Coding an efficient knockback (vJass) - The Hive Workshop

Silvenon · 10-13-2007, 11:34 AM

INTRODUCTION + REQUIREMENTS

Knockback is probably one of the most popular things in JASS, because it can be made to be very smooth. Now, the purpose of this tutorial is how to make an efficient knockback - using vJass.

Just a good knowledge of structs is required. You can find more info on the structs in the jasshelper manual. We aren't going to use the game cache at all, and it will still be MUI.

PARAMETERS

First, let's examine our four main parameters:

u = the unit that is being knockbacked
d = the distance the unit is knockbacked to
a = the angle (direction) - in RADIANS
w = the duration of the knockback

RADIANS

Now, why am I using radians instead of degrees? Simple, so I don't have to convert degrees to radians and other way around. I have to convert degrees to radians when I'm using trigonometrical functions and I have to convert radians to degrees when I'm using Atan2() function. Example:

function IHateDegrees takes nothing returns nothing
    local real a = 30 // degrees
    call Sin(a * bj_DEGTORAD)
    // ...
    // ...
    set a = Atan2(y, x) * bj_RADTODEG
endfunction

It's more efficient to work with radians all the time. All natives that work with angles take radians, except these two: SetUnitFacing(), GetUnitFacing(). That could be annoying sometimes, but get used to it.Now, let's actually code something here, for starters, let's make a function that will take our four main parameters:

function Knockback takes unit u, real d, real a, real w returns nothing

TIMER

The basic thing you must have when you're making a knockback code is a timer, because a knockback is moving a unit a bit a time. That timer will periodically fire a function that will move the knockbacked unit a bit. We are going to make a global timer using the global block (vJass feature), you'll see later why:

globals
timer Tim = CreateTimer()
endglobals

INTERVAL

Now, when having a timer, you must also have to have the interval of timer's periodic executions. This will tell the timer how fast he has to execute.

The best way is to make a constant interval, we have 2 ways of achieving that, we can:

Make a global block, then create a (constant) global real which will hold the interval
Make a constant function which will return the interval

There is actually no difference between those two methods afaik, so I'll choose the second one, because I want the interval to be separated from the other globals for some reason. The question you are probably (or probably not) asking is: what interval is best to put? I, personally, use 0.035, but it seems like 0.04 is the most popular one, so we are going to use that one (you can use any you like, but recommended is somewhere between 0.03 and 0.04 so we can achieve a smooth effect):

constant function Interval takes nothing returns real
return 0.04
endfunction

NUMBER OF EXECUTIONS

Now when that is clear, we have one last thing we need to have: the number of executions. We have to calculate how many executions the timer has to perform, that is necessary for the timer to know when to stop. We will going to name that variable q and it will be an integer (makes sense). How to calculate q? That's simple, it's the duration divided by the interval. I hope you know math well enough to know why is it like that, if not, then learn. We will convert the result to integer, so we can put it in our integer variable:

function Knockback takes unit u, real d, real a, real w returns nothing
local integer q = R2I(w / Interval())
endfunction

What now? First, we need to create a function that will be assigned to moving the unit (that function will be executed periodically by the timer). Now we must name that function somehow:

function Knockback_Execute takes nothing returns nothing

STRUCT

The function takes nothing and returns nothing because it will be executed by the timer, and we all know that when executing a function like that, we can not transfer parameters. We have to think of a way to transfer the values from Knockback to Knockback_Execute. Hmm.... I know! Lets use structs, we will create a simple struct that will hold the values we need to transfer:

struct Knockback_Data
unit u
real a
endstruct

And let's create a global array of Knockback_Datas (again, later you'll see why):

globals
timer Tim = CreateTimer()
Knockback_Data array Ar
endglobals

What about the rest of the parameters? Well, this part gets a little complicated, but don't lose hope! We're going to go through this together.

A knockback is a process in which the unit is pushed back and then it is slowing down until it reaches the speed of 0, in other words, until it stops.

