Develop Reference

alignment arrows do not appear in relative layout of my android-studio

i'm new to android and android studio and i'm trying to build a relative layout.
when i choose a relative layout and try to put a for example button inside the layout it sticks to the top left corner of the screen, and the arrows for aligning the element do not appear on the screen and all choosen elements stick together at the top left corner of the screen.
how do you think i can fix the problem??

Considering the picture, you cannot "align" elements in design editor using RelativeLayout.
Look, you are probably talking about ConstraintLayout.
With ConstraintLayout you can align elements and constrain elements where you need.
https://developer.android.com/training/constraint-layout Check out this tutorial.
And also, currently not many people use RelativeLayout as it a little bit complex, has more performance issues than ConstraintLayout.

Split activity with adjustable children height

I am trying to create an activity layout with two parts, where their respective size is dynamic. The activity should have two states:
Starting state:
The user scrolls the bottom part up to the second state (and back):
It's important that the change will be animated. I tried a few solutions so far but didn't manage to find the exact way to do that:
AndroidSlidingUpPanel - The panel acts as another layout, covering the upper view and moving the toolbar out of the screen.
Android Split Pane Layout - Correct behavior, but the splitter is dragged and not the bottom part (I don't want to have a visible splitter).
CoordinatorLayout with CollapsingToolbarLayout - I didn't find a way to limit the upper part from totally disappearing. Anyway I think that it's a bit abusing because I don't want to collapse a toolbar but just change the children's height with animation.
Is there a good way to implement this using another library, one of these that I perhaps didn't use correctly or with simple layouts? Thanks!

I found a custom version of the SlidingPaneLayout created by VLC, which is basically a vertical SlidingPaneLayout - the exact functionality that I was looking for.

What is the best practise for padding or margin in Android?

I am relatively new to Android UI. I always get confuse in providing margin to different view like should i provide bottom margin or should i use top margin(to view below it). Also should i use RelativeLayout or LinearLayout if both can solve my problem.
Thanks

It Depends on your need
Linear Vs Relative
If you just want to stack your TextView and Button horizontally or vertically you should go with LinearLayout.
If you want to make a layout that is more complex for example you can have an ImageView covering all of the screen and Button over the ImageView or if you want your layout elements to be stack on corners or at bottom, RelativeLayout is your Guy.
Top margin vs Bottom Margin
It doesn't make much a difference its a personal preference, I Use margin-bottom on first element rather than margin-top for second element.

One noticeable difference is when you are working with Show layout bounds during development. You can see here those pink coloration indicate that it is using margin on its view while padding has no coloration. Recently I prefer to use padding if applicable with my requirements as it seems more cleaner to inspect UI when Show layout bounds is enabled from Developer option.
The image is not mine and was just use as a quick sample.

If both Linear and RelativeLayout will solve your layout, then you should use Linear as it is faster to render.
With regards to top vs. bottom Margin. That's entirely your preference and how you want to think about the elements. Does Item A always sit 40dp above the next item or does Item B always sit 40dp below the previous item?

Using Layout - Android

I have a layout, but I need put more views (Buttons, EditText, etc), but how you know, the layout of the ADT is a bit small and I can't to set more views or know where is it, Somebody know How can I set more of these?
Thank you

Android isn't like iOS how you can just position elements with drag & drop.
RelativeLayout will position elements relative to each other and LinearLayout will lay out elements sequentially in a linear fashion, either vertically or horizontally. Both methods are better executed by actually writing the View XML yourself.
I suggest reading this: http://developer.android.com/guide/topics/ui/declaring-layout.html

If you want to actually see what you are adding to your layout without messing with the XML you could maybe change the device that the renderer is using to preview your layout.
I don't know what IDE you're using but in Eclipse and Android Studio you can change the device that your layout preview is rendered on. That way you can see what you're adding as you add it.
Then just make sure to put everything in a scroll view so users can access all the views and widgets you've put in that layout for your activity or fragment or dialog or whatever else it is.

How to place UI elements to the position we want in Android?

I have some buttons, textboxes etc. in my android application, but when i drag them with my mouse in the xml file, their place doesn't change, or changes but they are not placed where i exactly wanted. How can i adjust their positions in the screen?
Thanks

Unfortunatly there is no such thing as absolute positionning in android ( RIP AbsoluteLayout deprecated since years.)
instead you have to position views according to their parents and according to other views in the same parent.
first you have to define wich parent you need ( if you want some viens in a single line go for a LinearLayout. a more custom layout: use a RelativeLayout ...)
then you can drag and drop views inside, but they will always snap a position relative to their parent and/or relative to the other views.
you can of course play with margins.
A list of layout type with some advanced techniques can be found on this page
Hope that helps.

You RelativeLayout as a group layout for your layout so positioning can somewhat easy using mouse.
Best is to arrange them from the xml code. Just Learn about using the Relative layout, LinearLayout and TableLayout

Learn how the XML works. For a LinearLayout, the items come in the order listed. For a RelativeLayout, the items are related by the values of their layout_XXX properties. Then you don't have to worry about the WYSIWYG tool not working.
FYI, the tool bundled with eclipse is extremely buggy. Don't count on whats on there being what's on your phone for anything non-trivial.

Like the others wrote it is easier to edit layout using xml editor. You can read more here http://developer.android.com/guide/topics/ui/declaring-layout.html.