In Unity is a WWW class asynchronous or synchronous? - asynchronous

Question

I am trying to understand how to write a WWW call in Unity. According to the description here http://docs.unity3d.com/ScriptReference/WWW.html I can inspect the isDone property, but in the example on the same page, it makes no attempt to inspect isDone.
The question I have is, if I make a WWW call and it takes several seconds to reply, doesn't the game freeze?
I would like to think the right code is this, but is it?
StartCoroutine(WaitForResponse(myWWWObject));
private IEnumerator WaitForResponse(WWW aRequest ) {
while ( ! aRequest.isDone )
yield return aRequest;
}
Does the game freeze until aRequest is done? Or is it truly asynchronous?

Answer 1

You need to understand Coroutines - a fundamental feature of Unity that allows you to write long-duration code functions (eg: longer then a frame) that will not freeze your game.
http://docs.unity3d.com/Manual/Coroutines.html
In C# you can spot a coroutine function because it has a return type of IEnumerator. And you can spot the locations in the code where the function will suspend and continue from again by the C# keyword yield.
Each MonoBehaviour class can manage coroutines, and if you tell it to start one with StartCoroutine(), then MonoBehaviour will call the coroutine every frame (sometimes more then once) until the Coroutine reaches the end.
For WWW class (which supports co-routines), all you need is to call this:
WWW www = new WWW(url);
yield return www;
You create a WWW class with the URL to retrievve, and the yield will basically be called each frame automatically by the MonoDevelop coroutine manager until www object says it has completed (successfully or failure).
Your game will not freeze at all during this time.

Answer 2

In the linked documentation page for WWW, the following method is a coroutine method:
IEnumerator Start() {
WWW www = new WWW(url);
yield return www;
Renderer renderer = GetComponent<Renderer>();
renderer.material.mainTexture = www.texture;
}
The yield statement is used to pause execution and return control to Unity and then to resume execution in the next game frame. For example yield return null; will cause execution to resume in the next frame. Additionally, you can extend that behaviour by using one of the classes that derive from YieldInstruction class, for example WaitForSeconds.
Consider yield return new WaitForSeconds(1f); - this will resume execution after 1 second of game time has passed. The WWW class works in similar way. You can use an instance of that class to yield execution, returning only after the download has been completed, no need to manually poll the isDone property.
Keep in mind that you can only yield in Coroutines, I advise you read up on them. If you want to do a WWW request not in a Coroutine then you have to poll the isDone manually and proceed when completed.
To answer last question: Will the game freeze when creating an instance of the WWW class? - No. The class works asynchronously. You can use it in normal Update/Start etc. functions and inside Coroutines with yield.

Answer 3

if I make a WWW call and it takes several seconds to reply, doesn't the game freeze?
Not, Game Will not be freez try this code and you will see that log "after corou" will show immediately even the picture is downloading.
public class WWWDemo : MonoBehaviour {
bool b = true;
// Use this for initialization
void Start () {
}
// Update is called once per frame
void Update()
{
if (b) {
StartCoroutine(ExampleCoroutine());
Debug.LogWarning("after corou");
}
}
IEnumerator ExampleCoroutine()
{
b = false;
Debug.Log("Time : " + DateTime.Now);
string url = "http://images.earthcam.com/ec_metros/ourcams/fridays.jpg";
WWW www = new WWW(url);
yield return www;
Debug.Log("Time : " + DateTime.Now);
Renderer render = gameObject.GetComponent<Renderer>();
render.material.mainTexture = www.texture;
Debug.Log("Time : " + DateTime.Now);
}
}
but the execution of next line will definitely stop after yield return www statement it will wait unitl the picture download while the rest events of Unity will execute async.
Consider below example as well separately. Start Event executes ones it will wait but update will run continuously.
IEnumerator Start() {
Debug.Log("Time : " + DateTime.Now);
string url = "http://images.earthcam.com/ec_metros/ourcams/fridays.jpg";
WWW www = new WWW(url);
yield return www;
Debug.Log("Time : " + DateTime.Now);
Renderer render = gameObject.GetComponent<Renderer>();
render.material.mainTexture = www.texture;
Debug.Log("Time : " + DateTime.Now);
}
void Update()
{
Debug.Log("update is running while start is waiting for downloading");
}