Why is 0dp considered a performance enhancement?

First of all you have this,

<ListView    android:id="@android:id/list"    android:layout_width="match_parent"    android:layout_height="wrap_content"    android:layout_weight="1"></ListView>

Never take the ListView's height as wrap_content, that will lead into troubles. Here is the reason for that and this answer.

Further more,

I searched around but haven't found anything that really explains why Android Lint as well as some Eclipse hints suggests replacing some layout_height and layout_width values with 0dp.

Its because you are using layout_weight = "1" that means your ListView with take the height as much as is available to it. So, in that case there is no need of using layout_height = "wrap_content" just change it to android:layout_height="0dp" and ListView's height will be managed by layout_weight = "1".

So when android:layout_weight is used on View X and LinearLayout is horizontal, then X's android:layout_width is simply ignored.

Similar, when android:layout_weight is used on View X and LinearLayout is vertical, then X's android:layout_height is ignored.

This actually means, that you can put anything in those ignored fields: 0dp or fill_parent or wrap_content. It doesn't matter. But it's recommended to use 0dp so View's do not do extra calculation of their height or width (which is then ignored). This small trick simply saves CPU cycles.

from :

What is the trick with 0dip layout_height or layouth_width?

as far as i know , there is a difference between using 0dp (or 0px btw, it's the same since 0 is 0 no matter what is the unit here ) and the wrap_content or fill_parent (or match_parent, it's the same).

it depends on the weight you use . if you only use weight of 1 , they all look the same , but the meaning is always different , and it is important for performance.

in order to show this , try the following:

<LinearLayout 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:orientation="vertical">  <TextView android:id="@+id/textView1" android:layout_width="match_parent"    android:layout_height="0px" android:text="1" android:background="#ffff0000"    android:layout_weight="1" android:gravity="center"    android:textColor="#ffffffff" android:textSize="20sp" />  <TextView android:id="@+id/textView2" android:layout_width="match_parent"    android:layout_height="0px" android:text="2" android:background="#ff00ff00"    android:layout_weight="2" android:gravity="center"    android:textColor="#ffffffff" android:textSize="20sp" />  <TextView android:id="@+id/textView3" android:layout_width="match_parent"    android:layout_height="0px" android:text="3" android:background="#ff0000ff"    android:layout_weight="3" android:gravity="center"    android:textColor="#ffffffff" android:textSize="20sp" /></LinearLayout>

and then try to replace the 0px with match_parent . you will see that the result is very different.

usually , for both better understanding and for better performance , you would want to use 0px.