Develop Reference

tools:listheader doesn't work although tools:listitem does

I have a fragment layout that contains the following code:
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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"
tools:context=".BlankFragment">
<ListView
android:id="#+id/blankList"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:listheader="#layout/menu_header"
tools:listitem="#layout/fragment_item" />
</FrameLayout>
My fragment_item contains the following code:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="bottom"
android:orientation="horizontal"
android:background="#f0f0f0">
<ImageView
android:id="#+id/image_menu"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="24dp"
android:paddingBottom="3dp"/>
<TextView
android:id="#+id/content"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="#dimen/menu_article_text_size"
android:layout_marginStart="16dp"
android:textAppearance="?attr/textAppearanceListItem" />
</LinearLayout>
My menu_header contains the following code:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v7.widget.CardView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:elevation="12dp"
android:id="#+id/view2"
app:cardCornerRadius="40dp"
android:layout_centerHorizontal="true">
<ImageView
android:layout_height="80dp"
android:layout_width="match_parent"
android:id="#+id/imageView1"
android:src="#drawable/ic_launcher_background"
android:scaleType="centerCrop"/>
</android.support.v7.widget.CardView>
</LinearLayout>
My problem is that for some reason when I run it the app the fragment works, the list appears and the items of it are fine, but the header simply isn't there.
any help anyone?
* IMPORTANT FACT *
on the preview, it does work but it doesn't when I run it

My problem is that for some reason when I run it the app the fragment
works, the list appears and the items of it are fine, but the header
simply isn't there.
IMPORTANT FACT * on the preview, it does work but it doesn't when I run it
=> Looks like you have misunderstood about tools attribute. Those are design time attributes, means that it's helpful while designing layouts. Yes those are not meant for runtime and so obviously it won't be appeared in app.
To clarify it by giving you an example, android:text="Hello World and tools:text="Hello World". You should put demo/testing data in layout using tools attribute so user will not be seeing those data but it will be helpful to developers to see that there are views available in layout. In fact, they can adjust margins, paddings and all other layout design time operations!

Why is the look of the activity different in the Android Studio preview than on the device?

I created an navigation drawer activity and found that its content page looks different in the preview.
This is the preview.
And this is the AVD appearance (it looks like a real device too).
XML code is:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:context="com.xy.xy.HomePage">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="160sp"
android:layout_marginLeft="5sp"
android:layout_marginRight="5sp"
android:layout_marginTop="5sp">
<ImageView
android:id="#+id/TestMenuImage"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="#string/app_name"
app:srcCompat="#drawable/bg1"
tools:scaleType="centerCrop" />
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
tools:background="#color/hbm_text_background">
<TextView
android:id="#+id/TestMenuName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5sp"
android:contentDescription="#string/app_name"
android:text="#string/test"
android:textColor="#color/white" />
</RelativeLayout>
<Button
android:id="#+id/TestMenuButton"
style="?android:attr/buttonBarButtonStyle"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
</LinearLayout>
What do you think where the problem is?

Try adding android:scaleType="fitXY" in ImageView like this:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
app:layout_behavior="#string/appbar_scrolling_view_behavior"
tools:context="com.xy.xy.HomePage">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="160sp"
android:layout_marginLeft="5sp"
android:layout_marginRight="5sp"
android:layout_marginTop="5sp">
<ImageView
android:id="#+id/TestMenuImage"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="#string/app_name"
app:srcCompat="#drawable/bg1"
android:scaleType="fitXY"/>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
tools:background="#color/hbm_text_background">
<TextView
android:id="#+id/TestMenuName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5sp"
android:contentDescription="#string/app_name"
android:text="#string/test"
android:textColor="#color/white" />
</RelativeLayout>
<Button
android:id="#+id/TestMenuButton"
style="?android:attr/buttonBarButtonStyle"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
</LinearLayout>
That's probably a little bug with ImageViews´ previews in Android Studio,
Hope it helps

EDIT: I see you've included the style file and it does appear that your app is running the same style (AppTheme) as your preview so I think we can possibly rule out that as a culprit.
I doubt this makes a difference but I noticed your preview is running api 27 and your device is running api 27.
Try removing the
tools:context="com.xy.xy.HomePage" and see if it makes a difference.
The preview can only show you the current view. It doesn't know the parent view. The child view may have a parent view that constricts the size of the child view. Let's say that your image is set to width="match_parent". In this case, the preview may expand the image to fill the full width but at runtime your image my be placed inside a parent with a width="100dp" which would cause the image to have a width of only 100dp. Padding and margin of the parent view can also cause an issue.
If this isn't the problem then it may be a styling issue. I'd have to see more of your Theme and Style setup in order to know for sure. Your preview might be showing you a different style/theme than what you are actually using at runtime.

Output Design of a layout is not exact what is set while developing the Layout

I am trying to design a layout ( basically a splash screen ) which shows the App logo in almost middle of the screen and the company tagline under the logo on the start of the App. While working in Android Studio the design of layout looks like this. I positioned it 200dp downwards from the top.
As you can see, logo is right in the middle of the screen.
But when i run the app on my emulator the screen appears like this.The logo is not at the exact position where i wanted it.
I want my logo at the same relative position, no matter how big the screen size is.
Assume if logo is positioned at 200dp at 1000dp long screen. i want it at 400dp position on a 2000dp screen size.
Simulator : 5.0.0 API-21 768 X 1280
What are the possible solutions ??
Following image contains both outputs.
Follow this Link for the image
Here is the XML code :
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#f84343"
android:weightSum="1">
<ImageView
android:layout_width="100dp"
android:layout_height="100dp"
android:id="#+id/AppIcon"
android:src="#drawable/unnamed"
android:layout_marginTop="200dp"
android:layout_centerHorizontal="true" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/simpler_better_faster"
android:id="#+id/textView"
android:layout_gravity="center_horizontal"
android:layout_below="#+id/AppIcon"
android:layout_centerInParent="true"
android:layout_marginTop="50dp"
android:textColor="#ffffff"
android:textSize="20dp" />
</RelativeLayout>

