# 12. Responding to Touch Events

In this chapter, we will improve stage script to draw judgment line at different opacity responding to touch events.

# touch Callback

For each point of contact player touches the screen, a touch event will be broadcasted. Entities can receive them using touch callback.

In touch callback, Temporary Data and Temporary Memory block are also available, and touch related information is stored in Temporary Data block which you can access using various imports.

To quickly try it out, let's make our stage script DebugLog the touch starting time when a touch starts.

const touch = And(TouchStarted, DebugLog(TouchST))

return {
    // ...
    touch,
    // ...
}

const touch = And(TouchStarted, DebugLog(TouchST))

return {
    // ...
    touch,
    // ...
}

(Note: here we use And as a shorthand for If without false branch)

# Storing Value in Entity Memory Block

Our drawing logic is in updateParallel callback, however we can only know touch information in touch callback, how do we pass data from one to another?

We can using Entity Memory block to store and share data.

We are going to use 0th value in Entity Memory block as whether there is any touch event, so we first obtain a pointer to it.

const anyTouch = EntityMemory.to<boolean>(0)

const anyTouch = EntityMemory.to(0)

Next whenever touch callback is called, we check if the touch is ended, if not then we set anyTouch to true.

const touch = Or(TouchEnded, anyTouch.set(true))

const touch = Or(TouchEnded, anyTouch.set(true))

(Note: here we use Or as a shorthand for If without true branch)

Lastly, we DebugLog it and reset it to false at the end of each frame in updateParallel.

const updateParallel = [
    DebugLog(anyTouch),
    // ...
    anyTouch.set(false),
]

const updateParallel = [
    DebugLog(anyTouch),
    // ...
    anyTouch.set(false),
]

Now if we test the level, we can see debug value changed to 1 whenever a touch is held, and back to 0 when released.

# Improve Judgment Line

To make use of it, let's change our judgment line drawing to draw at 100% opacity when there is a touch, and 50% when there isn't.

const updateParallel = [
    Draw(
        SkinSprite.JudgmentLine,
        left,
        bottom,
        left,
        top,
        right,
        top,
        right,
        bottom,
        0,
        If(anyTouch, 1, 0.5)
    ),
    anyTouch.set(false),
]

const updateParallel = [
    Draw(
        SkinSprite.JudgmentLine,
        left,
        bottom,
        left,
        top,
        right,
        top,
        right,
        bottom,
        0,
        If(anyTouch, 1, 0.5)
    ),
    anyTouch.set(false),
]