So we have to have create two new variables:

d1 = the "bit" in "moving a unit a bit a time"
d2 = will decrease d1 with each execution

CALCULATIONS

The variable d2 is required because it will decrease the d1 until it reaches 0 (or slightly less than that), that will cause deceleration in unit movement and then the unit will eventually stop. Now we'll gonna do some calculations, which is a little advanced math, but don't worry, I'll try to explain everything. I suggest you take a paper and a pencil/pen, because this calculations look ugly when written like this. I'm going to use jass tags so it's more readable (I hope):

d1 = (d - d2) + (d - 2 * d2) + (d - 3 * d2) + ... + (d - q * d2)
A math rule allows us to write that like this:

d1 = q * d - q * (q + 1) * d1 / 2
We'll just make a substitute for 'q + 1' so the calculation looks a little nicer (yeah, right...), lets name it x:

d1 = q * (d - (x * d2) / 2)
Lets fix it a little bit:

d1 = q * ((2 * d - x * d2) / 2)
When d1 reaches 0:

0 = q * ((2 * d - x * d2) / 2)
Now we'll multiply the equation by 2:

0 = q * (2 * d - x * d2)
0 = 2 * d * q - x * d2 * q
We'll going to return 'q + 1' now:

0 = 2 * d * q - (q + 1) * d2 * q
We'll going to move '- (q + 1) * d2 * q' to the other side:

(q + 1) * d2 * q = 2 * d * q
On the both sides we have q so if we divide the equation with 'q', they will be gone (meaning they will turn to 1 and we all know that 'x * 1' is 'x'):

(q + 1) * d2 = 2 * d
d2 = 2 * d / (q + 1)
Now we have:

d1 = 2 * d / (q + 1)
d2 = d1 / q

So we'll add those two in our struct:

struct Knockback_Data
    unit u
    real a
    real d1
    real d2
endstruct

So, now we have to create that struct as a local in the Knockback function and set its values:

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q
endfunction

I think everything is clear here, we need to transfer the knockbacked unit, the angle, d1 and d2 to the Knockback_Execute function.

The important part in knockbacking is to pause the knockbacked unit, because not pausing it can cause undesirable effects (like not moving in a straight line):

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    call PauseUnit(u, true)
endfunction

STOPPING ANY ORDERS

For moving the unit, we'll going to use SetUnitX/Y, because it has (in this case) a better effect than SetUnitPosition. But SetUnitPosition can break channeling spells, moreover, stop the unit's orders (among other things), and it is important for the knockback to be able to do that. We'll going to move the unit to the same point it already is, because we just want to stop its orders:

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)
endfunction

NUMBER OF KNOCKBACKS

Now, lets get back to that mysterious global timer. The reason I'm using a global timer is because I don't want to use game cache. Instead of creating a local timer every time, attaching the struct to it and execute the movement in the Knockback_Execute function for each unit separately, I'm going to use a single timer that will perform each active knockback. I'm going to pause it when there are no active knockbacks (and unpause it when there are). I'll do this by creating a new integer global that will tell me how many units do I have to move. I'll set it's initial value to 0, because in the start, there will be 0 active knockbacks:

globals
    timer Tim = CreateTimer()
    Knockback_Data array Ar
    integer Total = 0
endglobals

Then I'm going to start the timer in the Knockback function if Total is equal to 0, because that will mean it is paused and it needs to be started again because the function was called (which means there is a knockback to be performed):

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif
endfunction

Now we have to tell the system that a new knockback is active, we'll going to do that by increasing Total by 1. Another thing we also have to do is add the Knockback_Data that was created (kd) in the Knockback_Data array (Ar), but the index of the first one will be 0:

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd
endfunction

"KNOCKBACK_EXECUTE" FUNCTION

We have to move to the Knockback_Execute function now, what we'll do is create a local Knockback_Data variable, so we can handle each knockback (that needs to be processed) more easily:

function Knockback_Execute takes nothing returns nothing
local Knockback_Data kd
endfunction

We are going to have a loop now, because we need to process each active knockback, so we'll also have to create a local integer for the loop:

function Knockback_Execute takes nothing returns nothing
local Knockback_Data kd
local integer i = 0
endfunction

And because we are going to move units, we are going to add two coordinate variables (x and y):

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y
endfunction

Now we will make a loop that will stop when i (the variable, not me) reaches Total, and just in case of it skipping Total, we'll make it stop when i is equal or greater than Total:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set i = i + 1
    endloop
endfunction

In the beginning of the loop we'll set kd to Ar[i] (so it's easier to handle):

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]
        set i = i + 1
    endloop
endfunction

MOVING THE UNIT

Remember what we said about d1? It is the "bit" in "moving a unit a bit a time", we will put its destination coordinates in variables, then we'll move the unit to those coordinates with SetUnitX/Y:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set i = i + 1
    endloop
endfunction

I hope we all know how to do a polar projection with coordinates, because that is what I did here.

