docs: simple cocoa API usage example

DOCS/client_api_examples/cocoabasic.m

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+// Plays a video from the command line in a window provided by mpv.
+// You likely want to play the video in your own window instead,
+// but that's not quite ready yet.
+// You may need a basic Info.plist and MainMenu.xib to make this work.
+
+#include "../../libmpv/client.h"
+#include "shared.h"
+
+#import <Cocoa/Cocoa.h>
+
+@interface AppDelegate : NSObject <NSApplicationDelegate>
+{
+    mpv_handle *mpv;
+}
+@end
+
+@implementation AppDelegate
+
+- (void) applicationDidFinishLaunching:(NSNotification *)notification {
+    // Read filename
+    NSArray *args = [NSProcessInfo processInfo].arguments;
+    if (args.count < 2) {
+        NSLog(@"Expected filename on command line");
+        exit(1);
+    }
+    NSString *filename = args[1];
+
+    // Run MPV loop on its own queue
+    dispatch_async(dispatch_queue_create("mpv", DISPATCH_QUEUE_SERIAL), ^{
+
+        // Set up MPV
+        mpv = mpv_create();
+        if (!mpv) {
+            printf("failed creating context\n");
+            exit(1);
+        }
+        check_error(mpv_initialize(mpv));
+
+        const char *cmd[] = {"loadfile", filename.UTF8String, NULL};
+        check_error(mpv_command(mpv, cmd));
+
+        // Listen for events
+        mpv_event *event;
+        do {
+            event = mpv_wait_event(mpv, -1);
+            switch (event->event_id) {
+                case MPV_EVENT_NONE:
+                    break;
+                default:
+                    printf("event: %s\n", mpv_event_name(event->event_id));
+            }
+        } while (event->event_id != MPV_EVENT_SHUTDOWN);
+
+        // Clean up and shut down
+        mpv_terminate_destroy(mpv);
+        mpv = nil;
+        dispatch_async(dispatch_get_main_queue(), ^{
+            [[NSApplication sharedApplication] terminate:nil];
+        });
+    });
+}
+
+// Ostensibly, mpv's window would be hooked up to this.
+- (BOOL) windowShouldClose:(id)sender
+{
+    if (mpv) {
+        const char *args[] = {"quit", NULL};
+        mpv_command(mpv, args);
+    }
+    return YES;
+}
+@end
+
+
+
+// Delete this if you already have a main.m.
+int main(int argc, const char * argv[]) {
+    @autoreleasepool {
+        NSApplication *app = [NSApplication new];
+        AppDelegate *delegate = [AppDelegate new];
+        app.delegate = delegate;
+        [app run];
+    }
+    return EXIT_SUCCESS;
+}