Bug 606695 - Helper for chaining async functions [r=mconnor]

services/sync/modules/util.js

@@ -53,6 +53,36 @@ Cu.import("resource://services-sync/log4moz.js");
  */
 
 let Utils = {
+  /**
+   * Execute an arbitrary number of asynchronous functions one after the
+   * other, passing the callback arguments on to the next one.  All functions
+   * must take a callback function as their last argument.  The 'this' object
+   * will be whatever asyncChain's is.
+   * 
+   * @usage this._chain = Utils.asyncChain;
+   *        this._chain(this.foo, this.bar, this.baz)(args, for, foo)
+   * 
+   * This is equivalent to:
+   *
+   *   let self = this;
+   *   self.foo(args, for, foo, function (bars, args) {
+   *     self.bar(bars, args, function (baz, params) {
+   *       self.baz(baz, params);
+   *     });
+   *   });
+   */
+  asyncChain: function asyncChain() {
+    let funcs = Array.slice(arguments);
+    let thisObj = this;
+    return function callback() {
+      if (funcs.length) {
+        let args = Array.slice(arguments).concat(callback);
+        let f = funcs.shift();
+        f.apply(thisObj, args);
+      }
+    };
+  },
+
   /**
    * Wrap a function to catch all exceptions and log them
    *

services/sync/tests/unit/test_utils_asyncChain.js

@@ -0,0 +1,27 @@
+Cu.import("resource://services-sync/util.js");
+
+function run_test() {
+  _("Chain a few async methods, making sure the 'this' object is correct.");
+
+  let methods = {
+    save: function(x, callback) {
+      this.x = x;
+      callback(x);
+    },
+    addX: function(x, callback) {
+      callback(x + this.x);
+    },
+    double: function(x, callback) {
+      callback(x * 2);
+    },
+    neg: function(x, callback) {
+      callback(-x);
+    }
+  };
+  methods.chain = Utils.asyncChain;
+
+  // ((1 + 1 + 1) * (-1) + 1) * 2 + 1 = -3
+  methods.chain(methods.save, methods.addX, methods.addX, methods.neg,
+                methods.addX, methods.double, methods.addX, methods.save)(1);
+  do_check_eq(methods.x, -3);
+}