Now, after that we need to decrease d1 by d2 (that's what I explained in the "calculations" part):

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        set i = i + 1
    endloop
endfunction

DESTROYING THE STRUCT

When a knockback finishes, we have to destroy its struct, right? The knockback stops when d1 reaches 0, because that means that the unit has stopped moving. We'll check if d1 reached 0 (or less) with an if/then/else:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 then
            call kd.destroy()
        endif

        set i = i + 1
    endloop
endfunction

But when we are destroying that struct, it will leave an empty space, and that is definitely not good. We will have to fill it out somehow, I think the easiest way would be to move the last struct to that empty place (we'll going to do that before destroying the struct):

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 then
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop
endfunction

I used Ar[Total - 1] because the first index of the array is 0, not 1 (I hope that's clear). I'm setting Total to Total - 1 because when a struct is destroyed, that means there is a finished knockback that is no longer active, but done.

One thing we're forgetting (or maybe not) is unpausing the unit when the knockback is over:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop
endfunction

Also, when there are no active knockbacks (meaning Total is equal to 0), we need to pause the timer:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * Cos(kd.a)
        set y = GetUnitY(kd.u) + kd.d1 * Sin(kd.a)

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

SINE AND COSINE

Pay attention to that Cos() and Sin() in the Knockback_Execute function, we don't have to call them every 0.04 seconds, right? That's really inefficient, we can just store them in a variable. Moreover, we are going to put them in our struct:

struct Knockback_Data
    unit u
    real a
    real d1
    real d2

    real sin
    real cos
endstruct

Now, what we're going to do now is give those variables a value:

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    set kd.sin = Sin(a)
    set kd.cos = Cos(a)

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd
endfunction

But note that then we don't need the angle variable in our struct anymore, so we can remove it:

struct Knockback_Data
    unit u
// real a
    real d1
    real d2

    real sin
    real cos
endstruct

function Knockback takes unit u, real d, real a, real w returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
// set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    set kd.sin = Sin(a)
    set kd.cos = Cos(a)

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd
endfunction

Also, don't forget to do use them in the Knockback_Execute function:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * kd.cos
        set y = GetUnitY(kd.u) + kd.d1 * kd.sin

        call SetUnitX(kd.u, x)
        call SetUnitY(kd.u, y)

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

MAP BOUNDS CAUTION

I think we all know that when a unit (or anything else) gets outside the map bounds, the game crashes. What we're going to do is prevent our knockback code to push the unit outside those bounds. Instead, we'll make the unit stop if it reaches the bounds. First, we will implement Vexorian's CheckPathability function, which is very useful I must say. Then lets put the function on the top of our script (or in the custom script section, wherever you're coding this).

Now when we have that implemented, here's what we're going to do: in the Knockback_Execute function we're going to check if the point that the unit is supposed to move to is pathable, if it's not, then that means something is already there (tree, building, map bound, cliffs etc.). Here's how we're going to do that:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * kd.cos
        set y = GetUnitY(kd.u) + kd.d1 * kd.sin

        if CheckPathability(x, y) then
            call SetUnitX(kd.u, x)
            call SetUnitY(kd.u, y)
        endif

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 or not CheckPathability(x, y) then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

The problem here is that we're creating an item and removing it every 0.04 seconds, that's inefficient. We can make a single global item and just move it around in each execution:

globals
    timer Tim = CreateTimer()
    Knockback_Data array Ar
    integer Total = 0

    item I = CreateItem('ciri', 0, 0)
endglobals

function CheckPathabilityTrickGet takes nothing returns nothing
    set bj_rescueChangeColorUnit = bj_rescueChangeColorUnit or GetEnumItem() != I
endfunction

function CheckPathabilityTrick takes real x, real y returns boolean
    local integer i = 30
    local real X
    local real Y
    local rect r
    call SetItemPosition(I, x, y)
    set X = GetItemX(I) - x
set Y = GetItemY(I) - y
    if X * X + Y * Y <= 100 then
        return true
    endif
    set r = Rect(x - i, y - i, x + i, y + i)
    set bj_rescueChangeColorUnit = false
    call EnumItemsInRect(r, null, function CheckPathabilityTrickGet)
    call RemoveRect(r)
    set r = null
    return bj_rescueChangeColorUnit
endfunction

function CheckPathability takes real x, real y returns boolean
    local boolean b = CheckPathabilityTrick(x, y)
    call SetItemVisible(I, false)
    return b
endfunction

I also got rid of those darn Pow() functions, I don't know why Vex used them.

