why marginBottom not working?

marginBottom has no effect if you set android:layout_height="wrap_content" for <RelativeLayout>, instead set it as match_parent and check.

It is realy just a bug of the <RelativeLayout>, so you could try to wrap the RelativeLayout inside of a <LinearLayout> and set the margin there.

I am not sure in which revision of android you are experiencing this issue. I looked at RelativeLayout in Android 4.2.2 (https://android.googlesource.com/platform/frameworks/base/+/android-4.2.2_r1.1/core/java/android/widget/RelativeLayout.java) and I believe there is a bug in the onMeasure.

In lines 486 to 488, we have following code:

if (isWrapContentHeight) {
    height = Math.max(height, params.mBottom);
}

In my opinion, the above should read:

if (isWrapContentHeight) {
    height = Math.max(height, params.mBottom + params.bottomMargin);
}

With android:layout_height="wrap_content", RelativeLayout does not appear take into account the bottomMargin of the last vertically laid out view.

I suggest you try one of the following:

*) Add a dummy view that will be the very last vertically laid out view, and ensure that it has the android:layout_below attribute. Note that the dummy view does not have a bottom margin, it is there so that RelativeLayout considers the bottom margin of views laid out above it. For your case, the dummy view would look like this:

<View
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_below="@id/messageSender"
/>

*) As others have mentioned, achieve the same effect in other ways, such as Padding.

*) A not-so-trivial solution is bring in RelativeLayout, its platform style definitions and Pool management classes into your project, perform the bug fix I mentioned above and use this anywhere you would normally use RelativeLayout.

*) File a bug with Google

Hope this helps.

Maybe you can set android:paddingBottom="" of <RelativeLayout> to get the same effect.

Make sure the layout_height of your root container is set to match_parent instead of wrap_content.