In android layouts what is the effect/meaning of layout_height="0dip"

I've seen several examples that use android:layout_height="0px" or "0dip" but i do not understand the impact of this. It seems that would make the layout 0 pixels tall. Is the value mitigated but some other factor like 'weight' or the height of any parent views?

Solution 1:

Yep you are right about the weight, when you want the width or height to be controlled by weight its convention to set that value to 0dip and let the weight control the actual value. Although I am pretty sure 0 is just arbitrary here you could put anything but putting 0 makes your intention more clear.

Solution 2:

When using a LinearLayout if you set the layout_weight to a non-zero value and set the layout_height (or layout_width) to 0px or 0dip then the LinearLayout distributes any unassigned space along the appropriate axis based on the weights. So for example, if you look at the layout below the View with id *gestures_overlay* it has layout_height 0dip and layout_weight 1 so the parent LinearLayout stretches it to fill the available vertical space between the 2 surrounding LinearLayouts. If there was another View with the same 0dip layout_height and a layout_weight value then they would share the vertical space based on their weight values.

<?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">

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"

        android:orientation="horizontal">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginRight="6dip"

            android:text="@string/prompt_gesture_name"
            android:textAppearance="?android:attr/textAppearanceMedium" />

        <EditText
            android:id="@+id/gesture_name"
            android:layout_width="0dip"
            android:layout_weight="1.0"
            android:layout_height="wrap_content"

            android:maxLength="40"
            android:singleLine="true" />

    </LinearLayout>

    <android.gesture.GestureOverlayView
        android:id="@+id/gestures_overlay"
        android:layout_width="fill_parent"
        android:layout_height="0dip"
        android:layout_weight="1.0"

        android:gestureStrokeType="multiple" />

    <LinearLayout
        style="@android:style/ButtonBar"

        android:layout_width="fill_parent"
        android:layout_height="wrap_content"

        android:orientation="horizontal">

        <Button
            android:id="@+id/done"

            android:layout_width="0dip"
            android:layout_height="wrap_content"
            android:layout_weight="1"

            android:enabled="false"

            android:onClick="addGesture"
            android:text="@string/button_done" />

        <Button
            android:layout_width="0dip"
            android:layout_height="wrap_content"
            android:layout_weight="1"

            android:onClick="cancelGesture"
            android:text="@string/button_discard" />

    </LinearLayout>

</LinearLayout>

Solution 3:

A relevant example from the official developer docs (http://developer.android.com/guide/topics/ui/layout/linear.html):

Equally weighted children

To create a linear layout in which each child uses the same amount of space on the screen, set the android:layout_height of each view to "0dp" (for a vertical layout) or the android:layout_width of each view to "0dp" (for a horizontal layout). Then set the android:layout_weight of each view to "1".