使用Xamarin.Forms和Xamarin Studio的基本存储

Question

I have been going around and around for hours with a seemingly simple task of storing very basic information in a Xarmarin.Forms PCL app. All I want to do is have a user enter one thing into an Entry field and then save that to Android and IOS devices.

Everything I see I can't get to work in Xamarin Studio (Such as installing nuget packages).

Answer 1

Performing file I/O using a shared PCL project is a little trickier than you might expect; the core Xamarin Forms framework doesn't expose any of the System.IO classes one would use to do this, which can leave you pretty puzzled at first. If you think about it a bit, it makes a certain amount of sense; after all, each platform is going to have its own implementation for the I/O infrastructure. However, that doesn't make it any less frustrating when all you're trying to do is write to a bleeping file.

This is where the DependencyService comes in! The designers of the Xamarin Forms framework understood that people would want to write code in a shared assembly but still need access to platform-specific implementations of things such as low-level file access. To support this use case, Xamarin Forms exposes a simple MEF-like dependency-injection framework that allows you to register implementations of interfaces via attribution.

The DependencyService framework has a simple recipe: define an interface in an assembly that will be referenced by your platform-specific code (the Android, iOS, WinPhone projects), define implementation classes in the platform-specific asssemblies, and register the implementation classes with the Dependency attribute (see the Xamarin Docs for more information).

For a simple project, the shared PCL will be fine. In more complex projects, I'd recommend a dedicated "Common" assembly that contains interfaces, shared types, etc. In this case, we can define an interface that exposes some simple functionality:

using System;

namespace MyDemo
{
    public interface IFileService
    {
        void WriteData(String data);
        String ReadData();
    }
}

Next up, we'll create an implementation for our Android and iOS platforms. Create a class that implements the interface. It is important that it have a parameterless constructor, as the DependencyService currently does not provide a way to pass in any constructor arguments:

namespace MyDemo
{
    public class FileService : IFileService
    {
        public FileService()
        {
        }

        public void WriteData(String data)
        {
            string filePath = Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.Personal),
            "Data.txt");
            System.IO.File.WriteAllText(filePath, data);
        }

        public String ReadData()
        {
            string filePath = Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.Personal),
            "Data.txt");
            var data = System.IO.File.ReadAllText(filePath);
            return data;
        }
    }
}

Finally, add an attribute that exports this implementation:

[assembly: Xamarin.Forms.Dependency(typeof(MyDemo.FileService))]
namespace MyDemo
{
    public class FileService : IFileService
    ...

It's VERY important that the Dependency attribute be placed outside the namespace declaration; you can be left scratching your head trying to understand why you're getting null references even though you have apparently done everything right.

Finally, in your shared project's code, use the DependencyService to resolve the implementation at runtime:

var fileService = DependencyService.Get<IFileService>();
fileService.WriteData("Hello!");
var data = fileService.ReadData();
System.Diagnostics.Debug.WriteLine(data);

Obviously, you will want something a little more advanced than my example, but this should get you started.

Hope this helps!

Answer 2

Charles Petzold在他的电子书《 使用Xamarin创建移动应用程序》中还使用DependencyService为所有三个平台的File helper编写了大量代码。