Develop Reference

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>

how to align Second Item with respect to parent in LinerLayout in android?

I am having problem with LinearLayout. I have two items in LinearLayout ..one is Linearlayout with wrapped width and another is simple textview...My wrapped width LinearLayout get width from Activity ..and it is working good..But I have a problem with textview which is second item ..It get start from my wrapped LinearLayout, whatever width my wrapped Layout get, textview start from it.I want to make that textview align in center with respect to Parent Linearlayout.Here is my xml ..`
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:background="#drawable/graph_panel_bg_one"
android:orientation="horizontal" >
<LinearLayout
android:id="#+id/looser_left_jab_force_panel"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:background="#drawable/force_panel_gradient" >
</LinearLayout>
<TextView
android:id="#+id/looser_left_jab_force"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text=""
android:textColor="#F18B86"
android:textSize="10sp" />
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:background="#drawable/graph_panel_bg_one" >
<LinearLayout
android:id="#+id/looser_left_jab_speed_panel"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:background="#drawable/speed_panel_gradient" >
</LinearLayout>
<TextView
android:id="#+id/looser_left_jab_speed"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:text="12222"
android:textColor="#CEE2E9"
android:textSize="10sp" />
</LinearLayout>
</LinearLayout>`

LinearLayout is a view group that aligns all children in a single direction, vertically or horizontally. You can specify the layout direction with the android:orientation attribute.
All children of a LinearLayout are stacked one after the other, so a vertical list will only have one child per row, no matter how wide they are, and a horizontal list will only be one row high (the height of the tallest child, plus padding). A LinearLayout respects margins between children and the gravity (right, center, or left alignment) of each child.
It get start from my wrapped LinearLayout, whatever width my wrapped Layout get
This is because your parent linear layout "android:orientation="horizontal", If you set "android:orientation="vertical" it may get start from your wrapped LinearLayout, whatever height your wrapped Layout gets.
It is probably hard to achieve the way you want through the LinearLayout.
I too suggest to go with Relative Layout. It's helps to align the child the way you want.
Note: Take proper actions of your textview which possible overlaps with the LinearLayout.

Set your TextView width to "wrap_content"

android:gravity="top" doesn't align child view to top, but "center" aligns to center?

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:gravity="top" <!--this line-->
android:orientation="horizontal" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:textSize="56dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:textSize="36dp" />
</LinearLayout>
Shouldn't that make the smaller "0" t-view align to the top of the inner LinearLayout?
Since the other "0" is larger in height, it increases the overall height of the inner LinearLayout.
Alternatively, if you included this:
android:layout_gravity="top"
within the t-view of the smaller 0, it also does nothing.
Why is this?
Does the wrap_content of the LinearLayout wrap each individual view independent of others?
If so, then why does setting gravity to "center" work? In the sense that the smaller zero is vertically centered to its parent.
I know you can just set the smaller 0's height to match parent and set its own gravity to top. I'm just trying to understand this. Thanks.

The default behavior of the layout is to align the baselines of the text, which in English are at the bottom of the view. Adding android:baselineAligned="false" to your LinearLayout will align the second TextView at the top of the view.

gravity attribute applies to widgets within the LinearLayout, while layout_gravity tells the parent of LinearLayout where to place the child (LinearLayout).
Also, in a vertical LinearLayout the gravity="top" attribute won't work.
In the layout you have now, both text views will be stacked one on top of the other, wrapped with a linear layout with no space in-between - so the "top" or "center" values won't do anything because there is no extra space to move the text views up or down.
If you want to understand this better, try giving your linear layout and both text views backgrounds of different colors, like this:
android:background="#color/mycolor"
Then you will see the bounds of each widget.

You can use android:layout_weight="10" and distribute on the other components.

