Top-bar main menu is a big screen as bottom-bar main menu is an small phone screens. Here is an example of how to build a smartphone interface with multiple interfaces. I just added two interfaces the first screen has a welcoming message. The second on a keyboard to dial.

The buttons controls the screens through the ScreenManager and the current property. All we have to do is assign a a name to the screens and change `current` property to the desire screen when the `on_press` event is triggered, like this:

    on_press: _screen_manager.current = 'screen1'

_screen_manager is just a kivy id so we can reference it (the ScreenManager)inside the Kivy Language. So, here is the code.

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout

Builder.load_string("""
<Phone>:
    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'top'
        ScreenManager:
            size_hint: 1, .9
            id: _screen_manager
            Screen:
                name: 'screen1'
                Label: 
                    markup: True
                    text: '[size=24]Welcome to [color=dd88ff]THE APP[/color][/size]'
            Screen:
                name: 'screen2'
                GridLayout: 
                    cols: 3
                    padding: 50
                    Button:
                        text: "1"
                    Button:
                        text: "2"
                    Button:
                        text: "3"
                    Button:
                        text: "4"
                    Button:
                        text: "5"
                    Button:
                        text: "6"
                    Button:
                        text: "7"
                    Button:
                        text: "8"
                    Button:
                        text: "9"
                    Button:
                        text: "*"
                    Button:
                        text: "0"
                    Button:
                        text: "#"
    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'bottom'
        BoxLayout:
            orientation: 'horizontal'
            size_hint: 1, .1
            Button:
                text: 'Go to Screen 1'
                on_press: _screen_manager.current = 'screen1'
            Button:
                text: 'Go to Screen 2'
                on_press: _screen_manager.current = 'screen2'""")

class Phone(FloatLayout):
    pass

class TestApp(App):
    def build(self):
        return Phone()

if __name__ == '__main__':
    TestApp().run()

Hope somebody found it useful.

I already posted before about the how to use transformations with Kivy. There was still some gaps in the explanation. After a while of being working with Kivy I have a much better understanding of what it is happening so I came with a list of 10 things that you should know about the Kivy canvas.

1. A Kivy canvas is not the space in which you paint. 

   The major problems I had at the beginning with the canvas were due to its name. Particularly considering all the buzz about HTML5 canvas. I initially visualize the canvas as the cloth that painters paint. Since all the widgets has one canvas, I started to get confused and asked why did the Widget seem to share the same canvas?

The answer needs, first, to clarify what is the canvas? (2nd bullet point) and where is it that the widgets share? (3rd bullet point)

2. A Kivy canvas is a set of instructions! 

   A canvas is basically a container of instructions. More like a recipe that tells you how to draw, rather than a place to draw. In other words the bitmap is not saved (like .bmp), but the vectors (like .gif). The canvas of the Widget basically keep the instructions to draw that performs the drawing.

3. The widgets share the same coordinate space, not the same canvas.

   The final answer to my previous question is that all the widgets have a different canvas but all the canvas draw in exactly the same drawing space, i.e. the coordinate space. Moreover, the drawing space is not limited to the position and size of the widget. The 0,0 of the drawing space is always (except for the RelativeLayout and the ScatterLayout ) the bottom-left corner. If we want to draw a Rectangle that covers the whole area of the Widget we should do something like this:

   ...
   Widget:
       canvas:
           Rectangle:
               pos: self.pos
               size: self.size

   Notice that we need to specify explicitly that the size and pos of the Rectangle is the same of the Widget. There lies...

4. ... the little secret of RelativeLayout.

   The previous code doesn't work with RelativeLayout. Instead we actually use the coordinate (0,0) to achieve the same result:

   ...
   Widget:
       canvas:
           Rectangle:
               pos: 0,0
               size: self.size

   The RelativeLayout includes a Translate instruction:

   Translate:
       xy: self.pos
   

   That instruction is going to translate the canvas to the self.pos position. In other words, the (0,0) is now self.pos.

5. PushMatrix and PopMatrix to control the effects of sharing the same drawing space.

   Sharing the same drawing space has its consequences. The problem is that the instructions of one Widget canvas may affect the next one. If I rotate the drawing space, the drawing space will be rotated until some other instructions rotate it back again. It is up to us to guarantee that the context of the drawing space remains the same when the canvas stop drawings. Kivy provides two instructions PushMatrix and PopMatrix. RelativeLayout uses a PushMatrix before the instructions of the canvas and a PopMatrix after it.

