Spawn delay between enemies

Question

I am working on a tower defence game in XNA, and I have a class called Tank which creates an enemy that follows a route around a map that is found via A*. Right now, I can create multiple instances of Tank, but they all spawn at the same time. This means that they are in exactly the same place as they follow the route, making it appear like there is only one enemy.

Here is my code for loading a new wave:

if (spawn == true)
{
    maxTanks += 2;
    killsInWave += 2;

    tanks = new Tank[maxTanks];
    for (int i = 0; i < maxTanks; i++)
    {
        tanks[i] = new Tank();
        tanks[i].Initialize(map);
        tanks[i].LoadContent(Content);                                  
    }
    waveNum += 1;
    spawn = false;
}

Could anyone give me a little help making each enemy spawn with a delay between it and the last one?

An easy way for delaying is `Thread.Sleep(int milliseconds)`, but there might be something more suitable in an XNA context. — phipsgabler, May 07 '12 at 11:09

Blau · Answer 1 · 2012-05-07T11:57:23.230

You need a spawn creation rate based in time, something similar to this...

   SpawnUnitsCount = n;
   Units_Per_Second = 5; // 5 Per second

   void Update(float ElapsedSeconds)  // (float) gametime.ElapsedTime.TotalSeconds
   {      
    if (SpawnUnitCount>0)
    {
      CreationRate += Units_per_second * ElapsedSeconds;
      while ( CreationRate >= 1 )
      {
         CreationRate--;
         MakeNewUnit(); 
         SpawnUnitsCount --;
         if (SpawnUnitCount == 0)
         {
            CreationRate = 0; // The wave is finished
         }
      }
    }

score 0 · Answer 2 · answered May 07 '12 at 11:29

I would do it like that:

//need to be defined in your loop
int nextTankToCreate(0);
int iterationCount(0);
int CreateTime(500); //create every 500 iteration

iterationCount++;
if (spawn == true && iterationCount >= createTime)
        {
            iterationCount = 0;
            maxTanks += 2;
            killsInWave += 2;

            tanks = new Tank[maxTanks];
            tanks[nextTankToCreate] = new Tank();
            tanks[nextTankToCreate].Initialize(map);
            tanks[nextTankToCreate].LoadContent(Content);                                  
            nextTankToCreate++;

            if (nextTankToCreate == maxTanks)
            {
                nextTankToCreate = 0;
                waveNum += 1;
                spawn = false;
            }
        }

define the 3 first `int`s as static if it's within a function. — Roee Gavirel, May 07 '12 at 11:30

the_joric · Answer 3 · 2012-05-07T12:00:16.737

Not sure about XNA particulary, but if you need to execute some repetitive actions with interval you may use something like that:

        Action addTank = null;
        int interval = 1000; // 1000 milliseconds
        int requiredNumberOfTanks = 10;
        int tanksCreated = 0;

        var timer = new Timer(o => addTank());
        addTank = () =>
                         {
                             CreateTankMethod();  // your method to create tank
                             tanksCreated = tanksCreated + 1;
                             if (tanksCreated < requiredNumberOfTanks)
                                 timer.Change(interval, 0);
                         };
        timer.Change(interval, 0);

This code will utilize threads in most effective way. It is much better than Thread.Sleep()

You may also want to make interval between tank appears more random.

Xna provides a loop game with gives you the elapsed time between frames... this solution is ok but is not ideal for this scenario — Blau, May 07 '12 at 12:00

Spawn delay between enemies

3 Answers3