This post was authored by guest blogger André Obelink, a Visual Basic MVP, and published by the VBTeam on his behalf.
In this first blog post of a series of two, I explain what dependency injection (DI) is and why you might want to use this design principle in your software. The target audience of this post is the junior / medium experienced software developer, with no knowledge of dependency injection or related techniques. In the second post, I’ll describe the use of Inversion of Control Containers (IoC containers), to use dependency injection in a much more flexible way.
In this first blog post of a series of two, I explain what dependency injection (DI) is and why you might want to use this design principle in your software. The target audience of this post is the junior / medium experienced software developer, with no knowledge of dependency injection or related techniques. In the second post, I’ll describe the use of Inversion of Control Containers (IoC containers), to use dependency injection in a much more flexible way.
- Visual Studio Mac Xamarin
- Visual Studio Community For Mac
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- Visual Studio With Xamarin
- Visual Studio Xamarin Sample Applications
EmguCV - Resolving Visual C++ Dependency in Mono/Xamarin - Mac OSX.In addition to this I need visual studio 2015 dependency installed as mentioned in the question.I don't understand how I can use the.dylib to solve this problem. Browse other questions tagged c# xamarin mono emgucv xamarin.mac or ask your own question. Using Xamarin.Forms and Visual Studio, the sample app constructed is implemented for Android, iOS, and Windows 10 UWP, with 99% of the code shared across the platforms. What you’ll learn: Fundamental concepts of localization and internationalization of apps. Autofac, Ninject, TinyIoC & Unity are 4 widely used dependency injection containers that can be used with Xamarin Forms Labs. This post is going to quickly run over how to get started with each of them.
- As of May 10th, 2017, Visual Studio for Mac replaced Xamarin Studio (Xamarin Studio 6.3 being the final release) and offers all the features of Xamarin Studio, plus cloud and web development, and contains improvements for cross-platform mobile development.
- Visual UI Tests using Xamarin and Appium In this tutorial, you will learn how to integrate automated visual user interface tests into a development toolchain that includes Xcode iOS simulator using Xamarin, Appium, Applitools, and MacOS (formerly OSX).
What is dependency injection?
Dependency injection isn’t new. It’s been around since the early 90s. Because it’s a design principle, it isn’t directly related to a specific programming language. For example, you can use this principle in Java, C#, or Visual Basic .NET. It’s one of the principles of SOLID. SOLID is a mnemonic acronym for the five basic principles of object-oriented programming and design. In my daily life as a software developer, I try to commit myself to apply the principles of SOLID. They were, years ago, defined by Robert C. Martin (also known as ‘Uncle Bob’). He’s the author of many books, including ‘Clean Code’. A highly recommended book!
Let’s start from the beginning by defining the term ‘dependency injection’. However, this is easier said than done. Wikipedia says: “In software engineering, dependency injection is a technique whereby one object supplies the dependencies of another object. A dependency is an object that can be used (a service). An injection is the passing of a dependency to a dependent object (a client) that would use it. The service is made part of the client’s state. Passing the service to the client, rather than allowing a client to build or find the service, is the fundamental requirement of the pattern.”
If you’re new to this subject, you may read this definition more than once and still have no clue what exactly is meant. I think an example from real life might help.
When you were a child, and you asked your mother to give you something to drink, you were happy when she handed you a cup of milk, a glass of orange juice, or a bottle of apple juice, for example. Your mother was in charge. You asked for a drink, but you didn’t know what you’d eventually get. The fact that you knew that you’d get some drink, without knowing exactly which drink it would be, describes more-or-less the core concept of dependency injection. You don’t know the exact type you’ll get, but you know about, for example, the methods on the interface.
Let’s look at another example from daily life, more related to computer software. Perhaps you’re familiar with Adobe Photoshop and their mechanism of plug-ins. You want to apply a filter to a photo. You can install a filter or plug-in, published by a third-party vendor. Adobe Photoshop doesn’t know anything about that filter. However, when this plug-in is installed, it can execute the filter logic, and apply that specific implementation of the filter to the active photo. Again, there’s a concept of not knowing about the filter that is installed, and the ability to apply that ‘unknown’ filter.
