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How do I make android layout margin auto-adjust to the length of the page

So in my activity_page.xml, I have this originally android:layout_marginBottom="75dp" in my bottom-most linear layout. It works for most pages, but in some cases where the article is too long, some of the elements disappear and I have to change the size to at 100dp or 125dp for the elements to appear again, but in the cases where the article is shorter, the extra whitespace becomes obnoxious. Is there a way to automatically adjust the size?

Use scroll view.
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:id="#+id/content_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal">
</LinearLayout>
</ScrollView>

Why does the orientation of a linearlayout determine the centering of a child element?

Why doesn't this center a button both horizontally and vertically on the screen?
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
android:background="#FFFFFFFF">
<Button android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#color/red"
android:textColor="#color/white"
android:text="click"
android:layout_gravity="center"/>
</LinearLayout>
layout_gravity specifies alignment for the button within it's parent. But this only centers the button vertically, not horizontally. If I change the orientation of the linearlayout to vertical, the button is centered horizontally, not vertically. I don't see why the orientation matters here because I only have 1 child element.
I know I can achieve this by specifying the gravity in the LinearLayout with android:gravity="true" or using a RelativeLayout and have the Button android:centerInParent="true", but I'd like to know how android came up with the layout in the code above.
P.S. Why does the background color still show as gray if that's the hex code for white?

LinearLayout will only allocate the minimum amount of space needed for a view in the direction of its orientation. That's why you can't seem to center a view in the same direction as the orientation. LinearLayout generally assumes that you want to put multiple things adjacent to each other, not occupy an entire space unconditionally for a single item.
P.S. I see the entire background of the LinearLayout as white in my preview view in Android Studio, so I don't know what you mean in your P.S.

Don't use a linear layout to display items in the middle of the screen, as these are meant to list items in a row. Use a relative layout instead. So your code should look like this:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal"
android:background="#FFFFFF">
<Button android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#color/red"
android:textColor="#color/white"
android:text="click"
android:layout_centerInParent="true"/>
</RelativeLayout>

ScrollView goes on top of another layout Android

In the following code I've got two main containers, a RelativeLayout and a ScrollView. I need the RelativeLayout to be on top fixed and below that I need the ScrollView with Images to be scrollable. I've got the following two questions:
Though my ScrollView is scrollable but it goes on top of the RelativeLayout. !important
The view of my images present within the vertical LinearLayout are thumbnail sized. If I have 15-20 images, they take a lot of vertical space. How do I have the images to fit the maximum in a row based on the screen size and then go to the next row?
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
</LinearLayout>
</RelativeLayout>
<ScrollView
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical">
<ImageView>
</ImageView>
<ImageView>
</ImageView>
<ImageView>
</ImageView>
<ImageView>
</ImageView>
</LinearLayout>
</ScrollView>

Follow these in simple way:
You need to use a fixed height for the parent, i.e., <RelativeLayout>.
Set the height of the ScrollView to match_parent.
Make sure you put everything inside a LinearLayout.
Finally you would end up with this:
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
</RelativeLayout>
</LinearLayout>

First, you can set your RelativeLayout's height to a fixed value. Then set your ScrollView's height to match_parent, so it takes up all the available space. Then both of these should be contained in a vertical LinearLayout, to avoid any overlapping.

Vertical Layout, 2 Elements, Make the Top One Fill the Remaining Space (Bottom One Has wrap_content)

I have the following dilemma. Two elements, one on top of each other. The bottom has wrap_content for height. I want the top element to fill the remaining space without the two elements overlapping (which happens if I use FrameLayout for example).
The width is match_parent for all elements.
Here's a mockup (some attributes truncated for clarity):
<LinearLayout android:layout_height="match_parent">
<CustomView android:layout_height="???????" />
<TextView android:layout_height="wrap_content">
</LinearLayout>

<CustomView
android:layout_height="0dp"
android:layout_weight="1"
.../>

Getting Views to fit

I have a main.xml with a LinearLayout with three items inside, LinearLayout, ListView and LinearLayout. I would like the ListView to be as big as possible while keeping the bottom LinearLayout always showing and not shrunk or knocked off the screen.
I have tried making the ListView height fillparent, but then the bottom LinearLayout doesn't show up, same with wrapcontent, since the ListView has lots of content.
When I make the ListView a set size, say 300dp, I have space below the bottom LinearLayout. I tried to fix this by making the top level LinearLayout gravity=fill, but that didn't help.
Also, depending on the android I try it on, the bottom LinearLayout will drop off the screen, or get shrunk.
In case it's relevant, the top level LinearLayout is set to fillparent for height.
My goal is to keep the top and bottom LinearLayout to wrap their content, and the middle ListView fill what's left... any suggestions?
Thanks in advance for your efforts!

I believe you can just add android:layout_weight="1" to the ListView, with the ListView set to a height of fill_parent, and the two LinearLayouts set to wrap_content. Regardless, I usually prefer to use a RelativeLayout. You can specify the header to align to the top of the screen, the footer to align to the bottom, and the ListView to fill the space in between, like so:
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<LinearLayout
android:id="#+id/header"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
>
//...insert stuff here
</LinearLayout>
<LinearLayout
android:id="#+id/footer"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
>
//...insert stuff here
</LinearLayout>
<ListView
android:id="#+id/listview"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_above="#id/footer"
android:layout_below="#id/header"
/>
</RelativeLayout>