maui-graphics-drawing（毛伊岛图形绘制）

.NET MAUI Graphics Drawing

Common gotchas

Canvas state — always pair Save/Restore

⚠️ Unpaired SaveState/RestoreState causes state leaks across draw calls.

// ✅ Correct — isolated state
canvas.SaveState();
canvas.StrokeColor = Colors.Red;
canvas.StrokeSize = 6;
canvas.DrawRectangle(10, 10, 80, 80);
canvas.RestoreState();
// Stroke reverts to previous values

// ❌ Wrong — state leaks to everything drawn after
canvas.StrokeColor = Colors.Red;
canvas.StrokeSize = 6;
canvas.DrawRectangle(10, 10, 80, 80);
// Every subsequent shape is now red with size 6

Saves/restores: stroke, fill, font, shadow, clip, and transforms. Nest calls for layered isolation.

Triggering redraws

// ✅ Correct — queue a redraw
graphicsView.Invalidate();

// ❌ Wrong — never call Draw() directly
myDrawable.Draw(canvas, rect);

• Invalidate() queues a redraw; the framework calls IDrawable.Draw on the next frame.
• ⚠️ Avoid calling Invalidate() in a tight loop — batch state changes, then invalidate once.

Performance tips

• Keep Draw() fast — pre-compute paths and data outside the draw method; Draw() is called on every frame.
• Reuse PathF objects — create them once, store as fields, draw repeatedly.
• Use MeasureFirstItem-style thinking — if drawing many identical items, calculate dimensions once.
• Minimize allocations — avoid new PathF() inside Draw() when the path doesn't change.

// ✅ Pre-computed path (field)
private readonly PathF _starPath = BuildStarPath();

public void Draw(ICanvas canvas, RectF dirtyRect)
{
    canvas.FillPath(_starPath);
}

// ❌ Allocating every frame
public void Draw(ICanvas canvas, RectF dirtyRect)
{
    var path = new PathF();
    // ...build path every frame...
    canvas.FillPath(path);
}

Shadows and clipping — sticky state pitfalls

Shadows apply to all subsequent draws until explicitly removed. Clips accumulate and can only be undone with RestoreState():

// ✅ Shadow: remove after use
canvas.SetShadow(new SizeF(5, 5), 4, Colors.Gray);
canvas.FillRectangle(20, 20, 100, 60);
canvas.SetShadow(SizeF.Zero, 0, null); // ← must remove

// ✅ Clip: isolate with SaveState/RestoreState
canvas.SaveState();
canvas.ClipRectangle(20, 20, 100, 100);
canvas.FillRectangle(0, 0, 200, 200); // clipped
canvas.RestoreState();                  // clip removed

// ❌ Clip persists — everything after is also clipped
canvas.ClipRectangle(20, 20, 100, 100);
canvas.FillEllipse(150, 150, 50, 50); // unintentionally clipped!

Set properties BEFORE draw calls

// ✅ Properties then draw
canvas.StrokeColor = Colors.Blue;
canvas.DrawRectangle(10, 10, 100, 50);

// ❌ Setting after draw has no effect on previous shape
canvas.DrawRectangle(10, 10, 100, 50);
canvas.StrokeColor = Colors.Blue;

Decision framework

Quick checklist

• GraphicsView has Drawable set and non-zero size
• Draw() is fast — no I/O, no heavy allocations
• Every SaveState() has a matching RestoreState()
• Shadows removed with SetShadow(SizeF.Zero, 0, null) after use
• Clips wrapped in SaveState/RestoreState blocks
• Properties set before the draw call they apply to
• Invalidate() used instead of calling Draw() directly