不使用任何自定义类或库:

Without using any custom classes or libraries:

<ImageView
    android:id="@id/img"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:adjustViewBounds="true"
    android:scaleType="fitCenter" />

scaleType="fitCenter"(省略时默认)

• 将使其尽可能宽，并根据需要保持纵横比.

scaleType="centerInside"

• 如果 src 的固有宽度小于父宽度
  将图像水平居中
• 如果 src 的固有宽度大于父级宽度
  将使其与父级允许的宽度一样宽，并按比例缩小以保持纵横比.

• if the intrinsic width of src is smaller than parent width
  will center the image horizontally
• if the intrinsic width of src is larger than parent width
  will make it as wide as the parent allows and down-scale keeping aspect ratio.

使用 android:src 或 ImageView.setImage* 方法没关系，关键可能是 adjustViewBounds.

It doesn't matter if you use android:src or ImageView.setImage* methods and the key is probably the adjustViewBounds.