ShapeDrawable

ShapeDrawable 用于绘制各种形状，使用 Shape 类来绘图。

Shape 类只负责 drawable 的形状和大小，颜色(材质 Shader) 以及 bounds 仍由 ShapeDrawable 本身管理。

Shape 类的主要方法和属性:

public abstract class Shape implements Cloneable {
    private float mWidth;
    private float mHeight;
   public abstract void draw(Canvas canvas, Paint paint);
    public final void resize(float width, float height) {
      //...
    }
    protected void onResize(float width, float height) {}
}

保存高宽，外部访问 resize，内部回调 onResize。

实现类 RectShape :

ublic class RectShape extends Shape {
    private RectF mRect = new RectF();

    @Override
    public void draw(Canvas canvas, Paint paint) {
        canvas.drawRect(mRect, paint);
    }

    @Override
    protected void onResize(float width, float height) {
        mRect.set(0, 0, width, height);
    }

    protected final RectF rect() {
        return mRect;
    }
}

实现类 OvalShape:

public class OvalShape extends RectShape {
    @Override
    public void draw(Canvas canvas, Paint paint) {
        canvas.drawOval(rect(), paint);
    }
}

其他 Shape 实现类也是类似的。

ShapeDrawable 通过 onDraw 函数把绘图的任务交给了 Shape 类：

@Override
    public void draw(Canvas canvas) {
        final Rect r = getBounds();
        final ShapeState state = mShapeState;
        final Paint paint = state.mPaint;

        final int prevAlpha = paint.getAlpha();
        paint.setAlpha(modulateAlpha(prevAlpha, state.mAlpha));

        // only draw shape if it may affect output
        if (paint.getAlpha() != 0 || paint.getXfermode() != null || paint.hasShadowLayer()) {
            final boolean clearColorFilter;
            if (mTintFilter != null && paint.getColorFilter() == null) {
                paint.setColorFilter(mTintFilter);
                clearColorFilter = true;
            } else {
                clearColorFilter = false;
            }

            if (state.mShape != null) {
                // need the save both for the translate, and for the (unknown)
                // Shape
                final int count = canvas.save();
                canvas.translate(r.left, r.top);
                onDraw(state.mShape, canvas, paint);
                canvas.restoreToCount(count);
            } else {
                canvas.drawRect(r, paint);
            }

            if (clearColorFilter) {
                paint.setColorFilter(null);
            }
        }

        // restore
        paint.setAlpha(prevAlpha);
    }

  protected void onDraw(Shape shape, Canvas canvas, Paint paint) {
        shape.draw(canvas, paint);
    }

注意这段代码:

canvas.translate(r.left, r.top);  
onDraw(state.mShape, canvas, paint);  
canvas.restoreToCount(count);

在绘画前，先平移了一段距离，因为 Shape 默认是在 (0,0) 位置的。

    @Override
    protected void onResize(float width, float height) {
        mRect.set(0, 0, width, height);
    }

大小变化触发过程：

View 重画， view.drawBackground，setBackgoundBounds，shapeDrawable.onBoundsChange，updateShape，

shape.resize，onResize，shapeDrawable.draw，onDraw，shape.draw。