Resolved GetKeyDown detection in Update and FixedUpdate.

if I detect a GetKeyDown in FixedUpdate, it will miss the input if the frame only runs Update.

Conversely, if I detect GetKeyDown in Update, could it be missed if the frame only runs FixedUpdate?

 

FixedUpdate() is a fraud. It doesn't run independent of the frame rate. Instead it runs as many times as needed each frame in order to catch up to its target speed. That means it could run multiple times or it could run zero times. If you store the value of GetKeyDown() in Update() it will be available to FixedUpdate().

GetKeyDown() and GetKeyUp() don't work in FixedUpdate() because the code that handles their current state isn't being executed with each FixedUpdate() but with Update(). So if you try to use them in FixedUpdate() you will see incorrect state info.

https://forum.unity.com/threads/the-truth-about-fixedupdate.231637/

 

There is no such thing, as far as I know. Update runs every frame.

 

Two good discussions on Update() vs FixedUpdate() timing:

https://jacksondunstan.com/articles/4824

https://johnaustin.io/articles/2019/fix-your-unity-timestep

 

thank you for all the helps.

after some research and some testing using the following script.

i believe that
1. the GetKeyDown always pickup by Update with no miss.
2. the GetKeyDown always detect at the Update.
3. If GetKeyDown called in FixedUpdate it will use the key-state at the last Update.

If the FixedUpdate run 10 times faster than Update, if key pressed at the last Update frame, all 10 FixedUpdate will consider the GetKeyDown is True. Even only press the key once.

Code (CSharp):

1. using UnityEngine;
2.  
3. public class test_fixed_update_update_getKeyDown : MonoBehaviour
4. {
5.     int total_key_down = 0;
6.     int keyDown_FixedUpdate = 0;
7.     int keyDown_Update = 0;
8.  
9.  
10.     void Awake()
11.     {
12.         // Time.fixedDeltaTime = 0.04f; //25fps
13.      
14.         // Application.targetFrameRate = -1;
15.         // Application.targetFrameRate = 50;
16.         // Application.targetFrameRate = 25;
17.         Application.targetFrameRate = 5;
18.     }
19.     void FixedUpdate()
20.     {
21.         if (Input.GetKeyDown(KeyCode.H))
22.         {
23.             keyDown_FixedUpdate++;
24.         }
25.         print($"Fixed:{keyDown_FixedUpdate} Update:{keyDown_Update}");
26.     }
27.     void Update()
28.     {
29.         if (Input.GetKeyDown(KeyCode.H))
30.         {
31.             keyDown_Update++;
32.         }
33.     }
34. }
35.  

 

Don't do it that way.

- check GetKeyDown() in Update()
- if true, set a boolean

- check the boolean in FixedUpdate()
- if true:
--> take action
--> clear the boolean

This is just basic input event processing logic. You will see it everywhere.

 

thank you Kurt-Dekker

i got your idea.

 

Code (CSharp):

1.    void FixedUpdate()
2.    {
3.       if (Input.GetKey("jump") && Grounded())
4.          rb.velocity.y+=10;
5.  
6.       if (Input.GetKey("shoot") && Time.time>shootTime)
7.       {
8.          Instantiate(bullet,transform.position,transform.rotation);
9.          shootTime=Time.time+1;
10.       }
11.  
12.       if (Input.GetKey("poop") && poopAvailable)
13.       {
14.          Instantiate(poop,transform.position,transform.rotation);
15.          poopAvailable=false;
16.       }
17.    }