android中由于图片素材尺寸和ImageView尺寸不一致，可以通过ScaleType设置不同的图片显示效果。
0、ImageView.ScaleType.MATRIX
/**
* Scale using the image matrix when drawing. The image matrix can be set using
* {@link ImageView#setImageMatrix(Matrix)}. From XML, use this syntax:
* <code>android:scaleType="matrix"</code>.
*/使用矩阵缩放图片
1、ImageView.ScaleType.FIT_XY
/**
* Scale the image using {@link Matrix.ScaleToFit#FILL}.
* From XML, use this syntax: <code>android:scaleType="fitXY"</code>.
*/图片铺满屏幕，非等比例缩放图片宽高
2、ImageView.ScaleType.FIT_START
/**
* Scale the image using {@link Matrix.ScaleToFit#START}.
* From XML, use this syntax: <code>android:scaleType="fitStart"</code>.
*/等比例缩放图像，居上或居左显示在ImageView上，缩放比例按照图片长边和ImageView长边的比值，保证显示图片全部内容
3、ImageView.ScaleType.FIT_CENTER
/**
* Scale the image using {@link Matrix.ScaleToFit#CENTER}.
* From XML, use this syntax:
* <code>android:scaleType="fitCenter"</code>.
*/ImageView的默认状态，等比例缩放图像，居中显示在ImageView上，缩放比例按照图片长边和ImageView长边的比值，保证显示图片全部内容
4、ImageView.ScaleType.FIT_END
/**
* Scale the image using {@link Matrix.ScaleToFit#END}.
* From XML, use this syntax: <code>android:scaleType="fitEnd"</code>.
*/等比例缩放图像，居下或居右显示在ImageView上，缩放比例按照图片长边和ImageView长边的比值，保证显示图片全部内容
5、ImageView.ScaleType.CENTER
/**
* Center the image in the view, but perform no scaling.
* From XML, use this syntax: <code>android:scaleType="center"</code>.
*/图片居中显示，不缩放，当图片图片尺寸超过ImageView尺寸则显示图片的中间部分内容；当图片尺寸小于ImageView尺寸则图片全部内容居中显示在图片上。
6、ImageView.ScaleType.CENTER_CROP
/**
* Scale the image uniformly (maintain the image's aspect ratio) so
* that both dimensions (width and height) of the image will be equal
* to or larger than the corresponding dimension of the view
* (minus padding). The image is then centered in the view.
* From XML, use this syntax: <code>android:scaleType="centerCrop"</code>.
*/图片居中显示，等比例缩放，使得图片长宽大于ImageView的宽高（即将图片短边缩放到与ImageView边一致，同时按照相同比例缩放长边），图片超出的部分剪裁掉。
7、ImageView.ScaleType.CENTER_INSIDE
/**
* Scale the image uniformly (maintain the image's aspect ratio) so
* that both dimensions (width and height) of the image will be equal
* to or less than the corresponding dimension of the view
* (minus padding). The image is then centered in the view.
* From XML, use this syntax: <code>android:scaleType="centerInside"</code>.
*/图片居中显示，等比例缩放，使得图片长宽小于或等于ImageView的宽高，当图片图片尺寸超过ImageView尺寸则等比例缩小图片尺寸到ImageView尺寸；当图片尺寸小于ImageView尺寸居中显示