# Canvas

> 2D drawing with shapes, paths, text and mouse events. A native Pyzed component for Python desktop GUIs, with a live preview, code examples and its API.

Source: https://albakore.github.io/pyzed/library/canvas/

:::tip[In plain words]
A blank sheet to draw shapes, lines and text on, for charts and games.
:::

A 2D drawing surface. `shapes` is a list made with [`pyzed.draw`](https://albakore.github.io/pyzed/reference/draw/) (rectangles, circles, arcs, paths, text, images…). Mouse callbacks receive coordinates relative to the canvas; with an `id` the geometry is cached between frames. The charts of this library are drawn with it.

`pz.canvas` · Category: [Content](https://albakore.github.io/pyzed/library/#content) · [Full reference →](https://albakore.github.io/pyzed/reference/widgets/#canvas)

**Code:**

```python
pz.canvas([
    draw.rect(10, 10, 120, 80, fill="#74ade8", radius=8),
    draw.circle(200, 50, 40, fill="#a1c181"),
    draw.arc(300, 50, 40, 0, 270, stroke="#e5c07b", width=8, cap="round"),
    draw.polygon([(360, 90), (400, 10), (440, 90)], fill="#d07277"),
    draw.line(10, 120, 440, 120, stroke="#5d636f", width=2, dash=[6, 4]),
    draw.text(225, 150, "pz.canvas + pyzed.draw", color="#a9afbc", size=16, align_x="center"),
], width=450, height=170)
```

## When to use

- For charts, games, diagrams, drawing apps and custom widgets.

## Usage

```python
import pyzed as pz
from pyzed import draw

pz.canvas([
    draw.rect(10, 10, 120, 80, fill="#74ade8", radius=8),
    draw.circle(200, 50, 40, fill="#a1c181"),
    draw.arc(300, 50, 40, 0, 270, stroke="#e5c07b", width=8, cap="round"),
    draw.polygon([(360, 90), (400, 10), (440, 90)], fill="#d07277"),
    draw.line(10, 120, 440, 120, stroke="#5d636f", width=2, dash=[6, 4]),
    draw.text(225, 150, "pz.canvas + pyzed.draw", color="#a9afbc", size=16, align_x="center"),
], width=450, height=170)
```

## Variants

:::note[About these captures]
Each capture is recorded by pyzed itself, off-screen at 2x, in One Dark or One Light to match this site's theme, and cropped to the component so it shows at its real size. The pointer hovers, presses or types into it; a component that does not react to the pointer cycles through a few themes. With reduced motion you see the first frame. Open **Code** under a capture to see the snippet that draws it; free names such as `on_save` stand for your own callbacks.
:::

### Shapes

```python
pz.canvas([draw.rect(10, 10, 140, 70, fill="#74ade8", radius=10)], width=160, height=90)
```

```python
pz.canvas([draw.circle(50, 45, 36, fill="#a1c181")], width=100, height=90)
```

```python
pz.canvas([draw.arc(50, 45, 34, 0, 270, stroke="#e5c07b", width=8, cap="round")], width=100, height=90)
```

```python
pz.canvas([draw.polygon([(10, 80), (50, 10), (90, 80)], fill="#d07277")], width=100, height=90)
```

```python
pz.canvas([draw.line(10, 45, 190, 45, stroke="#c678dd", width=3, dash=[8, 5])], width=200, height=90)
```

```python
pz.canvas([draw.text(100, 45, "pyzed.draw", color="#74ade8", size=20, align_x="center",
                     align_y="center")], width=200, height=90)
```

## Events

Canvas has **5 events of its own**, listed below. Like every widget it also has the **18 common events** (`click`, `double_click`, `right_click`, `mouse_enter`, `drag`, `key_down`, `resize`…), attached with `.on(...)`: see [Events](https://albakore.github.io/pyzed/guides/events/) and [Events of every component](https://albakore.github.io/pyzed/library/events/).

| Event | Handler gets | When |
| --- | --- | --- |
| `on_press` | `(x, y)` | Callable that receives `(x, y)` on press. |
| `on_release` | `(x, y)` | Callable that receives `(x, y)` on release. |
| `on_right_press` | `(x, y)` | Callable that receives `(x, y)` on right click. |
| `on_move` | `(x, y)` | Called with `(x, y)` when the mouse moves. |
| `on_scroll` | `(dx, dy)` | Callable that receives `(dx, dy)` in lines when the wheel is used. |

**Handler gets** is what Pyzed passes to your callable: `()` means nothing. Any handler can also be a message (a value that is not callable), which goes to `update()`.

