Moving Button Down Check from Update to separate Function

Currently, I am checking if buttons are being pressed in my update function - for some reason I just think that that is not the most efficient way to do thing.

Is there a way to morve checks for button presses to other functions?
Checking for everything each frame seems like it could cause potential issues down the track.

	void Update ()
	{
		if (Input.GetKeyDown(KeyCode.LeftShift)){
			selectedArrowPosition++;

			if (selectedArrowPosition >= ArrowTypes.Count){
				selectedArrowPosition = 0;
			}
		}

		if (Input.GetButton ("Fire1")){
			//add speed to projectile if the fire button is held down
			if (chargeSpeed < MAXCHARGESPEED) {
				chargeSpeed += 0.1f;
			}
		}
		
		if (Input.GetButtonUp ("Fire1")) {
			//...setting shoot direction
			Vector2 shootDirection = Input.mousePosition;
			shootDirection = Camera.main.ScreenToWorldPoint(shootDirection);
			shootDirection.x -= transform.position.x;
			shootDirection.y -= transform.position.y;
			
			GameObject arrow = Instantiate (Resources.Load (ArrowTypes[selectedArrowPosition])) as GameObject; //"BasicArrow"
			arrow.transform.position = firePosition.position;
			BasicArrow arrowScript = arrow.GetComponent<BasicArrow> ();
			arrowScript.chargeSpeed = chargeSpeed;
			arrowScript.shootDirection = shootDirection;
			if (player.transform.localScale.x < 0){
				arrowScript.playerBackward = true;
			}

			//reset chargeSpeed
			chargeSpeed = 0;
		}
	}

The most common approach that I’ve seen is to respond to input in Update(). In fact, Unity’s own documentation gives an example doing just that. However, I do understand your concern. To my knowledge Unity simply doesn’t handle events this way.

Considering the example you gave, I imagine it could get rather hard to deal with Shift key down in such a way. If pressing down the Shift key was bound to call Foo(), what would happen if you ever wanted it to call Bar()? Or if you didn’t want it to call anything at all? If it worked this way you’d probably have to check several conditions each frame anyway but instead of adding one extra conditional you would be changing some assignment on the Shift key.

My point is it can get complicated pretty quickly and one extra conditional is probably cheaper than many other methods.

Unity does have an EventSystem in place for handling EventTriggers. Something like this can be very useful for organizing and handling callbacks to execute some code, but even it functions on Update().