ios Multipeer Connectivity通过wifi传输图像缓慢

Question

I have an app where I am using multipeer connectivity framework to transfer image between multiple devices (peers). The issue that I have is that the image being transfer is slow. I would say a 500kb image takes about 2 secs to be sent over to the peers.

Following is my app architecture

I have multiple devices connecting to each other through wifi (without any router. Please note that the devices are not connected to any external network. I just turn on the wifi and does not connect it to any network.) I am not sure why but I have to start bluetooth as well on all the devices for them to be discovered by the device which is acting as a server and searching for other peers to connect. When I send an image from the server device to the peers, the image transfer is really slow. I thought the image should be transferring through the wifi channel which I suppose should be somewhere 5 MB/sec transfer speed. What I get is 500 KB in 2 seconds. Please check my code that I am using for multipeer connection.

Send Image code

  -(void)sendImage
    {
        UIGraphicsBeginImageContextWithOptions(_imgSize, NO, 0.0);
        [fullImage drawInRect:CGRectMake(0, 0, _imgSize.width, _imgSize.height)];
        UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

        NSData *sendData = UIImageJPEGRepresentation(fullImage, 0.2);
        NSLog(@"uiimage IMAGE data:- %lu",(unsigned long)sendData.length);
        NSArray *allPeers = appDelegate.mcManager.mySession.connectedPeers;

        NSString *strDeviceInfo=appDelegate.mcManager.mySession.description;
        NSLog(@"%@",strDeviceInfo);
        NSError *error;

        if(allPeers.count<=0)
        {
            [self.imgView setImage:fullImage];
        }
        else
        {
            [appDelegate.mcManager.mySession sendData:sendData toPeers:allPeers withMode:MCSessionSendDataReliable error:&error];
            if (error)
            {
                NSLog(@"%@", [error localizedDescription]);
            }
        }

       //imgView.layer.contentsRect=_myFrame;
        imgView.image=fullImage;
        imgView.backgroundColor=[UIColor darkGrayColor];
        imgView.contentMode = UIViewContentModeScaleAspectFit;
    }

Receive Image code

-(void)didReceiveData:(NSData *)data fromPeer:(MCPeerID *)peerID
{
    NSDictionary *dict = @{@"data": data,@"peerID": peerID};
    if([strPerform isEqualToString:@"orientation"])
    {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidReceiveContentSize" object:data userInfo:dict];
        [clientViewObj receivedContentSize:data];
    }
    else if ([strPerform isEqualToString:@"image"])
    {
        dispatch_async(dispatch_get_main_queue(), ^(void)
                       {
                           UIImage *image = [UIImage imageWithData:data];

                           clientViewObj.imgView.image = image;
        //[clientViewObj ReceivedData:data];
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidReceiveDataNotification" object:data userInfo:dict];
                       });
    }
    else if ([strPerform isEqualToString:@"pinch"])
    {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidReceivePinch" object:data userInfo:dict];
        [clientViewObj pinched:data];
    }
    else if ([strPerform isEqualToString:@"change"])
    {
        NSDictionary *dict = @{@"data": data,@"peerID": peerID};
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidChangeProcess" object:data userInfo:dict];
    }
    else if([strPerform isEqualToString:@"client"])
    {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidChangeServer" object:data userInfo:dict];
        //[_orientationViewObj receiveFromClient];
    }
    else if([strPerform isEqualToString:@"up"])
    {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidRotateImageUp" object:data userInfo:dict];
    }
    else if([strPerform isEqualToString:@"down"])
    {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"MCDidRotateImageDown" object:data userInfo:dict];
    }
}

I have tried the message and resource types to send the image. The resource is slower then message data.

Answer 1

In Multipeer Connectivity, transfers happen between both Bluetooth and WiFi. You can't guarantee any of the methods of data transfer, so if one device is on a WiFi network, the method of transfer will default to Bluetooth (slow transfers).

Answer 2

Wifi on iOS 8 appears to be much slower.

Today I implemented two apps that communicate together. One sends Core Motion data to the other via a Multipeer Connectivity session stream (NSOutputStream/NSInputStream).

With Wifi disabled on either device it flies on Bluetooth. Frequency was 50 gyroo motion objects a second from the Core Motion hardware and the connected devices was receiving 50 values a second.

On Wifi it's very slow, maybe 4–5x? Same code. No idea why. It defaults to Wifi, so if both devices have it enabled it seems to try that first.

I'm going to install iOS 9 beta tomorrow to see if they address this issue.

Answer 3

This is an old question, but since I just dealt with the same problem, I thought my experience might help someone else.

In my case, sending the image using a stream rather than the sendData method works about twice as fast. (Still note quite as fast as I would prefer, but this is a sizable gain).

Send function

func sendImage(imageURL: URL) {
    DispatchQueue.main.async {
        do {
            try self.mcSession.startStream(withName: "Image", toPeer: self.mcSession.connectedPeers[0])
            self.mcSession.sendResource(at: imageURL, withName: fileName, toPeer: self.mcSession.connectedPeers[0], withCompletionHandler: nil)
        }
        catch let error {
            print("error: \(error)")
        }
    }
}

Receive function

func session(_ session: MCSession, didFinishReceivingResourceWithName resourceName: String, fromPeer peerID: MCPeerID, at localURL: URL?, withError error: Error?) {
    do {
        let localData = try Data(contentsOf: localURL!)
        let image = UIImage(data: localData)
        DispatchQueue.main.async {
            // DispatchQueue.main.async necessary because modifying UI
            self.imageView.image = image
        }
    }
    catch let error {
        print("error \(error)")
    }
}