从源码角度分析 ImageView的ScaleType属性

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1.关注重点

• 图像能否完全显示（是否裁剪了）
• 视图有无空白（视图是否被图像充满）
• 图像是否变形（图像宽高比是否变化）

2.结论

CENTER_CROP、FIT_CENTER、FIT_START、FIT_END 均先做相同的scale（按比例缩放，使得缩放后，图像的长宽之一与视图相同，另一边比视图大），区别在于之后平移变换——FIT_CENTER 与 CENTER_CROP 相同，把此时比视图大的图像的中间部分显示到视图上；FIT_START把图像的左上角显示到视图上；FIT_END把图像的右下角显示到视图上。

2.源码分析

mDrawMatrix是关键。它决定 drawable 绘制的位置（相对于视图坐标系）与大小。

    private void configureBounds() {
        if (mDrawable == null || !mHaveFrame) {
            return;
        }
        int dwidth = mDrawableWidth;
        int dheight = mDrawableHeight;
        int vwidth = getWidth() - mPaddingLeft - mPaddingRight;
        int vheight = getHeight() - mPaddingTop - mPaddingBottom;
        boolean fits = (dwidth < 0 || vwidth == dwidth) &&
                       (dheight < 0 || vheight == dheight);
        if (dwidth <= 0 || dheight <= 0 || ScaleType.FIT_XY == mScaleType) {
            /* If the drawable has no intrinsic size, or we're told to
                scaletofit, then we just fill our entire view.
            */
            mDrawable.setBounds(0, 0, vwidth, vheight);
            mDrawMatrix = null;
        } else {
            // We need to do the scaling ourself, so have the drawable
            // use its native size.
            mDrawable.setBounds(0, 0, dwidth, dheight);
            if (ScaleType.MATRIX == mScaleType) {
                // Use the specified matrix as-is.
                if (mMatrix.isIdentity()) {
                    mDrawMatrix = null;
                } else {
                    mDrawMatrix = mMatrix;
                }
            } else if (fits) {
                // The bitmap fits exactly, no transform needed.
                mDrawMatrix = null;
            } else if (ScaleType.CENTER == mScaleType) {
                // Center bitmap in view, no scaling.
                mDrawMatrix = mMatrix;
                mDrawMatrix.setTranslate((int) ((vwidth - dwidth) * 0.5f + 0.5f),
                                         (int) ((vheight - dheight) * 0.5f + 0.5f));
            } else if (ScaleType.CENTER_CROP == mScaleType) {
                mDrawMatrix = mMatrix;
                float scale;
                float dx = 0, dy = 0;
                if (dwidth * vheight > vwidth * dheight) {
                    scale = (float) vheight / (float) dheight; 
                    dx = (vwidth - dwidth * scale) * 0.5f;
                } else {
                    scale = (float) vwidth / (float) dwidth;
                    dy = (vheight - dheight * scale) * 0.5f;
                }
                mDrawMatrix.setScale(scale, scale);
                mDrawMatrix.postTranslate((int) (dx + 0.5f), (int) (dy + 0.5f));
            } else if (ScaleType.CENTER_INSIDE == mScaleType) {
                mDrawMatrix = mMatrix;
                float scale;
                float dx;
                float dy;
                if (dwidth <= vwidth && dheight <= vheight) {
                    scale = 1.0f;
                } else {
                    scale = Math.min((float) vwidth / (float) dwidth,
                            (float) vheight / (float) dheight);
                }
                dx = (int) ((vwidth - dwidth * scale) * 0.5f + 0.5f);
                dy = (int) ((vheight - dheight * scale) * 0.5f + 0.5f);
                mDrawMatrix.setScale(scale, scale);
                mDrawMatrix.postTranslate(dx, dy);
            } else {
                // Generate the required transform.
                mTempSrc.set(0, 0, dwidth, dheight);
                mTempDst.set(0, 0, vwidth, vheight);
                mDrawMatrix = mMatrix;
                mDrawMatrix.setRectToRect(mTempSrc, mTempDst, scaleTypeToScaleToFit(mScaleType));
            }
        }
    }