About LinearLayout and how controls resize

Today I have been playing a bit with the LinearLayout and have been suprised with the results:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<TextView
android:id="#+id/textView1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Big Text"
android:gravity="center_vertical|center_horizontal"
android:textAppearance="?android:attr/textAppearanceLarge" />
<TextView
android:id="#+id/textView2"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="Medium Text"
android:textAppearance="?android:attr/textAppearanceMedium" />
<Button
android:id="#+id/button1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:text="Button"
/>
</LinearLayout>
This is a simple layout with a text view header, then a text view that I want it to cover all parent (but the space occupied by the bottom button) and a button that is placed at the botton side with the layout_gravity="bottom".
This produces a layout where header is shown correctly, center text view covers all remaining free space and the button does not appear. Why is this? Shouldn't the center text view just calculate its size taking into account the bottom button size?.

Use layout_weight="1" in your center TextView.

Always remember thumb rule
If you are using linear layout with vertical orientation as soon as it finds the control
with
android:layout_height="match_parent"
The layout will ignore all the controls present below it.
Hope this help
Vipul

In place of this
android:layout_width="match_parent"
android:layout_height="match_parent"
use 'wrap_content' like this
android:layout_width="wrap_content"/"fill_parent"
android:layout_height="wrap_content"/"fill_parent"

Shouldn't the center text view just calculate its size taking into account the bottom button size?.
No, because you tell the second TextView to match its parent's height, thus FILL_PARENT and hence it will fill up all remaining space, leaving none for the last TextView.
(...) and a button that is placed at the botton side with the layout_gravity="bottom".
Unfortunately, that's not how a LinearLayout works. If you set the orientation to vertical, basically only the left and right gravities will have effect. Vice versa, with the (default) horizontal orientation, only top and bottom work. The orientation determines in which direction the View children are dynamically positioned in order, which implies you cannot manually change the 'position' in that direction.
Now, to get the desired effect, you can give the second TextView a height of 0dp and a weight of 1, resulting in it dynamically filling up all remaining space without pushing the third TextView off the bottom. Alternatively, you can use a RelativeLayout, with which you can directly set the position, and simply instruct the middle TextView to sit below the first, but above the last.

Android - fill_parent not behaving as expected in RelativeLayout

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/frameLayout1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#drawable/background_gradient" >
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center" >
<ImageButton
android:id="#+id/buttonLog"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#drawable/log"
android:onClick="log" />
</RelativeLayout>
</FrameLayout>
I was expecting my button to appear in the center of the screen. However, it appears on the TOP center of the screen (that is, the button is center horizontally, but not vertically).
It seems to me that the RelativeLayout is behaving like it was defined with "wrap_content" instead of "fill_parent".
The funny thing is that, if I give an actual value to my RelativeLayout height property (android:layout_height), like:
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="100dp"
android:gravity="center" >
Then the button behaves correctly (i.e. the button is also centred vertically). But I don`t want to use actual values. I want to use fill_parent! Why doesn't it work with "fill_parent" ??
Does anybody know what's going on?
Thank you in advance!

RelativeLayout requires you to specify the position of the elements in the Layout. I don't see any layout_below or layout_toLeftOf tags. Gravity works on LinearLayouts. In general, LinearLayouts are easier to work with, and they scale much better to different screen sizes. I suggest you replace the RelativeLayout by a LinearLayout, and also the FrameLayout by a LinearLayout. You use a FrameLayout typically if you want to use multiple overlapping layouts, which you don't do.
I recommend you read up on using layouts in the Android sdk reference documentation, like here: http://bit.ly/djmnn7

You specified fill_parent for both the layout_width and layout_height of your RelativeLayout, therefore it fills up it's parent view.
By default, a relative layout arranges it's children to the top-left corner, regardless you use fill_parent for the size.
You should achieve the desired aspect by taking advantage of the RelativeLayout's own attribute set, which helps you arrange the child views relatively to each other or to their parent:
<ImageButton
android:id="#+id/buttonLog"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:background="#drawable/log"
android:onClick="log" />
Using the android:layout_centerInParent you can achieve this. This attribute if set true, centers this child horizontally and vertically within its parent.