HITTING TREES

You have probably seen that some knockbacked units take down trees if they bump into them. So that's what we're also going to do here, otherwise, the unit would stop if it ran into a tree also (the CheckPathability part). What we'll do is add a new parameter in our Knockback function:

function Knockback takes unit u, real d, real a, real w, real r returns nothing
This parameter represents the range in which the unit must come within the tree in order to take it down. We'll make it so if you put 0 as a range parameter, no trees will be destroyed. We have to put a new variable in our struct (the one that will hold the range):

struct Knockback_Data
    unit u
    real d1
    real d2

    real sin
    real cos

    real r
endstruct

function Knockback takes unit u, real d, real a, real w, real r returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    set kd.sin = Sin(a)
    set kd.cos = Cos(a)

    set kd.r = r

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd
endfunction

This is a function I wrote (actually copied from Hero12341234's Tree Revival tutorial, hehe, sorry Hero....):

function Knockback_TreeFilter takes nothing returns boolean
local integer d = GetDestructableTypeId(GetFilterDestructable())
return d == 'ATtr' or d == 'BTtw' or d == 'KTtw' or d == 'YTft' or d == 'JTct' or d == 'YTst' or d == 'YTct' or d == 'YTwt' or d == 'JTwt' or d == 'JTwt' or d == 'FTtw' or d == 'CTtr' or d == 'ITtw' or d == 'NTtw' or d == 'OTtw' or d == 'ZTtw' or d == 'WTst' or d == 'LTlt' or d == 'GTsh' or d == 'Xtlt' or d == 'WTtw' or d == 'Attc' or d == 'BTtc' or d == 'CTtc' or d == 'ITtc' or d == 'NTtc' or d == 'ZTtc'
endfunction

With this function we will check if the destructible (or, in JASS, destructable, I don't know which is right anymore) in range is a tree.

Ok, now we will go to the Knockback_Execute function, because from there we're going to destroy trees. This part is kinda weird because we'll have to create a rect, because the only way we could reach "destructibles in range" is through EnumDestructablesInRect() function. We will create a rect out of range, there's a BJ that does that:

function GetRectFromCircleBJ takes location center, real radius returns rect
    local real centerX = GetLocationX(center)
    local real centerY = GetLocationY(center)
    return Rect(centerX - radius, centerY - radius, centerX + radius, centerY + radius)
endfunction

We will use pure natives, I just showed you that function so it's easier to understand what I'm doing. So basically, in the Knockback_Execute function:

Code:

centerX = x
centerY = y
radius = kd.r

Now we'll create a local rect variable and we will check if a destructible is in the circle:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y
    local rect r

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * kd.cos
        set y = GetUnitY(kd.u) + kd.d1 * kd.sin

        if kd.r != 0 then
            set r = Rect(x - kd.r, y - kd.r, x + kd.r, y + kd.r)
        endif

        if CheckPathability(x, y) then
            call SetUnitX(kd.u, x)
            call SetUnitY(kd.u, y)
        endif

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 or not CheckPathability(x, y) then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

We'll create a new function that we will use for EnumDestructablesInRect() function, it will kill every picked destructible:

function Knockback_KillTree takes nothing returns nothing
call KillDestructable(GetEnumDestructable())
endfunction

We will put it above Knocback_Execute function, so we can reach it.