### Handling the events

The same events written three ways. **Imperative**: methods of the app change `self`. **Declarative**: the handlers produce messages and one `update()` decides. **Functional**: a pure function draws an immutable state and asks for changes with `set_state(...)`. See [Imperative, declarative and functional](https://albakore.github.io/pyzed/guides/code-styles/).

**Imperative:**

```python
class Paint(pz.App):
    def __init__(self):
        self.dots, self.drawing = [], False

    def press(self, x, y):               # on_press / on_move get the position
        self.drawing = True
        self.dots.append((x, y))

    def move(self, x, y):
        if self.drawing:
            self.dots.append((x, y))

    def view(self):
        return pz.canvas([draw.circle(x, y, 3, fill="#74ade8") for x, y in self.dots],
                         width=420, height=200, cursor="crosshair",
                         on_press=self.press, on_move=self.move,
                         on_release=lambda x, y: setattr(self, "drawing", False),
                         on_right_press=lambda x, y: self.dots.clear())
```

**Declarative:**

```python
from dataclasses import dataclass

@dataclass
class Dot:
    x: float
    y: float

class Paint(pz.App):
    dots = []

    def update(self, message):
        match message:
            case Dot(x, y):
                self.dots = self.dots + [(x, y)]
            case "clear":
                self.dots = []

    def view(self):
        return pz.canvas([draw.circle(x, y, 3, fill="#74ade8") for x, y in self.dots],
                         width=420, height=200, on_press=Dot, on_right_press=lambda x, y: "clear")
```

**Functional:**

```python
from dataclasses import dataclass

@dataclass(frozen=True)
class State:
    dots: tuple = ()
    zoom: float = 1.0

def view(state, set_state):
    shapes = [draw.circle(x, y, 3 * state.zoom, fill="#74ade8") for x, y in state.dots]
    return pz.canvas(shapes, width=420, height=200,
                     on_press=lambda x, y: set_state(dots=state.dots + ((x, y),)),
                     on_scroll=lambda dx, dy: set_state(zoom=max(0.5, state.zoom + dy * 0.1)))
```

## Properties and API

### `pz.canvas`

```python
canvas(
    shapes=None,
    *,
    width=None,
    height=None,
    id=None,
    on_press=None,
    on_release=None,
    on_right_press=None,
    on_move=None,
    on_scroll=None,
    cursor=None,
)
```

A 2D drawing surface. `shapes` is a list created with [`pyzed.draw`](https://albakore.github.io/pyzed/reference/draw/). Mouse callbacks receive coordinates relative to the canvas. With an `id`, the geometry is cached between frames and only recomputed when the shapes change. See the [drawing with canvas](https://albakore.github.io/pyzed/guides/canvas/) guide.

| Property | Default | Description |
| --- | --- | --- |
| `shapes` | `None` | List of `pyzed.draw` shapes (nested lists are accepted). |
| `width` | `None` | Width: pixels, `"fill"`, `"shrink"` or `"fill:N"` (proportion). |
| `height` | `None` | Height: pixels, `"fill"`, `"shrink"` or `"fill:N"` (proportion). |
| `id` | `None` | Enables the geometry cache. |
| `cursor` | `None` | Mouse cursor (`"crosshair"`, `"pointer"`…). |

Events: `on_press`, `on_release`, `on_right_press`, `on_move`, `on_scroll` ([events](#events)).

[Full reference →](https://albakore.github.io/pyzed/reference/widgets/#canvas)

## Related

- [Bar chart](https://albakore.github.io/pyzed/library/bar-chart/): A vertical bar chart with labels.
- [Donut](https://albakore.github.io/pyzed/library/donut/): A donut chart with a label in the middle.
- [Stack](https://albakore.github.io/pyzed/library/stack/): Layers children on top of each other; `pin` places one at x, y.