Dependency injection

1. Circumventing the DI system
2. Sharing services
3. Providers
4. DI tricks

In Angular one passes dependencies to a class through its constructor: this clearly defines and tells us what the dependencies of a class are and makes testing really easy.

Since those dependencies may need their own parameters, may have already been instanciated or just need to be mocked, we need a DI system to help us with these tasks.

• dependencies are unique per injector (so if you provide a dependency in a @Component and not in the topmost @NgModule then each and every component instance will have its own injected dependency)
• dependencies exist in a tree, just like the components
  • the dependency walker will walk from down to up, from child to parent
  • @Host will not walk further than the current host
  • @Optional set the non-existent dependency to null

❗ Place global dependencies in @NgModule - they are going to be instanciated only once.

💡 During testing one can easily inject mock dependencies instead of real ones.