如何在iOS上使用Xamarin未公开的方法？

Question

I want to use a method that exists on NSString that is called writeToFile:atomically:encoding:error: but this method is not exposed to Xamarin.iOS .

How can I create a binding that will allow me to use this method? All resources that I found explain how to bind libraries.

I am aware there are better ways to write a string to a file but so it is not so much about this method but rather how to bind such methods.

Answer 1

So at the end I have managed to solve it through a binding library.

I have created a new project of type "Binding Library (iOS)". After that I added a new file NSStringWriteToFileExtension.cs and in Properties I made sure the Build Action is set to ObjcBindingApiDefinition and put the following code in there.

using Foundation;
using ObjCRuntime;

namespace Foundation
{
    [Category]
    [BaseType(typeof(NSString))]
    public interface NSStringWriteToFileExtension
    {
        [Export("writeToFile:atomically:encoding:error:")]
        bool Write(string filePath, bool atomically, NSStringEncoding encoding, out NSError error);
    }
}

This creates a new extension on NSString that gives me access to the method.

Answer 2

Calling a method on an ObjC instance is about messaging. You do not directly invoke the method, but the ObjC objc_msgSend function will when called with the object to invoke it upon, the selector which describes the method to be invoked, and the parameters of the method.

Define the objc_msgSend entry point:

Note: This includes the object that you wish to invoke the selector on, the selector created with the selector's name, and then the parameters to be passed

// NSString writeToFile:atomically:encoding:error:
[DllImport(Constants.ObjectiveCLibrary, EntryPoint = "objc_msgSend")]
static extern bool NSString_bool_obj_msgSend_IntPtr_bool_NSStringEncoding_IntPtr(
        IntPtr target,
        IntPtr selector,
        IntPtr path,
        bool atomically,
        NSStringEncoding encoding,
        IntPtr error
);

Create the matching selector:

Note: Cache this if you will be calling it repeatedly

var nsStringWriteToFileSelector = new Selector("writeToFile:atomically:encoding:error:");

Calling Example:

// Selector for NSString writeToFile:atomically:encoding:error:
var nsStringWriteToFileSelector = new Selector("writeToFile:atomically:encoding:error:");

// Parameters to be passed via `objc_msgSend` 
var nsString = new NSString("StackOverflow");
var path = new NSString("nsstring.txt");
var error = new NSError();

// Call objc_msgSend with the object, selector and parameters
var success = NSString_bool_obj_msgSend_IntPtr_bool_NSStringEncoding_IntPtr(
    nsString.Handle,
    nsStringWriteToFileSelector.Handle,
    path.Handle,
    true,
    NSStringEncoding.UTF8,
    error.Handle);

if (success && File.Exists(path.ToString()))
    Console.WriteLine("success");
else
    Console.WriteLine(error.Description);