Develop Reference

Android Layout - when to use app: vs android:?

I've been writing some Android apps but I don't really understand when to use app: and when to use android:. When styles are not being applied the way they're supposed to, I use trial and error and sometimes find that using app: instead of android: solves the issue but I don't understand why. It'd be great if someone could point me in the right direction. Thanks!

You can use the app namespace to have app compatibility with older API versions.
For example
app:srcCompat="#drawable/customborder" has the same effects with
android:background="#drawable/customborder"
The difference is that the first will work correctly with older API's and the second will not display what you would like.

You are talking about custom namespace.In android we can create custom views in additional to already available views.
As per in Google developer docs..
To add a built-in View to your user interface, you specify it in an XML element and control its appearance and behavior with element attributes. Well-written custom views can also be added and styled via XML. To enable this behavior in your custom view, you must:
Define custom attributes for your view in a resource element
Specify values for the attributes in your XML layout
Retrieve attribute values at runtime
Apply the retrieved attribute values to your view
Once you define the custom attributes, you can use them in layout XML files just like built-in attributes. The only difference is that your custom attributes belong to a different namespace. Instead of belonging to the http://schemas.android.com/apk/res/android namespace, they belong to http://schemas.android.com/apk/res/[your package name]
So for if you use default views you can use android namespace and if you want to set and use attributes for custom view you can define your own name.
Refer this

If you take a look at the beginning of the your layout xml files (in which you used app:) you will (probably) find lines like this:
<?xml version="1.0" encoding="utf-8"?>
<SOME_LAYOUT xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
in this case app: namespace will be used for custom attributes, specified by you inside attrs.xml file or by someone else in one of used libraries.

Sometime the property with android only available in new Android version like
In this case, you should use app:... to make it work with older version.

moreover you will find two variants
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:app="http://schemas.android.com/apk/[packagename]"
the difference between xmlns lines is res-auto take care of resolving our package as sometime we will add .debug or .test in our package and we already provided the packageid of the app Ex:
xmlns:app="http://schemas.android.com/apk/com.test.io.debug"
xmlns:app="http://schemas.android.com/apk/com.test.io.test"

Simple hard coded ListView in Android

Is it possible to do something like this in Android? (basically the equivalent of select and option in HTML):
<ListView android:layout_height="match_parent"
android:layout_width="match_parent">
<ListViewItem>TEST</ListViewItem>
<ListViewItem>TEST1</ListViewItem>
</ListView>
I have tried this, it obviously doesn't work. All I can find online is information about using adapters, but as I have some hard coded values I don't need to use an adapter - I'm happy to put it directly in the XML file.
Is this possible?

1 ] Create a String array in strings.xml
2 ] Use the following attribute inside Listview tag
android:entries="#array/your_array_name"
Done !

No It is not possible in android but used android:entries properties to load list item using String resource array.

How to set z index by using some integer values

I have been working in android for past few months. The problem for me is now related to Z index value. I am having a layout with a textview,edittext and imageview.
Suppose i have a xml file something like this..
<Layout1>
<edittext><zindex>3</zindex></edittext>
<textview><zindex>2</zindex></textview>
<imageview><zindex>1</zindex></imageview>
</Layout1>
So my question is that am reading this xml file by DOM parser and i want to set the z index value for all these by the values defined in the xml. Now is there any function or property that i can use to do it.
I have learnt about coding it with xml, but that will make it hardcoded. I want a dynamic display so how do i adjust layout with the zindex values.... HELP PLZ

there is no Z-index in android layouts. You'll need to use FrameLayout or RelativeLayout if you need to place elements on top of each other in reverse order.
see Placing/Overlapping(z-index) a view above another view in android

You can use view.setZ(float) starting from API level 21. Here you can find more info.

How to hide the horizontal line at the bottom of each item in android listview? [duplicate]

This question already has answers here:
How do I remove lines between ListViews on Android?
(14 answers)
Closed 8 years ago.
It seems that there is a horizontal line by default at the bottom of each item in android listview. My problem is: how to let the line not display

You can do using this code..
lvlist.setDivider(null);
lvlist.setDividerHeight(0);

in xml:
android:divider="#null"
android:dividerHeight="0dp"
and in java you can use this:
myList.setDivider(null);
myList.setDividerHeight(0);

Check here: How to change color of Android ListView separator line?
You can try setting the divider height to 0px.

