android Canvas(2D)详解

</pre><p></p><p></p><pre name="code" class="java"> private Bitmap drawSubBitmap(int drawText,Drawable mIcon) {

        final Bitmap thumb = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        final Canvas canvas = new Canvas();
        canvas.setBitmap(thumb);
        Rect r = new Rect(0, 0, width, height);
        mIcon.setBounds(r);
        mIcon.draw(canvas);
        int x = (int) ((width - textWidth) * 0.5);
        FontMetrics fm = mPaint.getFontMetrics();
        int y = (int) ((height - (fm.descent - fm.ascent)) * 0.5 - fm.ascent);
        canvas.drawText(endstr, x, y - mOffsetY, mPaint);
        return thumb;
    }

上面函数的功能是在一个Drawable 的右上角 画上一个数字，然后返回一个bitmap，注意这个Canva是new 出来的。

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当我们通过 View.setScale() setAlpha()等方法 或者 调用scrollTo的时候，该方法内部会执行invalidate(),最终会执行View.onDraw()方法。

View.invalidate----->ViewParent:invalidateChild---------->ViewGroup:invalidateChild

------->ViewGroup:dispatchDraw

--->ViewGroup:drawChild(根据View的scale alpha left top等属性操作canvas[平移 缩放 旋转],重新绘制View)

---->View.draw(Canvas canvas)

View.draw(Canvas canvas)代码截取：

if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            // Step 6, draw decorations (scrollbars)
            onDrawScrollBars(canvas);

            // we're done...
            return;
        }

View绘制的三种方式：

1.设置View属性，执行draw

protected void dispatchDraw(Canvas canvas) {
canvas.save()
canvas.clipRect(mScrollX, mScrollY, mScrollX + mRight - mLeft, mScrollY + mBottom - mTop);	
canvas.translate(offset, 0);
  v.setTranslationX(0);
  v.setAlpha(1.0f);
  v.setRotationY(0);
  drawChild(canvas, getPageAt(childCount - 1), drawingTime);
 canvas.translate(-offset, 0);
canvas.restore();


}

2.直接操作canvas，执行draw

canvas.save()
canvas.clipRect(mScrollX, mScrollY, mScrollX + mRight - mLeft, mScrollY + mBottom - mTop);	
canvas.translate(offset, 0);
  
canvas.rotate(degrees)
canvas.scale(sx, sy)

  drawChild(canvas, getPageAt(childCount - 1), drawingTime);
 canvas.translate(-offset, 0);
canvas.restore();

3.构建Matrix 或者  Transformation（不是setTranslationX ），作用于cavas，执行draw

 Matrix matrix = new Matrix();
            matrix .setRotate(10);
            canvas.concat(matrix);
            drawChild(canvas, getPageAt(childCount - 1), drawingTime);