In my daily life as a developer, I use dependency injection a lot. Imagine, for example, the logging components we’re using in our software, like Log4net. Based on configuration, it logs to different – or a combination of – output destinations. In our code, we call something general like _logger.Log(“Hello!”). But when writing that code, we don’t know where this text will end up. Is it in a text file or is logged to the Windows Event log? Or both? Or will an email be sent containing the message?
Let’s start from the beginning by defining the term ‘dependency injection’. However, this is easier said than done. Wikipedia says: “In software engineering, dependency injection is a technique whereby one object supplies the dependencies of another object. A dependency is an object that can be used (a service). An injection is the passing of a dependency to a dependent object (a client) that would use it. The service is made part of the client’s state. Passing the service to the client, rather than allowing a client to build or find the service, is the fundamental requirement of the pattern.”
If you’re new to this subject, you may read this definition more than once and still have no clue what exactly is meant. I think an example from real life might help.
When you were a child, and you asked your mother to give you something to drink, you were happy when she handed you a cup of milk, a glass of orange juice, or a bottle of apple juice, for example. Your mother was in charge. You asked for a drink, but you didn’t know what you’d eventually get. The fact that you knew that you’d get some drink, without knowing exactly which drink it would be, describes more-or-less the core concept of dependency injection. You don’t know the exact type you’ll get, but you know about, for example, the methods on the interface.
Let’s look at another example from daily life, more related to computer software. Perhaps you’re familiar with Adobe Photoshop and their mechanism of plug-ins. You want to apply a filter to a photo. You can install a filter or plug-in, published by a third-party vendor. Adobe Photoshop doesn’t know anything about that filter. However, when this plug-in is installed, it can execute the filter logic, and apply that specific implementation of the filter to the active photo. Again, there’s a concept of not knowing about the filter that is installed, and the ability to apply that ‘unknown’ filter.
In my daily life as a developer, I use dependency injection a lot. Imagine, for example, the logging components we’re using in our software, like Log4net. Based on configuration, it logs to different – or a combination of – output destinations. In our code, we call something general like _logger.Log(“Hello!”). But when writing that code, we don’t know where this text will end up. Is it in a text file or is logged to the Windows Event log? Or both? Or will an email be sent containing the message?
Four roles
It’s important to understand that dependency injection involves four roles:
- The serviceobjects to be used.
- The clientobject that is depending on the services it uses.
- The interfaces that define how the client may use the services.
- The injector, which is responsible for constructing the services and injecting them into the client.
The serviceobjects are the actual implementations of the logic you want to execute. In the example of the child asking for a drink, there are three ‘DrinkServices’: the MilkDrinkService, the OrangeJuiceDrinkService, and the AppleJuiceDrinkService.
The Child is the client object. It calls the Drink() method in one of the services. You can ask yourself the question: “How does the Child know that it can call something like a Drink() method?” As you perhaps know, the Child is not aware – and not allowed to be aware – of the MilkDrinkService or one of the other services.
The Child knows about the Drink() method, because it calls a method on an interface, in our example the IDrinkService. An interface defines the public members, but doesn’t implement them. See it as a contract or a description of the signature of the class. Slendytubbies download mac os x. A service object, which implements an interface, contains the actual logic.
You can compare the mother of the child with the injector. She’s responsible for what kind of drink her child gets. The injector injects the actual logic.
The Child is the client object. It calls the Drink() method in one of the services. You can ask yourself the question: “How does the Child know that it can call something like a Drink() method?” As you perhaps know, the Child is not aware – and not allowed to be aware – of the MilkDrinkService or one of the other services.
The Child knows about the Drink() method, because it calls a method on an interface, in our example the IDrinkService. An interface defines the public members, but doesn’t implement them. See it as a contract or a description of the signature of the class. Slendytubbies download mac os x. A service object, which implements an interface, contains the actual logic.
You can compare the mother of the child with the injector. She’s responsible for what kind of drink her child gets. The injector injects the actual logic.
Do some coding
I think that some code will clarify this abstract theory much better. It’s a best practice to start with defining the interface. In this case, the IDrinkService.