There are different ways to achieve this, but I'm not sure which one is the best (I don't even know is there is a best way). I know at least 2 different ways to do this in a ListView:
1. Set divider to null:
1.1. Programmatically
yourListView.setDivider(null);
1.2. XML
android:divider="#null" (this goes inside your ListView element)
2. Set divider to transparent and set its height to 0 to avoid adding space between listview elements:
2.1. Programmatically:
yourListView.setDivider(new ColorDrawable(android.R.color.transparent));
yourListView.setDividerHeight(0);
2.2. XML
android:divider="#android:color/transparent"
android:dividerHeight="0dp"

Defining Z order of views of RelativeLayout in Android

I would like to define the z order of the views of a RelativeLayout in Android.
I know one way of doing this is calling bringToFront.
Is there are better way of doing this? It would be great if I could define the z order in the layout xml.

The easiest way is simply to pay attention to the order in which the Views are added to your XML file. Lower down in the file means higher up in the Z-axis.
Edit:
This is documented here and here on the Android developer site. (Thanks #flightplanner)

If you want to do this in code
you can do
View.bringToFront();
see docs

Please note, buttons and other elements in API 21 and greater have a high elevation, and therefore ignore the xml order of elements regardless of parent layout. Took me a while to figure that one out.

In Android starting from API level 21, items in the layout file get their Z order both from how they are ordered within the file, as described in correct answer, and from their elevation, a higher elevation value means the item gets a higher Z order.
This can sometimes cause problems, especially with buttons that often appear on top of items that according to the order of the XML should be below them in Z order. To fix this just set the android:elevation of the the items in your layout XML to match the Z order you want to achieve.
I you set an elevation of an element in the layout it will start to cast a shadow. If you don't want this effect you can remove the shadow with code like so:
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
myView.setOutlineProvider(null);
}
I haven't found any way to remove the shadow of a elevated view through the layout xml.

I encountered the same issues: In a relative layout parentView, I have 2 children childView1 and childView2. At first, I put childView1 above childView2 and I want childView1 to be on top of childView2. Changing the order of children views did not solve the problem for me. What worked for me is to set android:clipChildren="false" on parentView and in the code I set:
childView1.bringToFront();
parentView.invalidate();

Please note that you can use view.setZ(float) starting from API level 21. Here you can find more info.

Thought I'd add an answer since the introduction of the
android:translationZ
XML field changed things a tad. The other answers that suggest running
childView1.bringToFront();
parentView.invalidate();
are totally spot on EXCEPT for that this code will NOT bring childView1 in front of any view with a hardcoded android:translationZ in the XML file. I was having problems with this, and once I removed this field from the other views, bringToFront() worked just fine.

API 21 has view.setElevation(float) build-in
Use ViewCompat.setElevation(view, float); for backward compatibility
More methods ViewCompat.setZ(v, pixels) and ViewCompat.setTranslationZ(v, pixels)
Another way collect buttons or view array and use addView to add to RelativeLayout

childView.bringToFront() didn't work for me, so I set the Z translation of the least recently added item (the one that was overlaying all other children) to a negative value like so:
lastView.setTranslationZ(-10);
see https://developer.android.com/reference/android/view/View.html#setTranslationZ(float) for more

Or put the overlapping button or views inside a FrameLayout. Then, the RelativeLayout in the xml file will respect the order of child layouts as they added.

You can use custom RelativeLayout with redefined
protected int getChildDrawingOrder (int childCount, int i)
Be aware - this method takes param i as "which view should I draw i'th".
This is how ViewPager works. It sets custom drawing order in conjuction with PageTransformer.

Check if you have any elevation on one of the Views in XML. If so, add elevation to the other item or remove the elevation to solve the issue. From there, it's the order of the views that dictates what comes above the other.

You can use below code sample also for achieving the same
ViewCompat.setElevation(sourceView, ViewCompat.getElevation(mCardView)+1);
This is backward compatible.
Here mCardView is a view which should be below sourceView.