//jquery版本大于1.8
function runAsync(){
  var def = $.Deferred();
  setTimeout(function(){
    console.log('I am done');
    def.resolve('whatever');
  }, 1000);
  return def;
}
runAsync().then(function(msg){
  console.log(msg);//=>打印'whatever'
}).done(function(msg){
  console.log(msg);//=>打印'undefined'
});

jQuery.extend( {
  Deferred: function( func ) {
    var tuples = [
        // action, add listener, listener list, final state
        [ "resolve", "done", jQuery.Callbacks( "once memory" ), "resolved" ],
        [ "reject", "fail", jQuery.Callbacks( "once memory" ), "rejected" ],
        [ "notify", "progress", jQuery.Callbacks( "memory" ) ]
      ],
      state = "pending",
      promise = {
        state: function() {...},
        always: function() {...},
        then: function() {...},
        promise: function( obj ) {...}
      },
      deferred = {};
    // Keep pipe for back-compat
    promise.pipe = promise.then;
    // Add list-specific methods
    jQuery.each( tuples, function( i, tuple ) {} );
    // Make the deferred a promise
    promise.promise( deferred );
    // Call given func if any
    if ( func ) {
      func.call( deferred, deferred );
    }
    // All done!
    return deferred;
  }
}

tuples = [
  // action, add listener, listener list, final state
  [ "resolve", "done", jQuery.Callbacks( "once memory" ), "resolved" ],
  [ "reject", "fail", jQuery.Callbacks( "once memory" ), "rejected" ],
  [ "notify", "progress", jQuery.Callbacks( "memory" ) ]
]

jQuery.Callbacks = function( options ) {

  // Convert options from String-formatted to Object-formatted if needed
  // (we check in cache first)
  options = typeof options === "string" ?
    createOptions( options ) :
    jQuery.extend( {}, options );

  var // Flag to know if list is currently firing
    firing,

    // Last fire value for non-forgettable lists
    memory,

    // Flag to know if list was already fired
    fired,

    // Flag to prevent firing
    locked,

    // Actual callback list
    list = [],

    // Queue of execution data for repeatable lists
    queue = [],

    // Index of currently firing callback (modified by add/remove as needed)
    firingIndex = -1,

    // Fire callbacks
    fire = function() {

      // Enforce single-firing
      locked = options.once;

      // Execute callbacks for all pending executions,
      // respecting firingIndex overrides and runtime changes
      fired = firing = true;
      for ( ; queue.length; firingIndex = -1 ) {
        memory = queue.shift();
        while ( ++firingIndex < list.length ) {

          // Run callback and check for early termination
          if ( list[ firingIndex ].apply( memory[ 0 ], memory[ 1 ] ) === false &&
            options.stopOnFalse ) {

            // Jump to end and forget the data so .add doesn't re-fire
            firingIndex = list.length;
            memory = false;
          }
        }
      }

      // Forget the data if we're done with it
      if ( !options.memory ) {
        memory = false;
      }

      firing = false;

      // Clean up if we're done firing for good
      if ( locked ) {

        // Keep an empty list if we have data for future add calls
        if ( memory ) {
          list = [];

        // Otherwise, this object is spent
        } else {
          list = "";
        }
      }
    },

    // Actual Callbacks object
    self = {

      // Add a callback or a collection of callbacks to the list
      add: function() {
        if ( list ) {

          // If we have memory from a past run, we should fire after adding
          if ( memory && !firing ) {
            firingIndex = list.length - 1;
            queue.push( memory );
          }

          ( function add( args ) {
            jQuery.each( args, function( _, arg ) {
              if ( jQuery.isFunction( arg ) ) {
                if ( !options.unique || !self.has( arg ) ) {
                  list.push( arg );
                }
              } else if ( arg && arg.length && jQuery.type( arg ) !== "string" ) {

                // Inspect recursively
                add( arg );
              }
            } );
          } )( arguments );

          if ( memory && !firing ) {
            fire();
          }
        }
        return this;
      },

      // Remove a callback from the list
      remove: function() {
        jQuery.each( arguments, function( _, arg ) {
          var index;
          while ( ( index = jQuery.inArray( arg, list, index ) ) > -1 ) {
            list.splice( index, 1 );

            // Handle firing indexes
            if ( index <= firingIndex ) {
              firingIndex--;
            }
          }
        } );
        return this;
      },

      // Check if a given callback is in the list.
      // If no argument is given, return whether or not list has callbacks attached.
      has: function( fn ) {
        return fn ?
          jQuery.inArray( fn, list ) > -1 :
          list.length > 0;
      },

      // Remove all callbacks from the list
      empty: function() {
        if ( list ) {
          list = [];
        }
        return this;
      },

