Constructing View Layouts - TechGeekD/android_guides GitHub Wiki
Overview
View Layouts are a type of View class whose primary purpose is to organize and position other view controls. These layout classes (LinearLayout, RelativeLayout, etc. ) are used to display child controls, such as text controls or buttons on the screen.
Android activities (screens) use layouts as a container for view controls, and layouts can actually contain other nested layouts as well. Nearly all Android activities have layout containers similar to the way that most HTML documents use "divs" to contain other content.
There are a few very commonly used layouts and then many more specialized layouts that are used in only very particular cases. The bread and butter layouts are LinearLayout, RelativeLayout, and FrameLayout.
It's important to note the class hierarchy of these View Layouts. Each of them subclass ViewGroup
, which itself subclasses View
. That means it is perfectly legal to pass a layout such as LinearLayout
as an argument for something that takes View
as a parameter. ViewGroup
also contains the nested static class LayoutParams
which is used for creating or editing layouts in code. Keep in mind that each subclass of ViewGroup
, such as LinearLayout
, has its own nested static class LayoutParams
that's a subclass of ViewGroup.LayoutParams
. When creating View Layouts in code, beginners will often confuse the many different available LayoutParams
classes and run into hard to catch problems.
LinearLayout
In a linear layout, like the name suggests, all the elements are displayed in a single direction either horizontally or vertically and this behavior is specified in android:orientation
which is an attribute of the node LinearLayout
.
All children of a LinearLayout are displayed sequentially based on the order they are defined within the layout. A LinearLayout respects margins between children and the gravity (right, center, or left alignment) of each child.
Common view attributes you might see used in a LinearLayout:
android:gravity
- Controls the alignment of the view content (akin to text-align in CSS)android:layout_gravity
- Controls the alignment of the view within it's parent container (akin to float in CSS)android:layout_weight
- Specifies how much of the extra space in the layout to be allocated to a view.
Example of LinearLayout snippet:
<?xml version="1.0" encoding="utf-8"?>
<!-- Parent linear layout with vertical orientation -->
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView android:layout_width="match_parent" android:layout_height="wrap_content"
android:text="Email:" android:padding="5dp"/>
<EditText android:layout_width="match_parent" android:layout_height="wrap_content"
android:layout_marginBottom="10dp"/>
</LinearLayout>
If you want to setup a part of your layout, such that, for instance, 3 buttons appear in a row, occupying equal space (or if, for instance, you want to give 4/5 space to a map and 1/5 to another component below it), LinearLayout can be used to do the trick by leveraging android:layout_weight
:
...
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:layout_alignParentBottom="true">
<ImageButton
android:id="@+id/btnLocEnable"
android:src="@drawable/ic_location"
android:layout_width="0dp"
android:layout_weight="1"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:background="@color/twitter_light_blue"
/>
<ImageButton
android:id="@+id/btnUploadPhoto"
android:src="@drawable/ic_addimage"
android:layout_width="0dp"
android:layout_weight="1"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:background="@color/twitter_light_blue"/>
</LinearLayout>
...
But be careful in using nestedLinearLayout and layout_weight from a layout performance point of view!
RelativeLayout
In a relative layout every element arranges itself relative to other elements or a parent element. RelativeLayout positions views based on a number of directional attributes:
- Position based on siblings: layout_above, layout_below, layout_toLeftOf, layout_toRightOf
- Position based on parent: layout_alignParentTop, layout_alignParentBottom, layout_alignParentLeft, layout_alignParentRight, android:layout_centerHorizontal, android:layout_centerVertical
- Alignment based on siblings: layout_alignTop, layout_alignBottom, layout_alignLeft, layout_alignRight, layout_alignBaseline
An example of a RelativeLayout:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<TextView android:id="@+id/label" android:layout_width="match_parent"
android:layout_height="wrap_content" android:text="Email" />
<EditText android:id="@+id/inputEmail" android:layout_width="match_parent"
android:layout_height="wrap_content" android:layout_below="@id/label" />
<Button android:id="@+id/btnLogin" android:layout_width="wrap_content"
android:layout_height="wrap_content" android:layout_below="@id/inputEmail"
android:layout_alignParentLeft="true" android:layout_marginRight="5dp"
android:text="Login" />
</RelativeLayout>
Read this RelativeLayout tutorial for a more detailed overview. You can also see more about this layout by inspecting the RelativeLayout.LayoutParams docs and the official RelativeLayout guide.
Using Alignment to Control Width or Height
A less understood aspect of RelativeLayout is how the use of alignment can determine width or height. It may seem counterintuitive at first about how this works, so we'll walkthrough a few examples in this section. Using this approach is especially useful in matching the widths or heights relative to other elements.
Example 1: How alignment can determine width
Suppose we have two buttons of varying widths:
The corresponding XML would be:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="1245678901234567890"
android:id="@+id/button1"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="CANCEL IT"
android:id="@+id/button2"
android:layout_below="@id/button1"
android:layout_alignLeft="@id/button1" />
</RelativeLayout>
Suppose we also specify that the second button should be aligned left and right to the first button. If we add android:layout_alignRight="@id/button1"
to the second button's XML style, the change causes the second button to expand the width to match that of the first button. In other words, the only way to meet the requirements of specifying alignment on both sides is to expand the width of the second button.
In this way, when two elements are vertically positioned above or below the other, left and right alignments will control the width. When two elements are positioned horizontally next to each other, top and bottom alignments will control the height. We'll show how height can be impacted by specifying top and bottom alignments in the next example.