As you can see, it only defines which public methods are available. Because it always defines the public members, you must omit the keyword Public. And as I said earlier, it doesn’t contain any implementation.
The service objects implement the logic of the interface. The MilkDrinkService() implements IDrinkService. In practice, this means that we will write the specific implementation of the Drink() Commando full hd movie download. method in this class.
The service objects implement the logic of the interface. The MilkDrinkService() implements IDrinkService. In practice, this means that we will write the specific implementation of the Drink() Commando full hd movie download. method in this class.
After you type ‘Implements IDrinkService’ and hits Enter, Visual Studio helps you by creating the method for you. Replace the ‘Throw New NotImplementedException()’ with the specific logic you want to execute in that service object. We have now implemented one of the service objects. The other two are quite similar. They differ only in the specific logic. Let’s add the second service object.
And the last service object isn’t hard to guess:
We now have coded all service objects. The next step is to define the client object. As you can remember, we must inject the service object into the consuming client object. There are a couple of ways to inject dependencies, but I’m choosing to use ‘constructor injection’. Other techniques are, for example, ‘setter injection’ or ‘interface injection’. The first example is self-explanatory: the client object exposes a setter method that the injector uses to inject the dependency. The second example is harder to understand. Interface injection concludes that the dependency provides an injector method that will inject the dependency into any client passed to it. Clients must implement an interface that exposes a setter method that accepts the dependency. For now, forget these other two injection techniques and let’s focus on constructor injection. In most cases, I use this type of injection because it always brings my client object in a valid state. To understand what I mean with this last statement, we should take a close look at the code.
As you can notice, I added a constructor that forces to apply an argument of the type IDrinkService. This means that we can only create an instance of this object, when we provide an object that implements this interface. So, an instance of my client object has always the mandatory _drinkService set. When other developers use one of my DrinkServices, they’re ‘unable’ to instantiate them in a wrong manner because Visual Studio is already telling them at design time that some mandatory information is missing (In comparison with setter injection, where this missing piece of code is only noticed when you’re already running the code).
Note that the client object has no references to – or knowledge of – our service objects. It’s only aware of the methods of the interfaces that it implements.
The fourth step is creating the injector. The injector is responsible for defining and injecting the dependency, in our example on of our DrinkServices.
Note that the client object has no references to – or knowledge of – our service objects. It’s only aware of the methods of the interfaces that it implements.
The fourth step is creating the injector. The injector is responsible for defining and injecting the dependency, in our example on of our DrinkServices.
You can imagine that the Mother class returns, based on some business logic or external configuration, one of the other services. The last part of our code example is using our classes.
When executing this code, this results in the following output:
You can play with this example by changing the implementation of the Mother.GetDrinkService() method. Our client object isn’t aware of the actual services. And if you decide tomorrow to create a BeerDrinkService() method that implements the IDrinkService interface, all depending code will still run fine.
You can play with this example by changing the implementation of the Mother.GetDrinkService() method. Our client object isn’t aware of the actual services. And if you decide tomorrow to create a BeerDrinkService() method that implements the IDrinkService interface, all depending code will still run fine.
Why using dependency injection?
The concept of dependency injection introduces some abstraction to your business logic. Some developers experience this as adding extra complexity to the code base. Other developers, including myself, think that it just removes complexity. For me, the main reason to apply these design principles is that it makes my code more loosely coupled. Loosely coupling helps you make your code better maintainable, extensible, and testable. It helps your applications to get more configurable: you can even change logic without recompiling.
By applying dependency injection, you will find that your classes contain in most cases only one responsibility and that’s a good thing. Single responsibility is one of the other five principles of SOLID. Indeed, you need dependency injection for a couple of other techniques defined by SOLID. I’m convinced that applying these techniques helps you write better software.
When writing unit tests, you will love these techniques even more! Imagine you have business logic that calculates the total amount of an order. The order, including the order lines, is retrieved from the database by an OrderRepository that implements the IOrderRepository.GetOrderByNumber() method. When you want to write a unit test that calculates the total order amount, this code relies on data in the database. But this is not what you want, because when data in your database is changed, your unit test will fail. Also, when running this unit test as part of the automated build in, for example, the cloud service VSTS, it can’t even connect with your database on premise.
