B4J Library [BANanoCanvas] A HTML5 Canvas Library

Ola

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Using the BANanoElement & BANanoObject, one is able to script the HTML canvas to draw images etc. Here we follow the same methodology used to create canvas structures. To be able to do this, one uses the BANanoElement to create the <CANVAS> tab, then from that gets the 2d context to draw on the canvas.

The library uses the BANanoObject from the BANAnoElement and runs a series of .GetField, .SetField and .RunMethods internally. How the BANanoObject works was discussed on this thread.

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We want to change the page title and create some helper classes, so, we avail these.

B4X:
'change the title of the page
Sub SetTitle(tt As String)
    BANano.Window.GetField("document").SetField("title", tt)
End Sub

The skeleton sub will help with most creation of the canvas code we need to get the 2d context from.

B4X:
'create skeleton
Sub Skeleton(title As String, cid As String) As MashCanvas
    'add a paragraph to the body
    Dim p As BANanoElement = body.Append($"<p id="p${cid}">${title}</p>"$).Get($"#p${cid}"$)
    p.Append("<br/>")
    'add a canvas to the paragraph
    Dim c As BANanoElement = p.Append($"<canvas id="${cid}">Your browser does Not support the HTML5 canvas tag</canvas>"$).Get($"#${cid}"$)
    c.SetAttr("width", 300)
    c.SetAttr("height", 150)
    c.SetStyle(BANano.ToJson(CreateMap("border": "1px solid #d3d3d3")))
    p.Append("<div></div>")
    'create the context
    Dim ctx As MashCanvas
    ctx.Initialize(c, "2d")
    Return ctx
End Sub

As an example, the code for the canvas above, is written like this..

B4X:
' r1c0
    Dim ctx1 As MashCanvas = Skeleton("fillStyle", "ctx1")
    ctx1.fillStyle = "#ff0000"
    ctx1.fillRect1(20, 20, 150, 100)
   
    'r1c1
    Dim ctx2 As MashCanvas = Skeleton("fillStyle", "ctx2")
    Dim my_gradient As MashCanvas = ctx2.createLinearGradient1(0, 0, 0, 170)
    my_gradient.addColorStop1(0, "black")
    my_gradient.addColorStop1(1, "white")
    ctx2.fillStyle = my_gradient.Context
    ctx2.fillRect1(20, 20, 150, 100)
   
    'r1c2
    Dim ctx3 As MashCanvas = Skeleton("fillStyle", "ctx3")
    Dim my_gradient1 As MashCanvas = ctx3.createLinearGradient1(0, 0, 170, 0)
    my_gradient1.addColorStop1(0, "black")
    my_gradient1.addColorStop1(1, "white")
    ctx3.fillStyle = my_gradient1.Context
    ctx3.fillRect1(20, 20, 150, 100)
   
    'r1c3
    Dim ctx4 As MashCanvas = Skeleton("fillStyle", "ctx4")
    Dim my_gradient3 As MashCanvas = ctx4.createLinearGradient1(0, 0, 170, 0)
    my_gradient3.addColorStop1(0, "black")
    my_gradient3.addColorStop1(0.5, "red")
    my_gradient3.addColorStop1(1, "white")
    ctx4.fillStyle = my_gradient3.Context
    ctx4.fillRect1(20, 20, 150, 100)
   
    'r1c4
    Dim img As BANanoElement = body.Append($"<img id="lamp"></img>"$).Get("#lamp")
    img.SetAttr("src", "./assets/img_lamp.jpg")
    img.SetAttr("width", 32)
    img.SetAttr("height",  32)
    '
    Canvas("createPattern1", "ctx5")
    '
    Dim b1 As BANanoElement = body.Append($"<button id="b1">Repeat</button>"$).Get("#b1")
    b1.On("click", Me, "brepeat")

And the rest of the code...

B4X:
Sub brepeat(e As BANanoEvent)
    draw("repeat")
End Sub

Sub draw(direction As String)
    Dim c As BANanoElement
    c.Initialize("#ctx5")
    Dim ctx As MashCanvas
    ctx.Initialize(c, "2d")
    Dim cwidth As Int = c.GetAttr("width")
    Dim cheight As Int = c.GetAttr("height")
    ctx.clearRect1(0, 0, cwidth, cheight)
    Dim img As BANanoElement
    img.Initialize("#lamp")
    Dim pat As BANanoObject = ctx.createPattern1(img.ToObject, direction)
    ctx.rect1(0, 0, 150, 100)
    ctx.fillStyle = pat
    ctx.fill1
End Sub

Below is what we will create with this library...
 

Mashiane

Expert
Licensed User
This is what the library generates for your canvas. You can get any HTML5 Canvas book and explore its functionalities further. If you have questions and request more functionality, dont hesitate to start a new thread prefixed by [BANanoCanvas] in the B4J Questions.

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The source code will be in the pgIndex module of the demo project.

Ta!
 

Mashiane

Expert
Licensed User
To be able to draw the pie chart, we needed a helper function that finds the minimum value in an array:

B4X:
Sub minimum(items As List) As Int
    items.Sort(True)
    'get the first item
    Dim fi As Int = items.Get(0)
    fi = BANano.parseint(fi)
    'compare each item to the first item
    For Each item As String In items
        Dim ci As Int = BANano.parseInt(item)
        If ci < fi Then fi = ci
    Next
    Return fi
End Sub

NB: We are still exploring the issue of copying and pasting pixels, so far everything about the functionality of the canvas has been explored.

Enjoy!

PS: Updated library and demo on first post
 
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