Now we have an action and a condition, we can use EnumDestructablesInRect() now, but first we have to create a boolexpr that will check if the picked destructible is a tree:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y
    local rect r
    local boolexpr b

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        set x = GetUnitX(kd.u) + kd.d1 * kd.cos
        set y = GetUnitY(kd.u) + kd.d1 * kd.sin

        if kd.r != 0 then
            set r = Rect(x - kd.r, y - kd.r, x + kd.r, y + kd.r)
            set b = Filter(function Knockback_TreeFilter)
            call EnumDestructablesInRect(r, b, function Knockback_KillTree)
            call RemoveRect(r)
            call DestroyBoolExpr(b)
        endif

        if CheckPathability(x, y) then
            call SetUnitX(kd.u, x)
            call SetUnitY(kd.u, y)
        endif

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 or not CheckPathability(x, y) then
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

But instead of calling the Filter all the time, since the boolexpr is always the same, we can make a global boolexpr and set it in on map initialization:

globals
    timer Tim = CreateTimer()
    Knockback_Data array Ar
    integer Total = 0

    item I = CreateItem('ciri', 0, 0)

    boolexpr Filter
endglobals

function Trig_Map_Initialization_Actions takes nothing returns nothing
    set Filter = Filter(function Knockback_TreeFilter)
endfunction

Note that this would mean that you need to transfer the Knockback_TreeFilter function somewhere reachable by Trig_Map_Initialization_Actions, now it doesn't have to be reachable by Knockback_Execute function anymore.

SPECIAL EFFECTS

In some knockbacks there are special effects involved during the process. We're going to make two types of making a special effect:

Creating and destroying the special effect in the Knockback_Execute function
Creating a special effect attached to the knockbacked unit, and when the knockback ends, destroy it

First we are going to make a new integer parameter that will represent the type. 0 will be none (no special effects), 1 will be type 1 and 2 will be type 2:

function Knockback takes unit u, real d, real a, real w, real r, integer t returns nothing
Now, of course, we have to add the string parameter for the special effect path, and if the type is 2, a string parameter for the attachment point. We're going to make it so if you input 1 as the type, you can put "" as attachment point. Just to make something clear, in the type 1 we're going to create the special effect at the position of knockbacked unit (and destroy it) periodically:

function Knockback takes unit u, real d, real a, real w, real r, integer t, string s, string p returns nothing
If you input 0 as type, you can put "" for s and p.

We have to create two new variables in our struct (we're going to leave out type variable, you'll see why) and we're going to set their initial values to 'none':

struct Knockback_Data
    unit u
    real d1
    real d2

    real sin
    real cos

    real r

    string s = ""
    effect e = null
endstruct

What the hell is 'effect e'? This is used for the type 2, so it can be reachable when we want to destroy it.

Lets attach those parameters now, with few if/then/else's:

function Knockback takes unit u, real d, real a, real w, real r, integer t, string s, string p returns nothing
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    set kd.sin = Sin(a)
    set kd.cos = Cos(a)

    set kd.r = r

    if s != "" and s != null then
        if t == 2 then
            if p != "" and p != null then
                set kd.e = AddSpecialEffectTarget(s, u, p)
            else
                set kd.e = AddSpecialEffectTarget(s, u, "chest")
            endif
        elseif t == 1 then
            set kd.s = s
        endif
    endif

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd
endfunction

I made it so if you pick type 2, but use attachment point "", the attachment point will be "chest" (that's like a default value).

Now lets go to Knockback_Execute function:

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0
    local real x
    local real y
    local rect r
    local boolexpr b

    loop
        exitwhen i >= Total
        set kd = Ar[i]

        if kd.s != null and kd.s != null then
            set x = GetUnitX(kd.u)
            set y = GetUnitY(kd.u)

            call DestroyEffect(AddSpecialEffect(kd.s, x, y)

            set x = x + kd.d1 * kd.cos
            set y = y + kd.d1 * kd.sin
        else
            set x = GetUnitX(kd.u) + kd.d1 * kd.cos
            set y = GetUnitY(kd.u) + kd.d1 * kd.sin
        endif

        if kd.r != 0 then
            set r = Rect(x - kd.r, y - kd.r, x + kd.r, y + kd.r)
            set b = Filter(function Knockback_TreeFilter)
            call EnumDestructablesInRect(r, b, function Knockback_KillTree)
            call RemoveRect(r)
            call DestroyBoolExpr(b)
        endif

        if CheckPathability(x, y) then
            call SetUnitX(kd.u, x)
            call SetUnitY(kd.u, y)
        endif

        set kd.d1 = kd.d1 - kd.d2

        if kd.d1 <= 0 or not CheckPathability(x, y) then
            if kd.e != null then
                call DestroyEffect(kd.e)
            endif
            call PauseUnit(kd.u, false)
            set Ar[i] = Ar[Total - 1]
            set Total = Total - 1
            call kd.destroy()
        endif

        set i = i + 1
    endloop

    if Total == 0 then
        call PauseTimer(Tim)
    endif
endfunction

I did that weird if/then/else with s to avoid calling GetUnitX/Y twice. You see now why I put effect e in the struct, right? I did it because it was possible to do without attaching type variable too, with a few if/then/elses everything is possible ;).

