Objective-C sequentially Executing Two Completion Blocks

Question

I'm currently working on enabling a DJI product to execute waypoint missions autonomously, adapting from a DJI tutorial (https://developer.dji.com/mobile-sdk/documentation/ios-tutorials/GSDemo.html). So I'm trying to integrating all the processes into one function. Here are the two completion blocks that I would have to integrate:

[[self missionOperator] uploadMissionWithCompletion:^(NSError * _Nullable error) {
    if (error){
        ShowMessage(@"Upload Mission failed", error.description, @"", nil, @"OK");
    }else {
        ShowMessage(@"Upload Mission Finished", @"", @"", nil, @"OK");
    }
}];

and:

[[self missionOperator] startMissionWithCompletion:^(NSError * _Nullable error) {
    if (error){
        ShowMessage(@"Start Mission Failed", error.description, @"", nil, @"OK");
    }else
    {
        ShowMessage(@"Mission Started", @"", @"", nil, @"OK");
    }
}];

In order for the second one to run successfully, the first one must first execute completely. This doesn't seem like a hard problem, but I could not figure it out after trying to add delays or dispatches.

Any help is appreciate. Thanks.

Answer 1

From the iOS version of the docs you linked, the docs for -[DJIWaypointMissionOperator uploadMissionWithCompletion:] say:

If it is started successfully, use addListenerToUploadEvent:withQueue:andBlock to receive the detailed progress.

So, you would do something like this:

[[self missionOperator] uploadMissionWithCompletion:^(NSError * _Nullable error) {
    if (error)
    {
        ShowMessage(@"Upload Mission failed", error.description, @"", nil, @"OK");
    }
    else
    {
        ShowMessage(@"Upload Mission Started", @"", @"", nil, @"OK");

        [[self missionOperator] addListenerToUploadEvent:self
                                               withQueue:nil
                                                andBlock:^(DJIWaypointMissionUploadEvent *event){
            if (event.currentState == DJIWaypointMissionStateReadyToExecute)
            {
                [[self missionOperator] startMissionWithCompletion:^(NSError * _Nullable error) {
                    if (error)
                    {
                        ShowMessage(@"Start Mission Failed", error.description, @"", nil, @"OK");
                    }
                    else
                    {
                        ShowMessage(@"Mission Started", @"", @"", nil, @"OK");
                    }
                }];
            }
            else if (event.error)
            {
                ShowMessage(@"Upload Mission failed", event.error.description, @"", nil, @"OK");
            }
        }];
    }
}];

Answer 2

Your code looks correct. I ran into the same issue whereby after my mission finished uploading, my missionOperator's currentState would revert back to DJIWaypointMissionStateReadyToUpload rather than DJIWaypointMissionStateReadyToExecute. The mission had passed the validity check but in fact was invalid due to invalid curve requirements on individual waypoints (cornerRadiusInMeters property).

From the documentation:

/**
 *  Corner radius of the waypoint. When the flight path mode  is
 *  `DJIWaypointMissionFlightPathCurved` the flight path near a waypoint will be  a
 *  curve (rounded corner) with radius [0.2,1000]. When there is a corner radius,
 *  the aircraft will never  go through the waypoint. By default, the radius is 0.2
 *  m. If the corner is made of three adjacent waypoints (Short for A,B,C)  . Then
 *  the radius of A(short for Ra) plus radius of B(short for Rb) must be smaller
 *  than the distance between  A and B. The radius of the first and the last
 *  waypoint in a mission does not affect the flight path and it should keep the
 *  default value (0.2m).
 */

Hope this helps someone.