      // Disable .fire and .add
      // Abort any current/pending executions
      // Clear all callbacks and values
      disable: function() {
        locked = queue = [];
        list = memory = "";
        return this;
      },
      disabled: function() {
        return !list;
      },

      // Disable .fire
      // Also disable .add unless we have memory (since it would have no effect)
      // Abort any pending executions
      lock: function() {
        locked = true;
        if ( !memory ) {
          self.disable();
        }
        return this;
      },
      locked: function() {
        return !!locked;
      },

      // Call all callbacks with the given context and arguments
      fireWith: function( context, args ) {
        if ( !locked ) {
          args = args || [];
          args = [ context, args.slice ? args.slice() : args ];
          queue.push( args );
          if ( !firing ) {
            fire();
          }
        }
        return this;
      },

      // Call all the callbacks with the given arguments
      fire: function() {
        self.fireWith( this, arguments );
        return this;
      },

      // To know if the callbacks have already been called at least once
      fired: function() {
        return !!fired;
      }
    };

  return self;
};

jQuery.Callbacks = function(){
  var list = [],
    self = {
      add: function(){/*添加元素到list*/},
      remove: function(){/*从list移除指定元素*/},
      fire: function(){/*遍历list并触发每次元素*/}
    };
  return self;
}

promise = {
  state: function() {//返回状态值
    return state;
  },
  always: function() {//不管成功还是失败，最终都会执行该方法
    deferred.done( arguments ).fail( arguments );
    return this;
  },
  then: function( /* fnDone, fnFail, fnProgress */ ) {...},//重头戏，稍后会详讲
  promise: function( obj ) {//扩展promise，如不久我们会看见的promise.promise( deferred );
    return obj != null ? jQuery.extend( obj, promise ) : promise;
  }
}

/*
tuples = [
  [ "resolve", "done", jQuery.Callbacks( "once memory" ), "resolved" ],
  [ "reject", "fail", jQuery.Callbacks( "once memory" ), "rejected" ],
  [ "notify", "progress", jQuery.Callbacks( "memory" ) ]
]
*/
jQuery.each( tuples, function( i, tuple ) {
  var list = tuple[ 2 ],
    stateString = tuple[ 3 ];
  // promise[ done | fail | progress ] = list.add
  promise[ tuple[ 1 ] ] = list.add;
  // Handle state
  if ( stateString ) {
    list.add( function() {
      // state = [ resolved | rejected ]
      state = stateString;
    // [ reject_list | resolve_list ].disable; progress_list.lock
    }, tuples[ i ^ 1 ][ 2 ].disable, tuples[ 2 ][ 2 ].lock );
  }
  // deferred[ resolve | reject | notify ]
  deferred[ tuple[ 0 ] ] = function() {
    deferred[ tuple[ 0 ] + "With" ]( this === deferred ? promise : this, arguments );
    return this;
  };
  deferred[ tuple[ 0 ] + "With" ] = list.fireWith;
} );

promise = {
  then: function( /* fnDone, fnFail, fnProgress */ ) {  
    var fns = arguments;
    return jQuery.Deferred( function( newDefer ) {
      jQuery.each( tuples, function( i, tuple ) {
        var fn = jQuery.isFunction( fns[ i ] ) && fns[ i ];
        // deferred[ done | fail | progress ] for forwarding actions to newDefer
        deferred[ tuple[ 1 ] ]( function() {
          var returned = fn && fn.apply( this, arguments );
          if ( returned && jQuery.isFunction( returned.promise ) ) {
            returned.promise()
              .progress( newDefer.notify )
              .done( newDefer.resolve )
              .fail( newDefer.reject );
          } else {
            newDefer[ tuple[ 0 ] + "With" ](
              this === promise ? newDefer.promise() : this,
              fn ? [ returned ] : arguments
            );
          }
        } );
      } );
      fns = null;
    } ).promise();
  }
}

promise = {
  then: function( /* fnDone, fnFail, fnProgress */ ) {  
    var fns = arguments;
    return jQuery.Deferred( function( newDefer ) {
          ...
        } ).promise();
  }
}

 function SimpleDef(){
  var list = [],
    simpleDef = {
      done: function(func){
        list.push(func);
        return simpleDef;
      },
      then: function(func){
        list.push(func);
        return SimpleDef();
      },
      fire: function(){
        var i = list.length;
        while(i--){
          list[i]();
        }
      }
    };
  return simpleDef;
}

var def = SimpleDef();
var then1 = def.done(function(){
  console.log('self1-done1');
}).done(function(){
  console.log('self1-done2');
}).then(function(){
  console.log('self2-then1');
}).done(function(){
  console.log('self2-done1');
});
def.fire();//=>self2-then1 self1-done2 self1-done1
console.log('xxxxxxxxxxxxxxxxxxxx');
then1.fire();//=>self2-done1