I've been trying to get into the nitty-gritty with angular DI and really the bootstrap process at large, and I am a bit confused as to where things really happen. In my mind, the events are in this order. App starts.
Now is where I am lost.
The $injector, now having access to all the configured providers from $provide, calls the constructor functions (the $get function in each provider) to instantiate service instances.
Also, if that process is correct, how does the $injector handle cases where a service depends on another service?
Services are only instantiated at the moment when they are needed, rather than during Angular's initiation. For example, if you have a controller that isn't activated yet and it depends on services which haven't yet been used, those services will be instantiated and injected whenever that controller becomes active (like changing to a view that uses it). From then on, the same instance of each service will be used.
The same is true of services that depend on other services. All dependencies of anything are resolved before it is instantiated, so if a dependency has dependencies, the same process is applied (all of that dependency's dependencies will be instantiated first, and so on).
If a circular dependency is found (service foo has a dependency that depends on service foo), Angular will throw an exception and the functionality of those services will have to be refactored into different services that will not have this kind of circular chain.