So, to overcome this issue, when you want to unit test this piece code, the solution is to inject another service object that implements IOrderRepository too. For example, FakeOrderRepository. This fake or mock implementation will create an order and related lines in code. This code won’t normally change and can be executed independent from a database. Your unit tests will be much more predictable.
In this post, I have explained the concept of Dependency Injection. By introducing interfaces and implement them with a concrete and specific logic, your code will be more flexible, easier to understand, to extend and to maintain.
In the next post, I’ll show you the use of IoC containers, a technique that will help to assemble your application and other components by using dependency injection to a cohesive program.
Happy coding!
André Obelink
Twitter: @obelink
Blog: www.obelink.com
Links:
-->By applying dependency injection, you will find that your classes contain in most cases only one responsibility and that’s a good thing. Single responsibility is one of the other five principles of SOLID. Indeed, you need dependency injection for a couple of other techniques defined by SOLID. I’m convinced that applying these techniques helps you write better software.
When writing unit tests, you will love these techniques even more! Imagine you have business logic that calculates the total amount of an order. The order, including the order lines, is retrieved from the database by an OrderRepository that implements the IOrderRepository.GetOrderByNumber() method. When you want to write a unit test that calculates the total order amount, this code relies on data in the database. But this is not what you want, because when data in your database is changed, your unit test will fail. Also, when running this unit test as part of the automated build in, for example, the cloud service VSTS, it can’t even connect with your database on premise.
So, to overcome this issue, when you want to unit test this piece code, the solution is to inject another service object that implements IOrderRepository too. For example, FakeOrderRepository. This fake or mock implementation will create an order and related lines in code. This code won’t normally change and can be executed independent from a database. Your unit tests will be much more predictable.
In this post, I have explained the concept of Dependency Injection. By introducing interfaces and implement them with a concrete and specific logic, your code will be more flexible, easier to understand, to extend and to maintain.
In the next post, I’ll show you the use of IoC containers, a technique that will help to assemble your application and other components by using dependency injection to a cohesive program.
Happy coding!
André Obelink
Twitter: @obelink
Blog: www.obelink.com
Links:
Typically, a class constructor is invoked when instantiating an object, and any values that the object needs are passed as arguments to the constructor. This is an example of dependency injection, and specifically is known as constructor injection. The dependencies the object needs are injected into the constructor.
By specifying dependencies as interface types, dependency injection enables decoupling of the concrete types from the code that depends on these types. It generally uses a container that holds a list of registrations and mappings between interfaces and abstract types, and the concrete types that implement or extend these types. Visual code studio for mac.
There are also other types of dependency injection, such as property setter injection, and method call injection, but they are less commonly seen. Therefore, this chapter will focus solely on performing constructor injection with a dependency injection container.
Introduction to Dependency Injection
Dependency injection is a specialized version of the Inversion of Control (IoC) pattern, where the concern being inverted is the process of obtaining the required dependency. With dependency injection, another class is responsible for injecting dependencies into an object at runtime. The following code example shows how the
ProfileViewModel
class is structured when using dependency injection:The
ProfileViewModel
constructor receives an IOrderService
instance as an argument, injected by another class. The only dependency in the ProfileViewModel
class is on the interface type. Therefore, the ProfileViewModel
class doesn't have any knowledge of the class that's responsible for instantiating the IOrderService
object. The class that's responsible for instantiating the IOrderService
object, and inserting it into the ProfileViewModel
class, is known as the dependency injection container.Dependency injection containers reduce the coupling between objects by providing a facility to instantiate class instances and manage their lifetime based on the configuration of the container. Visual studio for mac extensions manager. During the objects creation, the container injects any dependencies that the object requires into it. If those dependencies have not yet been created, the container creates and resolves their dependencies first.
Note
Dependency injection can also be implemented manually using factories. However, using a container provides additional capabilities such as lifetime management, and registration through assembly scanning.
There are several advantages to using a dependency injection container:
- A container removes the need for a class to locate its dependencies and manage their lifetimes.
- A container allows mapping of implemented dependencies without affecting the class.