You don't need to use a <RelativeLayout> to achieve that. Below is the code using a <LinearLayout>. The key here is android:gravity="center" in the <LinearLayout> element:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#f84343"
android:orientation="vertical"
android:gravity="center">
<ImageView
android:id="#+id/AppIcon"
android:layout_width="100dp"
android:layout_height="100dp"
android:src="#drawable/unnamed"/>
<TextView
android:id="#+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:text="#string/simpler_better_faster"
android:textColor="#ffffff"
android:textSize="20dp"/>
</LinearLayout>
[EDIT]
OP - But if i go on using more widgets to the screen i might need to
position them relative to each other. how to overcome that with
Relative layout
Well, technically you can still add more widgets in their required order. But, if you really want to use a <RelativeLayout>, you can use something like:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#f84343"
android:gravity="center_vertical">
<ImageView
android:id="#+id/AppIcon"
android:layout_width="100dp"
android:layout_height="100dp"
android:layout_centerHorizontal="true"
android:src="#drawable/unnamed"/>
<TextView
android:id="#+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_below="#id/AppIcon"
android:text="#string/simpler_better_faster"
android:textColor="#ffffff"
android:layout_centerHorizontal="true"
android:textSize="20dp"/>
</RelativeLayout>

It looks like you've mixed up some RelativeLayout/LinearLayout attributes.
If you want to use RelativeLayout remove and modify some attributes.
Something like this instead
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#f84343">
<ImageView
android:layout_width="100dp"
android:layout_height="100dp"
android:id="#+id/AppIcon"
android:src="#drawable/unnamed"
android:layout_centerInParent="true"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/simpler_better_faster"
android:layout_below="#id/AppIcon"
android:id="#+id/textView"
android:layout_marginTop="50dp"
android:textColor="#ffffff"
android:textSize="20dp" />
</RelativeLayout>

Simplify it all down into a TextView if you ask me
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#f84343" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="#string/simpler_better_faster"
android:drawableTop="#drawable/arrow_right_fill_dark" />
</RelativeLayout>
This gets rid of the ImageView all together, the change being the drawableTop attribute that provides a drawable, centered above the text.

Contact card ui not displaying as expected

I'm currently trying to build a simple Contacs Card myself. I just want a Profile picture on the top left, about a quarter to a third of the width, and the Name + description on the right side.
No matter what i try, it always looks very similar to the screenshot below.
this is my Layout:
<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="#dimen/activity_standard_padding"
android:clipToPadding="false"
android:scrollbarStyle="outsideOverlay"
style="#style/AppBaseThemeCardBG">
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
style="#style/nowCardStyle">
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:orientation="vertical"
android:gravity="top"
android:paddingRight="#dimen/activity_horizontal_margin">
<ImageView
android:id="#+id/schuckCard"
android:scaleType="fitCenter"
android:layout_gravity="top"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</LinearLayout>
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="3"
android:orientation="vertical">
<com.mikebdev.douala.widget.RobotoTextView
android:id="#+id/schuckHeader"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:typeface="roboto_condensed_light"
android:text="JESCO SCHUCK"
android:textSize="#dimen/textSizeVeryLarge" />
<com.mikebdev.douala.widget.RobotoTextView
android:id="#+id/body1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:typeface="roboto_condensed_light"
android:text="Description" />
</LinearLayout>
</LinearLayout>
</ScrollView>
This is how it looks like:
I'd like the image to be approximately as wide as the description in the action bar. Obviously i brutally failed here

Use different values for the layout weights. Like 4 for the image and 6 for the content to give it 40‰ of the cards width

Consider using cardslib ( https://github.com/gabrielemariotti/cardslib) to do your card layout. It is very well put together.

Got my stupid Problem fixed:
In the ImageView I have declared android:scaleType="fitCenter" which apparently also centers the ImageView in my Layout.
When changing the scaleType to fitStart I get the result I wanted.

android imageview takes extra black space

the imageview takes extra [black] space and i dont know why
ok this imageview should wrap content but instead..
photo explains the problem
this is the code
<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
<ImageView
android:id="#+id/map"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/map" /><ImageView
android:id="#+id/blxx"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:paddingBottom="-20dp"
android:src="#drawable/photo" />
<ImageView
android:id="#+id/blxx"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:src="#drawable/blxx" />
</LinearLayout>
</ScrollView>

Try the below:
android:adjustViewBounds="true"
This will remove the extra padded space

first possible reason for your problem:
sometimes when the image is too small, it can't scratch anymore then some limit. after that limit - what happens is what you see - blank space claimed by the imageView where additional scratch of the image was expected. I advice you to take larger picture (with bigger resolution)
Second possible reason:
add to your imageView properties on the xml file the following attribute:
android:scaleType="fitXY"

android:layout_gravity="center"
Above line is centering the image to fit inside your ImageView. Try without it. If not, set gravity to "fill" and try again.
Also remove one of the duplicates of blxx as noted.

hope this ll help you
<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:layout_gravity="center">
<ImageView
android:id="#+id/map"
android:layout_width="fill_parent"
android:layout_height="fill_content"
android:src="#drawable/map" />
</LinearLayout>
</ScrollView>