An answer at the end of this question has been filled out, combining remarks and solutions.
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>
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
android:layout_height
android:layout_width
android:layout_weight
When a LinearLayout
is vertical, then the layout_weight
will effect the height of the child View
s (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 View
s (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 View
s 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 ListView
s, 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.
Equal distribution. 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).
match_parent means filling whats left where as 0dp means match constraints . In ConstraintLayout, match_parent is not supported. Save this answer.
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?
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