A library source to return a list of screen ids

ShareScreenMac.mm

//
//  ShareScreenMac.mm
//  ShareScreenLib
//
//  Created by Rick Cheng on 4/17/20.
//  Copyright © 2020 Agora. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <AppKit/AppKit.h>
#import <CoreGraphics/CoreGraphics.h>

#if _MSC_VER // this is defined when compiling with Visual Studio
#define EXPORT_API __declspec(dllexport) // Visual Studio needs annotating exported functions with this
#else
#define EXPORT_API // XCode does not need annotating exported functions, so define is empty
#endif

// ------------------------------------------------------------------------
// Plugin itself


// Link following functions C-style (required for plugins)
extern "C"
{
    // Just a test
    EXPORT_API int PrintANumber(){
        return 7;
    }
    
    ///  Obtain a list of DisplayIds
    /// @param d  - managed array passed from C# layer
    /// @param size  - assigned elements in the array
    /// @param length  - array length
    EXPORT_API void GetDisplayIds(unsigned int * d, int * size, int length)
    {
        int count = 0;
        for(NSScreen *screen in [NSScreen screens])
        {
            NSDictionary * dict = [screen deviceDescription];
            NSNumber * ddd = (NSNumber *) dict[@"NSScreenNumber"];
            
            if (ddd != nil)
            {
                UInt displayId = [ddd unsignedIntValue];
                d[count] = displayId;
            }
            count++;
            if (count > length) break;
        }
        
        *size = count;
    }
    


    ///  Obtain a list of Windows information
    /// @param output - A preallocated memory buffer to hold string data, represents windows list array in JSON format
    /// @param capacity - the length of the output allocated
    EXPORT_API void GetWindows(char * output, int capacity)
    {
        NSArray*  windows = (NSArray*)CFBridgingRelease(CGWindowListCopyWindowInfo(
                                                                                   kCGWindowListOptionOnScreenOnly | kCGWindowListExcludeDesktopElements, kCGNullWindowID));
        
        // NSLog(@"All on screen windows: %lu\n", (unsigned long)windows.count);
        
        // The windows filled, but we should filter out anything below the dock area
        // Find window ID of "Dock" window
        NSNumber *dockWindowNumber = nil;
        for (NSDictionary *window in windows) {
            if ([(NSString *)window[(__bridge NSString *)kCGWindowName] isEqualToString:@"Dock"]) {
                dockWindowNumber = window[(__bridge NSString *)kCGWindowNumber];
                break;
            }
        }
        // NSLog(@"dockWindowNumber: %@\n", dockWindowNumber);

        if (dockWindowNumber) {
            // Fetch on screen windows again, filtering to those "below" the Dock window
            // This filters out all but the "standard" application windows
            CFArrayRef windowListArray = CGWindowListCreate(kCGWindowListOptionOnScreenBelowWindow|kCGWindowListExcludeDesktopElements, [dockWindowNumber unsignedIntValue]);
            windows = CFBridgingRelease(CGWindowListCreateDescriptionFromArray(windowListArray));
            // NSLog(@"On screen application windows: %@", windows);
            CFRelease(windowListArray);
        }
        else {
            // NSLog(@"Could not find Dock window description");
        }
        
        // Turn the list into a JSON string to return
        NSError *error = nil;
        NSData *jsonData = [NSJSONSerialization dataWithJSONObject:windows options:0 error:&error];
        NSString *jsonString = [[NSString alloc] initWithData:jsonData
                                                     encoding:NSUTF8StringEncoding];
        
        // NSLog(@"JSON: %@\n", jsonString);
        
        NSUInteger length = [jsonString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
        if ( length >0 && length < capacity) {
            strcpy(output, jsonString.UTF8String);
        } else {
            NSLog(@"The output string can't hold the JSON:%ld", length);
        }
    }
} // end of export C block

TestMacScreenUnity.cs

#if UNITY_STANDALONE_OSX || UNITY_EDITOR_OSX

    [DllImport("ShareScreenLib")]
    private static extern int PrintANumber();
    [DllImport("ShareScreenLib")]
    public static extern void GetDisplayIds([In, Out] uint[] displays, ref int size, int length);

    void TestMacBridge()
    {
        int k = PrintANumber();
        Debug.LogWarning("ver number = " + k);

        uint[] ids = new uint[10];
        int size = 0;  // how much of array filled
        int length = 10;  // length of the allocated array
        GetDisplayIds(ids, ref size, length);

        Debug.LogWarning("Size = " + size);
        if (size > 0)
        {
            for (int i = 0; i < size; i++)
            {
                Debug.LogWarning("Id = " + ids[i]);
            }
        }
    }
#endif