服务定位器是框架的好模式吗？

Question

I'm building a GUI framework and I currently have a lot of lines like (usually in the constructor)

var renderer = locator.GetService<IRenderer>();
var input = locator.GetService<IMouseInput>();

Those services I'm getting are core services, I mean, services that are inherently part of what is considered the basics .

I have thought about using constructor injection, but some annoyances arise.

• I will have to modify almost every constructor to have its dependencies injected, and that means it will receive even the most basic "services". I'm afraid of having 3 or 4 arguments in each constructor.
• But another thing that is even worse is that end users of the framework won't be happy if they don't find parameterless constructors . Do you imagine that creating a TextBox required you to pass all the dependencies it really needs?

The end user could be overwhelmed with new TextBox(dep1, dep2, dep3) .

So, is there a good way to remove the Service Locator completely without making it too complex?

EDIT

In the team we are discussing and they still WANT to keep the Service Locator arguing the following. This is a fragment of the conversation:

We don't care about single resposibility in TextBox implementation. We arent' writing an application. It's a UI framework! It's supposed to hide stuff and details from end user, not to expose them. A fluent builder for a TextBox? Seriously? We aren't writing some web request handling pipeline.

I agree with all what he (Stack Overflow users) wrote, but regarding applications . Where you want stuff to be as extensible and reusable as possible. We have like 10 services. They are used everywhere . If we had like 50 dependencies with complex object graphs, we couldn't do anything without DI and stuff. The thing about DI is that you have pay for it. So for us Service Locator is free DI isn't It would be just unmaintainable- By having more complex API, by having backward API/ABI compatibility, etc. We are paying for Service Locator by not knowing what class needs as its dependencies and some problems with providing different dependency implementations for different instances. But we don't need either, so Service Locator is free. DI isn't.

Do you agree? Why?

Answer 1

I will have to modify almost every constructor to have its dependencies injected, and that means it will receive even the most basic "services". I'm afraid of having 4 or 5 arguments in each constructor.

If you have 4 or 5 services in a constructor, it is an indication you are violating the Single Responsibility Principle . At that point it is time to refactor to an Aggregate Service (otherwise known as a facade service) .

But another thing that is even worse is that end users of the framework will not have parameterless constructors. Do you imagine that creating a TextBox required you you pass all the dependencies it really needs?

Well, first of all it should be clear whether you are designing a framework or in fact that it is a library .

But either way, you should make it possible to inject services for overriding, but provide logical default behaviors. The best way to achieve that is by using a fluent builder to compose your services. That is, the public API will be comprised of builders that will do all of the configuration of the underlying services. You can then expose overloads that allow injection of custom services as required by the end user (ie one overload to build a default set of services with a builder, and one overload that just accepts the abstraction).

Here is a quick example that shows how that can be done:

public class VideoContentBuilder : IVideoContentBuilder
{
    private readonly string thumbnailLocation;
    private readonly string title;
    private readonly ICompressor compressor;

    public VideoContentBuilder()
        // Supply logical defaults
        : this(
            thumbnailLocation: string.Empty,
            title: string.Empty,
            compressor: new DefaultVideoCompressor(new Dependency())
        )
    {}

    private VideoContentBuilder(
        string thumbnailLocation,
        string title,
        ICompressor compressor)
    {
        this.thumbnailLocation = thumbnailLocation;
        this.title = title;
        this.compressor = compressor;
    }

    public IVideoContentBuilder WithThumbnailLocation(string thumbnailLocation)
    {
        return new VideoContentBuilder(thumbnailLocation, this.title, this.compressor);
    }

    public IVideoContentBuilder WithTitle(string title)
    {
        return new VideoContentBuilder(this.thumbnailLocation, title, this.compressor);
    }

    // Use a builder to configure defualt services
    //
    // Syntax:
    // .WithCompressor(compressor => compressor.WithLevel(Level.Maximum).WithEncryption(Encryption.None))
    public IVideoContentBuilder WithCompressor(Func<ICompressorBuilder, ICompressorBuilder> expression)
    {
        var starter = new CompressorBuilder(this.compressor);
        var builder = expression(starter);
        var compressor = builder.Create();

        return new VideoContentBuilder(this.thumbnailLocation, this.title, compressor);
    }

    // Allow a custom compressor to be injected.
    //
    // Syntax:
    // .WithCompressor(new CustomCompressor())
    public IVideoContentBuilder WithCompressor(ICompressor compressor)
    {
        return new VideoContentBuilder(this.thumbnailLocation, this.title, compressor);
    }

    // Create the configured service.
    public IVideoContent Create()
    {
        return new VideoContent(this.thumbnailLocation, this.title, compressor);
    }
)

Usage

var videoContentBuilder = new VideoContentBuilder()
    .WithThumbnailLocation("http://www.example.com/thumb.jpg")
    .WithTitle("The greatest video")
    .WithCompressor(compressor => compressor
        .WithQuality(Level.High)
        .WithAlgorithm(Algorithm.ReallyCool)
        .WithDependentService(new DependentService())
    );

var videoContent = videoContentBuilder.Create();

References:

• http://blog.ploeh.dk/2014/05/19/di-friendly-framework/
• http://blog.ploeh.dk/2014/05/19/di-friendly-library/