自定义 Canvas 渲染上下文文档

概述

set_custom_canvas 是 pretext_py 库提供的一个全局配置函数，允许您替换默认的 Canvas 渲染上下文实现。通过传入一个符合 CanvasRenderingContext2D 接口的类实例，您可以自定义文本测量、图形绘制等行为，从而适配不同的渲染后端或测试环境。

函数签名

set_custom_canvas(canvas_class: type) -> None

参数

参数	类型	说明
`canvas_class`	`type`	一个类（或类实例），必须实现 `measureText` 方法，并可选实现其他 Canvas 方法。

要求：

传入的类（或对象）至少应包含 measureText(text: str) -> dict 方法，返回一个包含 'width' 键的字典（或可索引对象），表示文本宽度。
如果您的实现还包含 font 属性，可在 measureText 中使用以计算正确的宽度。

返回值

None（该函数仅为全局配置，无返回值）。

示例

基础示例：自定义文本测量

from pretext_py import set_custom_canvas

# 假设有一个测量宽度的辅助函数
def measureWidth(text: str, font: str) -> float:
    # 自定义测量逻辑，例如基于字体和字符宽度表
    return len(text) * 8.0 # 简化示例

class TestCanvasRenderingContext2D:
    def __init__(self):
        self.font = '' # 可存储当前字体

    def measureText(self, text: str):
        # 必须返回包含 'width' 键的字典
        return {'width': measureWidth(text, self.font)}

# 设置自定义 canvas
set_custom_canvas(TestCanvasRenderingContext2D)

# 之后，所有使用 Canvas 的渲染操作都会调用此自定义类

重置默认 Canvas

若要恢复默认的 Canvas 实现，可传入 None 或重新调用不带参数（具体视库实现而定，通常允许传入 None）：

set_custom_canvas(None) # 恢复内置 Canvas

注意事项

接口兼容性：您的自定义类必须实现 measureText 方法，否则在需要测量文本宽度时会抛出 AttributeError。
返回值格式：measureText 返回的字典必须包含 'width' 键，其值为数值类型（int 或 float）。缺少该键会导致运行时错误。

Custom Canvas Rendering Context Documentation

Overview

set_custom_canvas is a global configuration function provided by the pretext_py library, allowing you to replace the default Canvas rendering context implementation. By passing a class that conforms to the CanvasRenderingContext2D interface, you can customize text measurement, drawing, etc., to adapt to different rendering backends or test environments.

Function Signature

set_custom_canvas(canvas_class: type) -> None

Parameters

Parameter	Type	Description
`canvas_class`	`type`	A class (or an instance) that must implement the `measureText` method, and optionally other Canvas methods.

Requirements:

The passed class (or object) must at least have a measureText(text: str) -> dict method, returning a dictionary (or indexable object) containing a 'width' key representing the text width.
If your implementation also includes a font attribute, you can use it inside measureText to compute accurate widths.
Other Canvas methods (such as fillText, strokeText, etc.) can be implemented as needed, but currently only measureText is required.

Return Value

None (this function is a global configuration, no return value).

Examples

Basic Example: Custom Text Measurement

from pretext_py import set_custom_canvas

# Assume a helper function for width measurement
def measureWidth(text: str, font: str) -> float:
    # Custom logic, e.g., based on font and character width table
    return len(text) * 8.0 # simplified example

class TestCanvasRenderingContext2D:
    def __init__(self):
        self.font = '' # can store current font

    def measureText(self, text: str):
        # Must return a dict with key 'width'
        return {'width': measureWidth(text, self.font)}

# Set custom canvas
set_custom_canvas(TestCanvasRenderingContext2D)

# From now on, all Canvas rendering operations will use this custom class

Restoring Default Canvas

To revert to the default Canvas implementation, pass None (or call without an argument, depending on library implementation, usually None is accepted):

set_custom_canvas(None) # Restore built-in Canvas

Important Notes

Interface Compatibility: Your custom class must implement measureText; otherwise, an AttributeError will be raised when text width measurement is needed.
Return Format: The dictionary returned by measureText must contain a 'width' key with a numeric value (int or float). Missing this key will cause runtime errors.

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