:task, FbVal:val, toString:function(){ var s=''; s+=' '+this.error.toString(); s+=' '+this.FbPath; if (this.FbVal) { s+=' '+this.FbVal.toSource(); }else{ s+=' null'; } return s; } });//reject }else{ task.resolve(fbr); } }); return task.promise; }; } </code></pre> <p>The tension builds... now, when I go to call that, I have a chainer mechanism:</p> <pre><code>add:function(w,type){ w=w.toLowerCase(); var fn0,fnLast; var fbr=FirebaseSvc.get(); var proms=[]; // var chain=function(fbrFn){ var wasFirst=false; if (typeof fn0=='undefined') { wasFirst=true; fn0=fbrFn; } proms.push(fbrFn.p); fbrFn.p.progress(logProgress); fbrFn.p.fail(logFail); if (!wasFirst) { var rv=fnLast.p.ok(fbrFn); fnLast=fbrFn; return rv; }else{ fnLast=fbrFn; return fbrFn.p; } }; // //.c() is .child() alias chain(fbr.root().c('words/upCase/'+w+'/id').s(w));//This is returing a promise, so I can still do promisy things, but I am not right now chain(fbr.root().c('words/upCase/'+w+'/type').s(type)); chain(fbr.root().c('words/words'+type+'/'+w+'/id').s(w)); // var ro={ promise:Q.all(proms), doIt:fn0 }; return ro; },//.add </code></pre> <p>From the client, it is:</p> <pre><code>var ro=WordSvc.add(w,type); ro.promise...// can attach handlers ro.doIt();// kick it all off </code></pre>
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