- A container facilitates testability by allowing dependencies to be mocked.
- A container increases maintainability by allowing new classes to be easily added to the app.
In the context of a Xamarin.Forms app that uses MVVM, a dependency injection container will typically be used for registering and resolving view models, and for registering services and injecting them into view models.
There are many dependency injection containers available, with the eShopOnContainers mobile app using Autofac to manage the instantiation of view model and service classes in the app. Autofac facilitates building loosely coupled apps, and provides all of the features commonly found in dependency injection containers, including methods to register type mappings and object instances, resolve objects, manage object lifetimes, and inject dependent objects into constructors of objects that it resolves. For more information about Autofac, see Autofac on readthedocs.io.
In Autofac, the
IContainer
interface provides the dependency injection container. Figure 3-1 shows the dependencies when using this container, which instantiates an IOrderService
object and injects it into the ProfileViewModel
class.Figure 3-1: Dependencies when using dependency injection
At runtime, the container must know which implementation of the
IOrderService
interface it should instantiate, before it can instantiate a ProfileViewModel
object. This involves:- The container deciding how to instantiate an object that implements the
IOrderService
interface. This is known as registration. - The container instantiating the object that implements the
IOrderService
interface, and theProfileViewModel
object. This is known as resolution.
Eventually, the app will finish using the
ProfileViewModel
object and it will become available for garbage collection. At this point, the garbage collector should dispose of the IOrderService
instance if other classes do not share the same instance.Tip
Write container-agnostic code. Always try to write container-agnostic code to decouple the app from the specific dependency container being used.
Registration
Before dependencies can be injected into an object, the types of the dependencies must first be registered with the container. Registering a type typically involves passing the container an interface and a concrete type that implements the interface.
Visual Studio Mac Xamarin
There are two ways of registering types and objects in the container through code:
- Register a type or mapping with the container. When required, the container will build an instance of the specified type.
- Register an existing object in the container as a singleton. When required, the container will return a reference to the existing object.
Tip
Dependency injection containers are not always suitable. Dependency injection introduces additional complexity and requirements that might not be appropriate or useful to small apps. If a class does not have any dependencies, or is not a dependency for other types, it might not make sense to put it in the container. In addition, if a class has a single set of dependencies that are integral to the type and will never change, it might not make sense to put it in the container.
The registration of types that require dependency injection should be performed in a single method in an app, and this method should be invoked early in the app's lifecycle to ensure that the app is aware of the dependencies between its classes. In the eShopOnContainers mobile app this is performed by the
ViewModelLocator
class, which builds the IContainer
object and is the only class in the app that holds a reference to that object. The following code example shows how the eShopOnContainers mobile app declares the IContainer
object in the ViewModelLocator
class:Types and instances are registered in the
RegisterDependencies
method in the ViewModelLocator
class. This is achieved by first creating a ContainerBuilder
instance, which is demonstrated in the following code example:Types and instances are then registered with the
ContainerBuilder
object, and the following code example demonstrates the most common form of type registration:Show box for mac. The
RegisterType
method shown here maps an interface type to a concrete type. It tells the container to instantiate a RequestProvider
object when it instantiates an object that requires an injection of an IRequestProvider
through a constructor.Visual Studio Community For Mac
Concrete types can also be registered directly without a mapping from an interface type, as shown in the following code example:
When the
ProfileViewModel
type is resolved, the container will inject its required dependencies.Autofac also allows instance registration, where the container is responsible for maintaining a reference to a singleton instance of a type. For example, the following code example shows how the eShopOnContainers mobile app registers the concrete type to use when a
ProfileViewModel
instance requires an IOrderService
instance:The
RegisterType
method shown here maps an interface type to a concrete type. The SingleInstance
method configures the registration so that every dependent object receives the same shared instance. Therefore, only a single OrderService
instance will exist in the container, which is shared by objects that require an injection of an IOrderService
through a constructor.Instance registration can also be performed with the
RegisterInstance
method, which is demonstrated in the following code example:The
RegisterInstance
method shown here creates a new OrderMockService
instance and registers it with the container. Therefore, only a single OrderMockService
instance exists in the container, which is shared by objects that require an injection of an IOrderService
through a constructor.Following type and instance registration, the
IContainer
object must be built, which is demonstrated in the following code example: Heiti tc font download mac.Invoking the
Build
method on the ContainerBuilder
instance builds a new dependency injection container that contains the registrations that have been made.Tip
Consider an
IContainer
as being immutable. While Autofac provides an Update
method to update registrations in an existing container, calling this method should be avoided where possible. There are risks to modifying a container after it's been built, particularly if the container has been used. For more information, see Consider a Container as Immutable on readthedocs.io.Resolution
After a type is registered, it can be resolved or injected as a dependency. When a type is being resolved and the container needs to create a new instance, it injects any dependencies into the instance.
