Custom painters (CustomPaint) in Flutter - UI Render Trace

Component - Custom painters (CustomPaint)

The CustomPaint widget lets you draw shapes, lines, and designs directly on the screen using a CustomPainter. It is like having a blank canvas where you can paint anything you want, giving you full control over the look of your app.

Widget Tree

Scaffold
├─ AppBar
│  └─ Text
└─ Center
   └─ CustomPaint
      └─ SizedBox

The screen has a Scaffold with an AppBar showing a title text. The body centers a CustomPaint widget, which contains a SizedBox to define the drawing area size. The CustomPaint uses a CustomPainter to draw on this canvas.

Render Trace - 4 Steps

Step 1: Scaffold

Step 2: Center

Step 3: CustomPaint

Step 4: CustomPainter.paint()

State Change - Re-render

Trigger:User taps a button to change circle color

Before

Circle color is blue with red border

→

After

Circle color changes to green with red border

Re-renders:CustomPaint widget and its CustomPainter repaint the canvas

UI Quiz - 3 Questions

What does the CustomPaint widget provide in this UI?

A A button to trigger actions

B A blank canvas area to draw custom shapes

C A text input field

D A list of items

Key Insight

CustomPaint with a CustomPainter gives you full freedom to draw anything on the screen. It is perfect for creating unique graphics, charts, or animations that standard widgets cannot provide. Remember to keep the painting code efficient to keep your app smooth.