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Where you give `ng-class` an object, whose keys are CSS class names and whose values are conditional expressions using `$scope` variables.
The element will then have all the classes whose conditions are truthy, and none of those whose conditions are falsy.
Note also the handy `ng-class-even` and `ng-class-odd`, and the related though somewhat different `ng-style`.
### `$watch` and `$apply`
Angular's two-way data binding is the root of all awesome in Angular.
However, it's not magic, and there are some situations where you need to give it a nudge in the right direction.
When you bind a value to an element in Angular using `ng-model`, `ng-repeat`, etc., Angular creates a `$watch` on that value.
Then whenever a value on a scope changes, all `$watch`es observing that element are executed, and everything updates.
Sometimes, usually when you're writing a custom directive, you will have to define your own `$watch` on a scope value to make the directive react to changes.
On the flip side, sometimes you change a scope value in some code but the app doesn't react to it.
Angular checks for scope variable changes after pieces of your code have finished running; for example, when `ng-click` calls a function on your scope, Angular will check for changes and react.
However, some code is outside of Angular and you'll have to call `scope.$apply()` yourself to trigger the update.
This is most commonly seen in event handlers in custom directives.
### Combining `ng-repeat` with other directives
`ng-repeat` is extremely useful, one of the most powerful directives in Angular.
However the transformation it applies to the DOM is substantial.
Therefore applying other directives (such as `ng-show`, `ng-controller` and others) to the same element as `ng-repeat` generally leads to problems.
If you want to apply a directive to the whole repeat, wrap the repeat in a parent element and put it there.
If you want to apply a directive to each inner piece of the repeat, put it on a child of the element with `ng-repeat`.
### `$rootScope` exists, but it can be used for evil
Scopes in Angular form a hierarchy, prototypically inheriting from a root scope at the top of the tree.
Usually this can be ignored, since most views have a controller, and therefore a scope, of their own.
Occasionally there are pieces of data that you want to make global to the whole app.
For these, you can inject `$rootScope` and set values on it like any other scope.
Since the scopes inherit from the root scope, these values will be available to the expressions attached to directives like `ng-show` just like values on your local `$scope`.
Of course, global state sucks and you should use `$rootScope` sparingly, like you would (hopefully) use with global variables in any language.
In particular, don't use it for code, only data.
If you're tempted to put a function on `$rootScope`, it's almost always better to put it in a service that can be injected where it's needed, and more easily tested.
Conversely, don't create a service whose only purpose in life is to store and return bits of data.