Generally, when a type is resolved, one of three things happens:
- If the type hasn't been registered, the container throws an exception.
- If the type has been registered as a singleton, the container returns the singleton instance. If this is the first time the type is called for, the container creates it if required, and maintains a reference to it.
- If the type hasn't been registered as a singleton, the container returns a new instance, and doesn't maintain a reference to it.
The following code example shows how the
RequestProvider
type that was previously registered with Autofac can be resolved: https://greatperks.weebly.com/blog/android-studio-for-mac.In this example, Autofac is asked to resolve the concrete type for the
IRequestProvider
type, along with any dependencies. Typically, the Resolve
method is called when an instance of a specific type is required. For information about controlling the lifetime of resolved objects, see Managing the Lifetime of Resolved Objects.The following code example shows how the eShopOnContainers mobile app instantiates view model types and their dependencies:
In this example, Autofac is asked to resolve the view model type for a requested view model, and the container will also resolve any dependencies. When resolving the
ProfileViewModel
type, the dependency to resolve is an IOrderService
object. Therefore, Autofac first constructs an OrderService
object and then passes it to the constructor of the ProfileViewModel
class. For more information about how the eShopOnContainers mobile app constructs view models and associates them to views, see Automatically Creating a View Model with a View Model Locator.Note
Registering and resolving types with a container has a performance cost because of the container's use of reflection for creating each type, especially if dependencies are being reconstructed for each page navigation in the app. If there are many or deep dependencies, the cost of creation can increase significantly.
![Dependency Injection Xamarin Visual Studio For Mac Dependency Injection Xamarin Visual Studio For Mac](/uploads/1/2/6/3/126330695/978288972.png)
Managing the Lifetime of Resolved Objects
After registering a type, the default behavior for Autofac is to create a new instance of the registered type each time the type is resolved, or when the dependency mechanism injects instances into other classes. In this scenario, the container doesn't hold a reference to the resolved object. However, when registering an instance, the default behavior for Autofac is to manage the lifetime of the object as a singleton. Therefore, the instance remains in scope while the container is in scope, and is disposed when the container goes out of scope and is garbage collected, or when code explicitly disposes the container.
Xamarin Visual Studio For Mac
An Autofac instance scope can be used to specify the singleton behavior for an object that Autofac creates from a registered type. Autofac instance scopes manage the object lifetimes instantiated by the container. The default instance scope for the
RegisterType
method is the InstancePerDependency
scope. However, the SingleInstance
scope can be used with the RegisterType
method, so that the container creates or returns a singleton instance of a type when calling the Resolve
method. The following code example shows how Autofac is instructed to create a singleton instance of the NavigationService
class:Visual Studio With Xamarin
The first time that the
INavigationService
interface is resolved, the container creates a new NavigationService
object and maintains a reference to it. On any subsequent resolutions of the INavigationService
interface, the container returns a reference to the NavigationService
object that was previously created.Note
The SingleInstance scope disposes created objects when the container is disposed.
Autofac includes additional instance scopes. For more information, see Instance Scope on readthedocs.io.
Summary
Dependency injection enables decoupling of concrete types from the code that depends on these types. It typically uses a container that holds a list of registrations and mappings between interfaces and abstract types, and the concrete types that implement or extend these types.
Autofac facilitates building loosely coupled apps, and provides all of the features commonly found in dependency injection containers, including methods to register type mappings and object instances, resolve objects, manage object lifetimes, and inject dependent objects into constructors of objects it resolves.