Why is 0dp considered a performance enhancement?

问题

An answer at the end of this question has been filled out, combining remarks and solutions.

Question

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

For example, I have a ListView that was suggested to be changed

Before

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

After

<ListView
    android:id="@android:id/list"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1">
</ListView>

Similarly, it suggested changes to a ListView item. These all look the same before and after the changes, but I'm interested in understanding why these are performance boosters.

Anyone have an explanation of why? If it helps, here is general layout with the ListView.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <ImageView
        android:id="@+id/logo_splash"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    </ImageView>

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:orientation="vertical"
        android:background="@color/background"
        android:layout_below="@id/logo_splash">

        <ListView
            android:id="@android:id/list"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1">
        </ListView>

        <TextView
            android:id="@android:id/empty"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/no_upcoming" />

    </LinearLayout>        
</RelativeLayout>

Answer

I'm putting in an answer here because it's really a combination of answers and referenced links below. If I'm wrong on something, do let me know.

From What is the trick with 0dip layout_height or layouth_width?

There are 3 general layout attributes that work with width and height

1. android:layout_height
2. android:layout_width
3. android:layout_weight

When a LinearLayout is vertical, then the layout_weight will effect the height of the child Views (ListView). Setting the layout_height to 0dp will cause this attribute to be ignored.

Example

<LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">
    <ListView
        android:id="@android:id/list"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1">
    </ListView>
</LinearLayout>

When a LinearLayout is horizontal, then the layout_weight will effect the width of the child Views (ListView). Setting the layout_width to 0dp will cause this attribute to be ignored.

Example

<LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal">
    <ListView
        android:id="@android:id/list"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1">
    </ListView>
</LinearLayout>

The reason to want to ignore the attribute is that if you didn't ignore it, it would be used to calculate the layout which uses more CPU time.

Additionally this prevents any confusion over what the layout should look like when using a combination of the three attributes. This is highlighted by @android developer in an answer below.

Also, Android Lint and Eclipse both say to use 0dip. From that answer below, you can use 0dip, 0dp, 0px, etc since a zero size is the same in any of the units.

Avoid wrap_content on ListView

From Layout_width of a ListView

If you've ever wondered why getView(...) is called so many times like I have, it turns out to be related to wrap_content.

Using wrap_content like I was using above will cause all child Views to be measured which will cause further CPU time. This measurement will cause your getView(...) to be called. I've now tested this and the number of times getView(...) is called is reduced dramatically.

When I was using wrap_content on two ListViews, getView(...) was called 3 times for each row on one ListView and 4 times for each row on the other.

Changing this to the recommended 0dp, getView(...) was called only once for each row. This is quite an improvement, but has more to do with avoiding wrap_content on a ListView than it does the 0dp.

However the suggestion of 0dp does substantially improve performance because of this.

回答1:

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".

回答2:

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?

回答3:

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.

回答4:

LinearLayout measures all the children according to the layout_width/layout_height values, then divides up the leftover space (which may be negative) according to the layout_weight values.

0dp is more efficient than wrap_content in this case because it's more efficient to just use zero for the original height and then split the parent's full height based on the weight than to measure the child first and then split the remainder based on the weight.

So the efficiency comes from not measuring the child. 0dp should be exactly as efficient (and produce exactly the same result) as match_parent, or 42px, or any other fixed number.

回答5:

Caution re using android:layout_height="0dp"

I have found that in a ListView (with the recommended View recycling using convertView, see e.g. http://lucasr.org/2012/04/05/performance-tips-for-androids-listview/), setting android:layout_height="0dp" for the row TextView can lead to text truncation for multi-line text content.

Whenever a TextView object that was previously used to display a text that fitted in a single line is recycled to display a longer text that needs more than one line, that text is truncated to a single line.

The problem is cured by using android:layout_height="wrap_content"

来源：https://stackoverflow.com/questions/12016781/why-is-0dp-considered-a-performance-enhancement