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AndroidStudio ConstrainLayout not working

I am beginner in android studio and having headache with this if someone could help me am i missing something?.... I have problem with Constraint layout, the problem is i put some element(ImageView, Button it does not mater) in layout and constraint it to parent but when I run it in emulator it just is not showing. also it is not showing me those buttons under elements connect BaseLine and remove Constrains for that element.
"screenShot":
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
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"
tools:context=".MainActivity">
<ImageView
android:id="#+id/imageView"
android:layout_width="343dp"
android:layout_height="118dp"
android:layout_marginTop="10dp"
android:scaleType="centerCrop"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_c`enter code here`onstraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
tools:srcCompat="#tools:sample/backgrounds/scenic[7]" />
</android.support.constraint.ConstraintLayout>

when i run it in emulator it just is not showing
The problem is here :
tools:srcCompat="#tools:sample/backgrounds/scenic[7]"
You need to understand what tools is, so I recommend to you to read a little bit the documentation.
Just a note of it :
It's a namespace that enable design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources). When you build your app, the build tools remove these attributes so there is no effect on your APK size or runtime behavior.
Instead you have to use to show it on the emulator.
app:srcCompat="#drawable/your_drawable"
Note: remember to add : xmlns:app="http://schemas.android.com/apk/res-auto"

I found the solution.it's difrent in new Android Studio 3.4, You have to Right click on element and option Show Base Line is there now.

Drag/Drop components are not visible in preview in android studio

I am a beginner in android development. I have created a android project. But in the preview section, I am not able to see the components. But components are visible when i run the same app on emulator i am able to see the components. I am missing some setting here? XML is given below.
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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"
tools:context=".MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

I've had have also this problem and solved it by only selecting another "Theme" cause
the default theme appTheme doesn't show added items.
If you i.e. select Transulcent, any item becomes visible ;-)
cheers
Oliwan

Maybe sound a weird answer, but self-tested this.
Change the XML file's (maybe activity_x.xml) layout's root tag android.support.constraint.ConstraintLayout to LinearLayout, then preview will start working again (as happened in my case). [If not, then perform force refresh (R)]
Then switch back to old one, I mean: Change the XML file's (maybe activity_x.xml) layout's root tag LinearLayout to android.support.constraint.ConstraintLayout. And force a refresh.

This does seem to be a bug in Android Studio. When it showed up for me, I fixed it by making a trivial change in the app build.gradle (change version code), do a gradle sync, then change back and sync again.
After that, the design view worked again.

Android ConstraintLayout generates absolute values

I have recently started learning the new ConstraintLayout in Android Studio 2.2 and noticed that when I add simplest of the views, the layout editor automatically generates some absolute coordinates. Here is a sample XML:
<android.support.constraint.ConstraintLayout 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:id="#+id/activity_portfolio"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.abc.Activity"
tools:layout_editor_absoluteX="0dp"
tools:layout_editor_absoluteY="81dp">
<TextView
android:text="#string/creator_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
tools:layout_editor_absoluteX="246dp"
tools:layout_editor_absoluteY="479dp"
android:id="#+id/first_textview"
app:layout_constraintRight_toRightOf="#+id/activity"
android:layout_marginEnd="16dp"
tools:layout_constraintRight_creator="0"
app:layout_constraintBottom_toBottomOf="#+id/activity"
android:layout_marginBottom="16dp"
tools:layout_constraintBottom_creator="0" />
</android.support.constraint.ConstraintLayout>
Notice the absolutes like 81dp, 246dp, 479dp... I tried to manually delete these, but when I go back to the "Design" tab and come back to the "Text" tab, these regenerate. Now, I have three questions:
Is there a way to tell Android Studio to not generate these?
Should I manually place them in dimens.xml?
Would these absolutes cause some layout problems in other devices?

You'll note that all of the absolute values are in the tools namespace - this means they are not compiled into your app, nor used in anything but in the tools (and in this case, the visual editor). They are simply to ensure that switching from the Design to Text tab is always consistent, with the underlying files remaining stable.
Is there a way to tell Android Studio to not generate these?
No.
Should I manually place them in dimens.xml?
These are only useful for the tools and therefore should not be added to a separate dimens.xml file that would be included in your final APK.
Would these absolutes cause some layout problems in other devices?
No, they are only used by the tools.