   PushMatrix:
   ...
   Translate:
       xy: self.pos
   ...
   PopMatrix:
   

   The PushMatrix saves the current context state (rotations, translations, zoomings) and the PopMatrix recovers it and let thing as they were before.

6. The color is an exception.

   That said, PushMatrix and PopMatrix doesn't have an effect over the Color instruction.

   PushMatrix:
   Color:
       rgb: 1,0,0
   Rectangle:
       ...
   PopMatrix:
   Line:
       ... 
   

   It doesn't matter which was the previous Color before PushMatrix, the Rectangle and and the Line would be red (rgb: 1,0,0).

7. The order in which canvas instructions are executed.

   The execution order follow a traversal order. There is recursivity involved but, let's stick with what you have to consider. When you are using a Widget canvas, the instructions of all the canvas of the parent and grandparents were executed. Also the instructions of the canvas of the first added siblings. The instructions of the canvas children hasn't been executed yet. Neither the ones of the siblings added after. When you are coding you actually perceive a normal top-down execution and thinking that way is ok for most of the cases. When you start using dynamic interfaces, adding and removing children, strange things seem to happen and you will need to remember the order of execution. Perhaps, more important is to know how to do to manipulate this order.

8. Drawing after the children with canvas.after

   What happens if we want to execute instructions after the children were added. In that case we have another canvas (called canvas.after) that is called after traverse all the children.

   Widget:
       canvas:
           ...
       canvas.after:
           ...
       Button:
           canvas.before:
              ...
           canvas:
              ...
           canvas.after:
              ...
   

   The canvas execution order is as follows:
   (1) The Widget canvas
   (2) All the Button canvas (canvas.before, canvas, canvas.after)
   (3) The Widget canvas.after
   Notice that all the canvas of the Button are executed in between canvas and canvas.after.

9. What about canvas.before?

   You probably notice that there is a third canvas (canvas.before). It executes just before the canvas. I found the canvas.before particularly useful when
   (1) I need to manipulate the Color of an instance and, (2) I need to manipulate inheritance. Let's say I have MyWidget that draws a Rectangle inside.

   <MyWidget@Widget>:
       canvas:
           Rectangle:
           ...
   

   So I can modify the Color of a particular instance doing something like this:

       ...
       mywidget = MyWidget
       mywidget.canvas.before:
           Color(rgba=(1,0,0,0))
   

   If I use canvas instead of canvas.before in RedWidget, the instruction is going to be executed after the Rectangle, so it won't apply to the Rectangle.
   (2) Similarly, when I need to manipulate inheritance. Let's say I have MyWidget that draws a Rectangle inside. I can do the following to change the color of the base class Rectangle:

   <RedWidget@MyWidget>:
       canvas.before:
           Color:
               rgb: 1,0,0
   

   The RedWidget inherits from MyWidget and I would have the same problem if I just use the canvas. Of course, similar applications goes for the canvas.after.

10. Drawing on top of a Button Widget with canvas.after.

    This works for any other Widget and not just for the Label but it is probably the most common case. If you want to draw over the Button background you can use the canvas.after. Otherwise the background will cover your drawings:

    Button:
        text:
        canvas.after:
            Rectangle:
                pos: self.pos
                size: self.size
    

    The rectangle of the previous code will actually cover its text.

This post turned out to be longer than I was expected. I hope I didn't make any mistake in my explanation. If so, please let me know.

In my previous post I explain how to create a smooth translation using the Kivy Clock and Properties. There is actually an easier way for doing this.

We can use Kivy animations. It is the simplest way. Below is the code. The interesting thing about the Animation class is that we can send any widget property as the animation parameter. For example, we can send the Label's font_size as the Animation's parameter. Moreover we can send two or more parameters.

label = Label(text = "increase and move", pos=(0,0))
animation = Animation(x=500, y=500, font_size=72px, d=2, t='in_out_quad');
animation.start(label) 

As you can see, I am sending three properties (x, y, and font_size). I added the t='in_out_quad' as a small surprise to add a transition. Just ask me if you have any question.

