Android Canvas

绘图4大基本要素：（参考sdk Canvas.java的描述）

A Bitmap to hold the pixels

A Canvas to host the draw calls ( writing into the bitmap)

A Drawing prinitive, e.g. Rect, Path, Bitmap

A Paint to describe the colors and styles of the drawing

那么， canvas提供的绘图操作有哪些呢？

1. 绘制Bitmap：drawBitmap、drawPicture
2. 绘制颜色：drawColor、drawARGB
3. 绘制基本形状：drawPoint、drawLine、drawCircle、drawArc、drawRect、drawRoundRect、drawPath
4. 绘制剪切区：clipPath、clipRect、clipRegion
5. 变换Canvas：save、restore、translate、scale、rotate、concat(Matrix matrix)、setMatrix(Matrix matrix)
6. 绘制顶点数据：drawVertices、drawBitmapMesh