What is android:weightSum in android, and how does

Question

I want to know: What is android:weightSum and layout weight, and how do they work?

Answer 1

The documentation says it best and includes an example, (highlighting mine).

android:weightSum

Defines the maximum weight sum. If unspecified, the sum is computed by adding the layout_weight of all of the children. This can be used for instance to give a single child 50% of the total available space by giving it a layout_weight of 0.5 and setting the weightSum to 1.0.

So to correct superM's example, suppose you have a LinearLayout with horizontal orientation that contains two ImageViews and a TextView with. You define the TextView to have a fixed size, and you'd like the two ImageViews to take up the remaining space equally.

To accomplish this, you would apply layout_weight 1 to each ImageView, none on the TextView, and a weightSum of 2.0 on the LinearLayout.

Answer 2

Per documentation, android:weightSum defines the maximum weight sum, and is calculated as the sum of the layout_weight of all the children if not specified explicitly.

Let's consider an example with a LinearLayout with horizontal orientation and 3 ImageViews inside it. Now we want these ImageViews always to take equal space. To acheive this, you can set the layout_weight of each ImageView to 1 and the weightSum will be calculated to be equal to 3 as shown in the comment.

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    <!-- android:weightSum="3" -->
    android:orientation="horizontal"
    android:layout_gravity="center">

   <ImageView
       android:layout_height="wrap_content"       
       android:layout_weight="1"
       android:layout_width="0dp"/>
  .....

weightSum is useful for having the layout rendered correctly for any device, which will not happen if you set width and height directly.

Answer 3

One thing which seems like no one else mentioned: let's say you have a vertical LinearLayout, so in order for the weights in layout/element/view inside it to work 100% properly - all of them must have layout_height property (which must exist in your xml file) set to 0dp. Seems like any other value would mess things up in some cases.

Answer 4

Layout Weight works like a ratio. For example, if there is a vertical layout and there are two items(such as buttons or textviews), one having layout weight 2 and the other having layout weight 3 respectively. Then the 1st item will occupy 2 out of 5 portion of the screen/layout and the other one 3 out of 5 portion. Here 5 is the weight sum. i.e. Weight sum divides the whole layout into defined portions. And Layout Weight defines how much portion does the particular item occupies out of the total Weight Sum pre-defined. Weight sum can be manually declared as well. Buttons, textviews, edittexts etc all are organized using weightsum and layout weight when using linear layouts for UI design.

Answer 5

Adding on to superM's and Jeff's answer,

If there are 2 views in the LinearLayout, the first with a layout_weight of 1, the second with a layout_weight of 2 and no weightSum is specified, by default, the weightSum is calculated to be 3 (sum of the weights of the children) and the first view takes 1/3 of the space while the second takes 2/3.

However, if we were to specify the weightSum as 5, the first would take 1/5th of the space while the second would take 2/5th. So a total of 3/5th of the space would be occupied by the layout keeping the rest empty.

Answer 6

If unspecified, the sum is computed by adding the layout_weight of all of the children. This can be used for instance to give a single child 50% of the total available space by giving it a layout_weight of 0.5 and setting the weightSum to 1.0. Must be a floating point value, such as "1.2"

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/main_rel"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="horizontal"
        android:weightSum="2.0" >

        <RelativeLayout
            android:id="@+id/child_one"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_weight="1.0"
            android:background="#0000FF" >
        </RelativeLayout>

        <RelativeLayout
            android:id="@+id/child_two"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_weight="1.0"
            android:background="#00FF00" >
        </RelativeLayout>

    </LinearLayout>