For the end I just want to make on more thing (I didn't know where to squeeze it):

function Knockback takes unit u, real d, real a, real w, real r, integer t, string s, string p returns Knockback_Data
    local Knockback_Data kd = Knockback_Data.create()
    local integer q = R2I(w / Interval())

    set kd.u = u
    set kd.a = a
    set kd.d1 = 2 * d / (q + 1)
    set kd.d2 = kd.d1 / q

    set kd.sin = Sin(a)
    set kd.cos = Cos(a)

    set kd.r = r

    if s != "" and s != null then
        if t == 2 then
            if p != "" and p != null then
                set kd.e = AddSpecialEffectTarget(s, u, p)
            else
                set kd.e = AddSpecialEffectTarget(s, u, "chest")
            endif
        elseif t == 1 then
            set kd.s = s
        endif
    endif

    call SetUnitPosition(u, GetUnitX(u), GetUnitY(u))
    call PauseUnit(u, true)

    if Total == 0 then
        call TimerStart(Tim, Interval(), true, function Knockback_Execute)
    endif

    set Total = Total + 1
    set Ar[Total - 1] = kd

    return kd
endfunction

I put that if you want to mess with the knockback during it, for example if you want to:

change the angle by changing kd.sin and kd.cos
change the unit knockbacked, though that can lead to some screwy side-effects
change the special effect by changing kd.s/kd.e

FEW TRICKS & TIPS

We can put this code in the map header by putting it in a library, so it can be reachable by anywhere:

library Knockback

struct Knockback_Data
    unit u
    real d1
    real d2

    real sin
    real cos

    real r

    string s = ""
    effect e = null
endstruct

globals
    timer Tim = CreateTimer()
Knockback_Data array Ar
    integer Total = 0

    item I = CreateItem('ciri', 0, 0)

    boolexpr Filter
endglobals

function CheckPathabilityTrickGet takes nothing returns nothing
    set bj_rescueChangeColorUnit = bj_rescueChangeColorUnit or GetEnumItem() != I
endfunction

function CheckPathabilityTrick takes real x, real y returns boolean
    local integer i = 30
    local real X
    local real Y
    local rect r
    call SetItemPosition(I, x, y)
    set X = GetItemX(I) - x
set Y = GetItemY(I) - y
    if X * X + Y * Y <= 100 then
        return true
    endif
    set r = Rect(x - i, y - i, x + i, y + i)
    set bj_rescueChangeColorUnit = false
call EnumItemsInRect(r, null, function CheckPathabilityTrickGet)
    call RemoveRect(r)
    set r = null
    return bj_rescueChangeColorUnit
endfunction

function CheckPathability takes real x, real y returns boolean
    local boolean b = CheckPathabilityTrick(x, y)
    call SetItemVisible(I, false)
return b
endfunction

function Knockback_TreeFilter takes nothing returns boolean
    local integer d = GetDestructableTypeId(GetFilterDestructable())
    return d == 'ATtr' or d == 'BTtw' or d == 'KTtw' or d == 'YTft' or d == 'JTct' or d == 'YTst' or d == 'YTct' or d == 'YTwt' or d == 'JTwt' or d == 'JTwt' or d == 'FTtw' or d == 'CTtr' or d == 'ITtw' or d == 'NTtw' or d == 'OTtw' or d == 'ZTtw' or d == 'WTst' or d == 'LTlt' or d == 'GTsh' or d == 'Xtlt' or d == 'WTtw' or d == 'Attc' or d == 'BTtc' or d == 'CTtc' or d == 'ITtc' or d == 'NTtc' or d == 'ZTtc'
endfunction

constant function Interval takes nothing returns real
    return 0.04
endfunction

function Knockback_Execute takes nothing returns nothing
    local Knockback_Data kd
    local integer i = 0