HTML5画布中的保存（）和Restore（）方法的目的是什么？

save()和restore()用于保存和恢复Canvas绘图状态，1. save()将当前状态（如样式、变换、透明度等）压入栈中；2. restore()将最近保存的状态弹出并恢复，避免样式污染；3. 适用于隔离旋转、透明度等临时变化，确保后续绘制不受影响；4. 支持嵌套使用，遵循后进先出原则；5. 应成对使用，以确保绘图环境的清洁与可控，从而安全实现复杂图形的独立绘制。

The save() and restore() methods in the HTML5 Canvas API are used to manage the canvas drawing state — they help you preserve and revert settings so you can make temporary changes without affecting the rest of your drawing.

Why Use save() and restore()?

When you're drawing on a canvas, you often change properties like fill color, stroke style, line width, transparency (via globalAlpha), or transformations (like rotation, scaling, or translation). These changes apply globally to all subsequent drawing operations. If you're working on complex drawings with multiple elements that need different styles or transformations, it's easy for one part to interfere with another.

That’s where save() and restore() come in.

How They Work

• save()
  Pushes a copy of the current canvas drawing state onto a stack. This includes:

  • Current transformation (scale, rotate, translate)
  • Clipping region
  • Global alpha (transparency)
  • Compositing settings
  • Styles (fillStyle, strokeStyle, lineWidth, etc.)

• restore()
  Pops the most recently saved state from the stack and restores all those settings. This effectively undoes any changes made since the last save().

  

Practical Use Case

Imagine you're drawing a shape that's rotated and semi-transparent, but you don't want those changes to affect other drawings:

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

ctx.fillStyle = 'blue';
ctx.fillRect(10, 10, 100, 100); // Draw a blue square

ctx.save(); // Save current state (blue fill, no rotation)

ctx.translate(200, 50);        // Move origin
ctx.rotate(Math.PI / 4);       // Rotate 45 degrees
ctx.fillStyle = 'red';
ctx.globalAlpha = 0.5;
ctx.fillRect(0, 0, 100, 100);  // Draw rotated, semi-transparent red square

ctx.restore(); // Restore: back to blue fill, no rotation, full opacity

ctx.fillRect(150, 150, 100, 100); // This will be blue and upright

Without restore(), the red color, rotation, and transparency would affect the final rectangle.

Key Benefits

• Prevents style leakage – keeps drawing operations isolated.
• Simplifies transformations – lets you rotate or scale just one element without manually reversing the transform.
• Supports nested states – you can call save() multiple times and restore() in reverse order (last in, first out).

Best Practices

• Always pair save() and restore() — think of them like opening and closing parentheses.
• Use them around complex or temporary drawing operations, especially when using transforms.
• Don’t rely on them for performance — they do add a small overhead, so use judiciously.

Basically, save() and restore() are like bookmarks for your canvas settings — they let you experiment or apply temporary changes safely.

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