Provider Collisions

Provider collisions occur when different providers that offer the same service are imported into the current module.

Let's take a closer look at a specific example. Imagine you have Module3, where you import Module2 and Module1. You made these imports because you need Service2 and Service1 from these modules, respectively. You review the results of these services operations, but for some reason, Service1 does not behave as expected. You start debugging and discover that Service1 is exported from both Module2 and Module1. You expected Service1 to be exported only from Module1, but in reality, the version exported by Module2 is being used:

import { featureModule, rootModule } from '@ditsmod/core';

class Service1 {}
class Service2 {}

@featureModule({
  providersPerMod: [Service1],
  exports: [Service1]
})
class Module1 {}

@featureModule({
  providersPerMod: [{ token: Service1, useValue: 'some value' }, Service2],
  exports: [Service1, Service2],
})
class Module2 {}

@rootModule({
  imports: [Module1, Module2],
})
class Module3 {}

To prevent this, if you import two or more modules that export non-identical providers with the same token, Ditsmod will throw an error similar to this:

Error: Importing providers to Module3 failed: exports from Module1, Module2 causes collision with Service1. You should add Service1 to resolvedCollisionsPerMod in this module. For example: resolvedCollisionsPerMod: [ [Service1, Module1] ].

In this specific scenario:

Module1 exports a provider with the token Service1.
Module2 overrides and then exports a provider with the token Service1.
The providers with the token Service1 in Module1 and Module2 are not identical.

And because both of these modules are imported into Module3, a "provider collision" occurs, leaving the developer uncertain about which provider will be used in Module3.

Collision resolution

If Module3 is declared in your application (it is not imported from node_modules), the collision is resolved by adding to resolvedCollisionsPer* an array of two elements, with the provider's token in the first place and the module from which the provider needs to be taken in the second place:

import { featureModule, rootModule } from '@ditsmod/core';

class Service1 {}
class Service2 {}

@featureModule({
  providersPerMod: [Service1],
  exports: [Service1]
})
class Module1 {}

@featureModule({
  providersPerMod: [{ token: Service1, useValue: 'some value' }, Service2],
  exports: [Service1, Service2],
})
class Module2 {}

@rootModule({
  imports: [Module1, Module2],
  resolvedCollisionsPerMod: [ [Service1, Module1] ]
})
class Module3 {}

If you have installed Module3 using packages manager (npm, yarn, etc.), there is no point in modifying this module locally to resolve the collision. This situation can only occur if Module2 and Module1 are exported from the root module, so you need to remove one of these modules from there. And, of course, after that you will have to explicitly import the deleted module into those modules where it is needed.

Collision resolution​

Collision resolution