Example 2: How alignment can determine height
Suppose we have this layout definition:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:height="200dp"
android:text="1245678901234567890"
android:id="@+id/button1"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="CANCEL IT"
android:id="@+id/button2"
android:layout_toRightOf="@id/button1" />
</RelativeLayout>
The corresponding preview looks like:
If we wish to match the height of the first button, we can specify layout_alignTop
and layout_alignBottom
on the second button.
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignTop="@id/button1"
android:layout_alignBottom="@id/button1"
android:text="CANCEL IT"
android:id="@+id/button2"
android:layout_toRightOf="@id/button1" />
The only way to fulfill this requirement is to expand the height of the second button as shown below:
PercentRelativeLayout
PercentRelativeLayout
, a recent addition to the Design Support Library, enables the ability to specify not only elements relative to each other but also the total percentage of available space. In the past, in order to position two elements next to each other with equal height, you would normally have to create a LinearLayout
within a RelativeLayout
. PercentRelativeLayout
helps solves this issue.
To use, follow the setup guide and make sure the Gradle dependency is included.
The layout_width
and layout_height
of the PercentRelativeLayout should determine the total width and height that can be used. Any elements contained within it should specify the width and height possible using layout_heightPercent
and/or layout_widthPercent
. Because this library is not part of the standard Android library, note that a custom attribute app
namespace being used.
An example of a layout used to describe the image above is shown below (taken from this sample code):
<android.support.percent.PercentRelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<View
android:id="@+id/top_left"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_alignParentTop="true"
android:background="#ff44aacc"
app:layout_heightPercent="20%"
app:layout_widthPercent="70%" />
<View
android:id="@+id/top_right"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_alignParentTop="true"
android:layout_toRightOf="@+id/top_left"
android:background="#ffe40000"
app:layout_heightPercent="20%"
app:layout_widthPercent="30%" />
<View
android:id="@+id/bottom"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_below="@+id/top_left"
android:background="#ff00ff22"
app:layout_heightPercent="80%" />
</android.support.percent.PercentRelativeLayout>
Margin Percentages
The margins can also be set to a percentage of the total widths as well:
app:layout_marginStartPercent
app:layout_marginEndPercent
app:layout_marginTopPercent
app:layout_marginBottomPercent
We can also define app:layout_marginPercent
that will be to all four values above.
Aspect Ratio
Similar to how ImageView's adjustViewBounds:true
can be used to scale the image according to its aspect ratio, we can also use PercentRelativeLayout
to define an aspect ratio for a layout. If one dimension is set to 0dp
and no percent scaling is associated with it, setting a percentage on the app:layout_aspectRatio
attribute can scale the other to meet the ratio:
<android.support.percent.PercentRelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:background="#ff00ff22"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- not using aspectRatio here -->
<View
android:id="@+id/view1"
android:background="#ff44aacc"
android:layout_width="300dp"
android:layout_height="wrap_content"
app:layout_heightPercent="50%"/>
<!-- using aspectRatio here -->
<View
android:layout_below="@id/view1"
android:background="#ffe40000"
android:layout_width="300dp"
android:layout_height="0dp"
app:layout_aspectRatio="160%"/>
</android.support.percent.PercentRelativeLayout>
The resulting layout appears as follows:
FrameLayout
In a frame layout, the children are displayed with a z-index in the order of how they appear. Put simply, the last child added to a FrameLayout
will be drawn on top of all the previous children. Think of it like a stack of items, the item last put on the stack will be drawn on top of the items below it. This layout makes it very easy to draw on top of other layouts, especially for tasks such as button placement.
To arrange the children inside of a FrameLayout
use the android:layout_gravity
attribute along with whatever android:padding
and android:margin
you need.
Example of FrameLayout snippet:
<FrameLayout
android:id="@+id/frame_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- Child1 is drawn first -->
<ImageView
android:id="@+id/child1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="Image"
android:src="@drawable/icon" />
<!-- Child2 is drawn over Child1 -->
<TextView
android:id="@+id/child2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Child 2"
android:layout_gravity="top|left" />
<!-- Child3 is drawn over Child1 and Child2 -->
<TextView
android:id="@+id/child3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Child 3"
android:layout_gravity="top|right" />
</FrameLayout>
In this example, an ImageView
is set to the full size of the FrameLayout
. We then draw two TextView
's over it.
Optimizing Layout Performance
To optimize layout performance, minimize the number of instantiated layouts and especially minimize deep nested layouts whenever possible. This is why you should generally use a RelativeLayout
whenever possible instead of nested LinearLayout
. Review the following references for more detail on optimizing your view hierarchy:
References
- http://developer.android.com/guide/topics/ui/declaring-layout.html
- http://developer.android.com/guide/topics/ui/layout/linear.html
- http://developer.android.com/guide/topics/ui/layout/relative.html
- http://www.learn-android.com/2010/01/05/android-layout-tutorial/
- http://mobile.tutsplus.com/tutorials/android/android-layout/
- http://www.androidhive.info/2011/07/android-layouts-linear-layout-relative-layout-and-table-layout/
- http://logc.at/2011/10/18/when-to-use-linearlayout-vs-relativelayout/
- http://developer.android.com/reference/android/widget/FrameLayout.html
- https://plus.google.com/+AndroidDevelopers/posts/C8oaLunpEEj