$watch API描述-第一个被描述为监听器函数的函数,为什么?

我正在阅读有关scope方法的文档 $watch here 。该方法接受: $watch(watchExpression, [listener] ; 然后,他们提供了示例:

// let's assume that scope was dependency injected as the $rootScope
var scope = $rootScope;
scope.name = 'misko';
scope.counter = 0;

expect(scope.counter).toEqual(0);
scope.$watch('name', function(newValue, oldValue) {
 scope.counter = scope.counter + 1;
});
expect(scope.counter).toEqual(0);

scope.$digest();
// the listener is always called during the first $digest loop after it was registered
expect(scope.counter).toEqual(1);

scope.$digest();
// but now it will not be called unless the value changes
expect(scope.counter).toEqual(1);

scope.name = 'adam';
scope.$digest();
expect(scope.counter).toEqual(2);



// Using a listener function
var food;
scope.foodCounter = 0;
expect(scope.foodCounter).toEqual(0);
scope.$watch(
 // This is the listener function --------- WHY ?????????????
 function() { return food; },
 // This is the change handler ---- THIS SHOULD BE A LISTNER FUNCTION
 function(newValue, oldValue) {
   if ( newValue !== oldValue ) {
     // Only increment the counter if the value changed
     scope.foodCounter = scope.foodCounter + 1;
   }
 }
);
// No digest has been run so the counter will be zero
expect(scope.foodCounter).toEqual(0);

// Run the digest but since food has not changed count will still be zero
scope.$digest();
expect(scope.foodCounter).toEqual(0);

// Update food and run digest.  Now the counter will increment
food = 'cheeseburger';
scope.$digest();
expect(scope.foodCounter).toEqual(1);

我不明白的是他们为什么提到 function() { return food; } 在第二个示例中 // This is the listener function 如果这是一个函数 should return the value that will be watched. 这是一个 watchExpression ?