如何在Unity中实例化对象？

Question

I am making a top down game in Unity, how do I instantiate objects based on the players current position from the top of the camera view in Unity?

So far I have tried:

Instantiate(prefab, player.transform.position * 2 * Space.World)

but this didn't work. This just spawned objects from the center of the camera.

Answer 1

Space is simply an enum and has nothing to do with what you are trying! (Its purpose is to define the relative transform space for eg Transform.Rotate or Transform.Translate )

in that enum afaik

• Space.World simply has the int value 0
• Space.Self has the int value 1

so what you actually do is

Instantiate(prefab, player.transform.position * 2 * 0);

which equals

Instantiate(prefab, Vector3.zero);

which means the object is instantiated at World position 0,0,0 .

Also using

Instantiate(prefab, player.transform.position * 2);

looks a bit strange. Are you sure you want to duplicate the actual position of the player ? This would mean the spawned object is always on a line with the player and the World 0,0,0 and always double as far from the center as the player.

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To me it sounds more like you rather want to spawn something in front of the player ... depending on the player's view direction (in topdown games usually player.transform.up or player.transform.right ) so I guess what you are trying to do instead is something like

Instantiate(prefab, player.transform.position + player.transform.forward * 2);

which would instead spawn the object 2 Unity units in front of the player object

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After your comment it sounds like instead you want to spawn the object at the player position on the X-axis but "over it" on the Y-axis so

Instantiate(prefab, player.transform.position + Vector3.Up * 2);

maybe you'll have to tweak the 2 depending how your player can move and how far it has to be to be "off screen". Alternatively you could also use a bit more complex using Camera.ScreenToWorldPoint

// get players position
var playerPos = player.transform.position;

// get camera's position
var cameraPos = Camera.main.transform.position;

// get difference on Z-axis
var cameraDistance = cameraPos.z - playerPos.z;

// Get the world 3d point for the upper camera border
// don't care about the X value
// and as distance we use the z-distance to the player
var cameraTopPoint = Camera.main.ScreenToWorldPoint(new Vector3(0, Camera.main.pixelHeight, cameraDistance);

// use the players X-axis position
cameraTopPoint.x = player.transform.x;

// to spawn it exactly on the Screen border above the player
Instantiate(prefab, cameraTopPoint);

// or to spawn it a bit higher
Instantiate(prefab, cameraTopPoint + Vector3.Up * 1);

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Update

you said you want the prefab to be spawned on the "opposide" of the player position on the X axis.

If your Camera is static on world x=0 than this is actually quite simple:

cameraTopPoint.x = -player.transform.x;

If your camera is moving or not on x=0 than we have to calculate it already on the screen position level:

// get players position
var playerPos = player.transform.position;

// additionally convert the player position to screen space
var playerScreenPos = Camera.main.WorldToScreenPoint(playerPos);

var centerX = Camera.main.pixelWidth / 2.0f;

// get the difference between camera an player X (in screen space)
var differenceX = Mathf.Abs(playerScreenPos.x - centerX);

// here calculate the opposide side
var targetX = centerX + differenceX * (playerScreenPos.x < centerX ? 1 : -1);
// or alternatively if you want e.g. that the prefab always
// spawn with a bit of distance to the player even though he is very close
// to the center of the screen you could do something like
//var targetX = centerX + centerX / 2 * (playerScreenPos.x < centerX ? 1 : -1);

// Get the world 3d point for the upper camera border
// with the calculated X value
// and as distance we use the z-distance to the player
var cameraTopPoint = Camera.main.ScreenToWorldPoint(new Vector3(targetX, Camera.main.pixelHeight, playerScreenPos.z);

// to spawn it exactly on the Screen border above the player
Instantiate(prefab, cameraTopPoint);

// or to spawn it a bit higher
Instantiate(prefab, cameraTopPoint + Vector3.Up * 1);

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Other Update

Ok so you want the prefab always spawn next to the player in a certain distance. The side depends on which side of the screen the player is so you could actually use the first approach again but just add the desired distance:

// Adjust this is the Inspector (in Unity units)
public float spawnDistanceToPlayer;

...

// get players position
var playerPos = player.transform.position;

// additionally convert the player position to screen space
var playerScreenPos = Camera.main.WorldToScreenPoint(playerPos);

var centerX = Camera.main.pixelWidth / 2.0f;

// Get the world 3d point for the upper camera border
// with the calculated X value
// and as distance we use the z-distance to the player
var cameraTopPoint = Camera.main.ScreenToWorldPoint(new Vector3(playerScreenPos.x, Camera.main.pixelHeight, playerScreenPos.z);

// now add or reduce spawnDistanceToPlayer 
// depending on which side of the screen he is
cameraTopPoint.x += spawnDistanceToPlayer * (playerScreenPos.x < centerX ? 1 : -1);

// OR if your camera sits static on X=0 anyway you also could compare
// the player position directly without calculating in screenspace:
cameraTopPoint.x += spawnDistanceToPlayer * (playerPos.x < 0 ? 1 : -1);

// to spawn it exactly on the Screen border above the player
Instantiate(prefab, cameraTopPoint);

// or to spawn it a bit higher
Instantiate(prefab, cameraTopPoint + Vector3.Up * 1);

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Typed on smartphone so might not be "copy-paste-able" ;)