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https://github.com/commons-app/apps-android-commons.git
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f12837650a
* Use JSON response for nearby places * Move okhttp calls to a different class * wip * Fetch picture of the day using JSON API * Search images using JSON APIs * tests * Fix injection based on code review comments
41 lines
2.3 KiB
Markdown
41 lines
2.3 KiB
Markdown
## Overview
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At its core, dependency injection is just the principle of `"tell, don't ask"` put into practice; for instance, if a class needs to use the `MediaWikiApi`, it should be handed an instance of the class rather than reaching out to get it. This has the effect of decoupling code, making it easier to test and reuse.
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## Dependency Injection in the Commons app
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We use Dagger 2 as our dependency injection engine. Dagger is a fully static, compile-time dependency injection framework for both Java and Android. Dagger aims to address many of the development and performance issues that have plagued reflection-based solutions that came before it, but it does come at something of a cost in complexity.
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For more information about Dagger, take a look at the [Dagger user guide](https://google.github.io/dagger/users-guide.html).
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## Dagger configuration in the Commons app
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The top level `CommonsApplicationComponent` pulls together configuration for injection across the app. The most important files to understand
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- if you need to add a new Activity, look at `ActivityBuilderModule` and copy how injection is configured. The `BaseActivity` class will take care of the rest.
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- if you are adding a new Fragment, look at `FragmentBuilderModule`
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- if you are adding a new ContentProvider, look at `ContentProviderBuilderModule`
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- if you are adding a new Service, look at `ServiceBuilderModule`
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- other dependencies are configured in `CommonsApplicationModule`
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## "Provider" methods
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Dagger will resolve the method arguments on provider methods in a module (or the constructor arguments when annotated with `@Inject`) and build the objects accordingly - either by calling another provider method or by looking for a constructor on a class that has the `@Inject` annotation. Dagger takes care of managing singletons, just annotate with the `@Singleton` annotation. For instance,
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```java
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@Provides
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@Singleton
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public SessionManager providesSessionManager(MediaWikiApi okHttpJsonApiClient) {
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return new SessionManager(application, okHttpJsonApiClient);
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}
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```
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If your code injects an interface (in this case, `MediaWikiApi`) then Dagger needs to know which concrete class to use. This comes by way of a provider method:
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```java
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@Provides
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@Singleton
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public MediaWikiApi provideMediaWikiApi() {
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return new ApacheHttpClientMediaWikiApi(BuildConfig.WIKIMEDIA_API_HOST);
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}
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```
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