I'm not sure your original question contains your entire layout, as it references a widget with an id of #+id/activity, so the issue might lie elsewhere in your layout.
Ensure that no widget that exists within a ConstraintLayout has a layout_width or layout_height of match_parent.
MATCH_PARENT is not supported for widgets contained in a ConstraintLayout, though similar behavior can be defined by using MATCH_CONSTRAINT with the corresponding left/right or top/bottom constraints being set to "parent".
Source
If you use match_parent, Android Studio will generate these absolute values, as well as replacing match_parent with an absolute dimension.
Based on the layout you posted, your TextView probably had a layout_width or layout_height of match_parent before Android Studio replaced it.
You should replace android:layout_width="match_parent" with
android:layout_width="0dp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndtOf="parent"
And android:layout_height="match_parent" with
android:layout_height="0dp"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toBottomtOf="parent"
In your specific layout, you probably want something like this:
<android.support.constraint.ConstraintLayout 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:id="#+id/activity_portfolio"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.abc.Activity">
<TextView
android:text="#string/creator_name"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:id="#+id/first_textview"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintBottom_toBottomOf="#+id/activity"
android:layout_marginEnd="16dp"
android:layout_marginBottom="16dp" />
</android.support.constraint.ConstraintLayout>

As a side notes to given answers, you can use the Magic Wand Icon in the toolbar menu above the design preview. Click on Infer Constraints button, this will automatically add some lines in the text field and the tools one will be converted to constrained.
Please see below picture :

Android: Why subtitle in ListView not grey?

I use latest Android Studio and SDK. In preview & real device i see this:
My code:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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="com.myappname.view.AboutActivity">
<ListView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/listViewAbout" />
</RelativeLayout>
How i make subtitle text color is gray? Like this:

I'm going out on a limb and assume that you're using the row layout simple_list_item_2.xml (based on the screenshot) which gives you two rows. The problem, if you may call it that, is that depending on the SDK version, the styling for this layout has changed.
On SDK 23, it looks like this:
However, on say SDK 19, it looks like this:
Why?
To understand this we first need to take a look at the xml that generates the rows from simple_list_item_2.xml, you'll see it's a pretty simple layout that uses the now deprecated view TwoLineListItem but that's just a plus on why to use your custom layout.
<TwoLineListItem xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:minHeight="?attr/listPreferredItemHeight"
android:mode="twoLine"
android:paddingStart="?attr/listPreferredItemPaddingStart"
android:paddingEnd="?attr/listPreferredItemPaddingEnd">
<TextView android:id="#id/text1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:textAppearance="?attr/textAppearanceListItem" />
<TextView android:id="#id/text2"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="#id/text1"
android:layout_alignStart="#id/text1"
android:textAppearance="?attr/textAppearanceListItemSecondary" />
</TwoLineListItem>
The reason is because of the way the style textAppearanceListItemSecondary is resolved in each SDK version. The style is what gives the text the size, the color, etc. The evolution of the interface in Android has given birth to a huge ecosystem of themes and relying on the default styling will result in inconsistencies like the one you stumbled upon.
What to do about it?
You should use your own layout for this to allow for uniform styling across versions. To do so, please refer to any of the multiple questions covering this matter. But in short it just means creating a layout file, call it for example custom_row.xml and having the layout look exactly as you please. This also gives you total control over placement of the items, extra Views that you may need, and overhead in terms of coding is minimal compared to the SimpleAdapter or ArrayAdapter that perhaps you were using.
Note
You should consider moving your code towards RecyclerView instead of ListView if you haven't already.

You can set Textview property
android:textColor="#color/grey"
in you Adapter layout to change colour of your sub item
Hope this will help

Android 5.0 does not respect view's margins

I have an app that runs well in any version of android, but in Lollipop seems that all views don't respect any margin or padding.
Does anyone know why??
Thanks in advance.
I'm declaring the CardView like this:
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:id="#+id/card_view1"
android:background="#color/backgroundItem"
android:layout_width="match_parent"
android:layout_height="fill_parent"
android:layout_marginRight="2dp"
android:layout_marginLeft="2dp"
card_view:cardCornerRadius="0dp"
android:layout_weight="1">

Per the CardView documentation:
Note that, if you specify exact dimensions for the CardView, because of the shadows, its content area will be different between platforms before L and after L. By using api version specific resource values, you can avoid these changes. Alternatively, If you want CardView to add inner padding on platforms L and after as well, you can set setUseCompatPadding(boolean) to true.