FlexLayout

The idea is simple, use java expressions in layout params like layout_left="view1.right+10dp". It is helpful when LinearLayout and RelativeLayout is not enough for you.

<TextView
	app:layout_left="icon.right+10dp"
	app:layout_right="100%-14dp"
	app:layout_centerY="icon.centerY"
	android:layout_height="wrap_content"
	.../>

Try the sample apk: FlexLayout.apk

Adding to project

Add dependencies in your build.gradle:

	dependencies {
	    compile 'com.github.mmin18:flexlayout:1.2.7'
	}

Or if you are using Eclipse, just copy FlexLayout.java and attrs.xml to your project.

Layout Params

Horizontal	Vertical
layout_left	layout_top
layout_right	layout_bottom
layout_centerX	layout_centerY
layout_width	layout_height

Remember the app:layout_width is different from android:layout_width
xmlns:app="http://schemas.android.com/apk/res-auto"

% Percentage

<Button
	app:layout_left="10%"
	app:layout_right="90%"
	app:layout_centerY="50%"
	android:layout_height="wrap_content"
	../>

or

<Button
	app:layout_width="80%"
	app:layout_centerX="50%"
	app:layout_centerY="50%"
	android:layout_height="wrap_content"
	../>

Reference other views

Reference previous view using prev, next view using next (Position in the XML layout file)

<View ../>        // prev = Previous view in xml layout file

<View
	app:layout_left="prev.right"
	app:layout_right="next.left"
	app:layout_top="prev.top"
	app:layout_bottom="next.bottom" />

<View ../>        // next = Next view in xml layout file

Reference a specific view using view's id

<View
	app:layout_left="view1.right"
	app:layout_right="android:text1.left"
	app:layout_top="view1.top"
	app:layout_bottom="android:text1.bottom" />

<View android:id="@+id/view1"
	../>
<View android:id="@android:id/text1"
	../>

You can also use parent to reference the FlexLayout and this to reference the child view itself. Use screen to reference screen size.

Keyword	Target
prev	Previous view in XML layout
next	Next view in XML layout
view_id	<View id="@+id/view_id" /> defined in the same layout
this	The view itself
parent	The parent FlexLayout, doesn't support left top right bottom centerX centerY
screen	Screen size (getResources().getDisplayMetrics(), only support width and height)

Properties		Value
left	top
right	bottom
centerX	centerY
width	height
visible		view.getVisibility() == View.VISIBLE
gone		view.getVisibility() == View.GONE
tag		view.getTag(), only support Number or Boolean. Other types or null returns 0

(When use with view.tag, after View.setTag() you should call View.requestLayout() to trigger layout.)

Expression

The syntax is the same as Java or C. Numbers can have units like 10dp, 15sp

(parent.height-view1.centerY)/2
100%-80dp
max(view1.right, view2.right)
screen.width<screen.height ? 64dp : 48dp
view1.visible && view2.visible ? max(view1.bottom, view2.bottom) : 0px

Operators (Order in precedence)

Operator	Associativity
() sp dp dip px pt mm in	Right
!	Right
* / %	Left
+ -	Left
<= < >= >	Left
== !=	Left
&&	Left
ll	Left
?=	Right

Functions

Name
max(a,b)
min(a,b)
round(a)
ceil(a)
floor(a)
abs(a)
mod(a)
pow(a)

dimens.xml

Of course you can reference dimensions defined in res/values/dimens.xml

<View
	app:left="@dimen/default_margin"
	app:bottom="50%-@dimen/default_margin"
	app:width="2*@android:dimen/app_icon_size"
	../>

wrap_content

You can use wrap_content and match_parent as a normal value in expression, like app:layout_width="min(wrap_content, 80dp)" which is equievalent to android:maxWidth="80dp".

Using wrap_content in expression is more flexable than using android:maxWidth / android:minWidth. For example, you want to put an icon to the right of a TextView:

<TextView
	app:layout_width="min(wrap_content, 100%-next.width)"
	android:layout_height="wrap_content"
	android:text="Either short or long text"
	android:singleLine="true"
	... />
<ImageView
	android:layout_width="wrap_content"
	android:layout_height="wrap_content"
	app:layout_left="prev.right"
	android:src="@drawable/info"
	... />

Benchmark

The following benchmark is done by Piasy, and you can check the details here.

Simple Layout

	inflate (ns)	measure (ns)	layout (ns)
RelativeLayout	3325842	947464	108585
FrameLayout	3159841	879161	112988
FlexLayout	5278923	796837	111414

Complex Layout

	inflate (ns)	measure (ns)	layout (ns)
RelativeLayout	17479435	2268045	822163
GridLayout	20350271	3270156	1177185
FlexLayout	21698676	2703914	1001549

You can check the layout xml files here

FlexLayout usually takes longer to inflate, but it's equally fast in measure and layout. Normally you use less hierarchy and views than RelativeLayout or LinearLayout, so the overall time spend is competitively, especially when comes to complex layouts.

Changelog

1.2.7 (2017-8-2)

Avoid crash when layout is empty.

1.2.6 (2016-9-28)

Support Arabic RTL (layoutDirection). Simply flip everything from right to left.

1.2.5 (2016-9-25)

Allow restriction conflict like both left, right and width is defined (width will be ignored)

1.2.4 (2016-6-02)

Fix #8, TextView clipped issue with wrap_content expression.

1.2.3 (2016-4-25)

Use wrap_content and match_parent as a normal value in expression.

1.2.2 (2016-4-17)

Support AndroidStudio Preview (Fix view reference in IDE preview)

1.2.1 (2016-4-8)

Support parent.visible, parent.gone, parent.tag

1.2.0 (2016-4-6)

Show source code position in XML when throw Exceptions. (Syntax exception, Circular dependency, etc.)

1.1.0 (2016-3-20)

Initial release to jcenter. Including percentage, view reference, ?= expressions, logic operators.