Here is the full example of my previous post but using animations:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.core.window import Window
from kivy.lang import Builder
from kivy.animation import Animation

Builder.load_string("""
<PongBall@Widget>:
    size: 50, 50 
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size

<PongGame>:
    ball: pong_ball

    PongBall:
        id: pong_ball
        pos: 0,0
""")


class PongGame(Widget):
    def animate(self):
        animation = Animation(pos=(Window.size), d=3)
        animation.start(self.ball)

class PongApp(App):
    def build(self):
        game = PongGame()
        game.animate()
        return game

if __name__ == '__main__':
    PongApp().run()

Somebody asked in StackOverflow how to do smooth movements of objects (widgets) in python game platforms. I didn't have the time to reply there because the question was closed because the question was posted in a way that promotes debate not answers. But here is an answer. I think. I hope. I just wanted to say that in Kivy is possible to do smooth movements, not complicated, very flexible and there is a tutorial about it: Pong Game Tutorial

This is a simplified code based of the tutorial where I just make the bool move slowly in diagonal direction from bottom left to top right. I hope it will help somebody.

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import NumericProperty, ReferenceListProperty,\
    ObjectProperty
from kivy.vector import Vector
from kivy.clock import Clock
from kivy.lang import Builder

Builder.load_string("""
<PongBall>:
    size: 50, 50 
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size

<PongGame>:
    ball: pong_ball

    PongBall:
        id: pong_ball
        pos: 0,0
""")

class PongBall(Widget):
    velocity_x = NumericProperty(1)
    velocity_y = NumericProperty(1)
    velocity = ReferenceListProperty(velocity_x, velocity_y)

    def move(self):
        self.pos = Vector(*self.velocity) + self.pos

class PongGame(Widget):
    ball = ObjectProperty(None)

    def update(self, dt):
        self.ball.move()

class PongApp(App):
    def build(self):
        game = PongGame()
        Clock.schedule_interval(game.update, 1.0 / 60.0)
        return game

if __name__ == '__main__':
    PongApp().run()

So yesterday I posted that I was puzzled by the way Kivy deal with transformations (Rotate, Translate and Scale). Especially, the  way RelativeLayout relates with all of this. Please read my previous post first: Rotate, Translate, Scale and RelativeLayout in Kivy .

I actually asked in StackOverflow and I get an insight and I think I am about to understand what is happening here.

This involves 2 Kivy basic instructions (PushMatrix and PopMatrix) that I completely ignored before. Their names are unfortunately very misleading from the way I am seeing things. Here is a what the kivy documentation indicates:

• PushMatrix: "PushMatrix on context’s matrix stack" [1]
• PopMatrix: 2Pop Matrix from context’s matrix stack onto model view" [2]

I don't think that is very helpful. This has something to see with OpenGl which I have never worked with and it is probably the source of all my misunderstandings here.

So I ll try to put things in the way I see it. I understand that Push and Pop are operations in a Stack [3]. But what are they pushing and popping? They push and pop the current context of the canvas. Let's see the example first. In my previous post I had two options to solve my transformation: undo all the transformations and use RelativeLayout. Here is a 3rd option using push and pop.

Option 3 - Using Push and Pop:

<Example>:
    Button:
        pos: [0,0]
        size_hint: [None, None]
        size: [200,200]
        canvas.after:
            PushMatrix:
            Line:
                circle: [50, 100, 50]
            Translate:
                x: 100
            Line:
                circle: [50, 100, 50]
            PopMatrix:
    Button:
        pos: [200,0]
        size_hint: [None, None]
        size: [200,200]
        background_color: [0,0,1,1]

So the PushMatrix and PopMatrix actually push and pop the original context. Moreover, this is what RelativeLayout is doing in its internal code, together with an expected translation to the position of the widget.

So what is this matrix? I resisted to believe that the matrix is a drawing space of the size of the screen. For example, a matrix of 800x600 that we are popping and pushing constantly. The Kivy documentation explains it here and I think I got the answer. It reminds me my course on linear Maths 10 years ago.

The idea is that you can save many transformations in a simple matrix and then applied to any vertex instruction (lines, polygons, etc) you were about to draw. For example, before you are going to display a Line,you apply (by mathematical matrix operators) the matrix to the points of the line. A matrix can save many transformations, I honestly don't remember if all of them but surely can rotate, translate and scale figures.

I started to see my general problem here. A Kivy canvas is not a canvas like when you paint but this will be another post.

[1]  http://kivy.org/docs/api-kivy.graphics.html?highlight=pushmatrix#kivy.graphics.PushMatrix

[2] http://kivy.org/docs/api-kivy.graphics.html?highlight=popmatrix#kivy.graphics.PopMatrix

[3] http://en.wikipedia.org/wiki/Stack_(abstract_data_type)