Develop Reference

What LayoutParams do or pass in Android?

I read often something like this:
RelativeLayout.LayoutParams lParams = (RelativeLayout.LayoutParams) view.getLayoutParams();
What exactly LayoutParams do?
I've read the Documentary but I wasn't smarter after reading!
Hope someone can explain me what LayoutParams do or pass!
Kind Regards!

LayoutParams are the Java Object representation of all the params you give to your View in the .xml layout file, like layout_width, layout_height and so on. Getting this object from a View allows you to look up those params on runtime, but also to change them in your Java code, when you need to move the View, change it's size etc.

LayoutParams are used by views to tell their parents how they want to be laid out.
The base LayoutParams class just describes how big the view wants to be for both width and height. For each dimension, it can specify one of:
FILL_PARENT (renamed MATCH_PARENT in API Level 8 and higher), which means that the view wants to be as big as its parent (minus padding)
WRAP_CONTENT, which means that the view wants to be just big enough to enclose its content.
That's all folks.

LayoutParams is use for the dynamically change the layout width and height. and also use the create custom view without the xml by using the directly by use of the LayoutParams for Relative or Linear type layout.

Basically when you set an xml with 'match_parent' or anything like layout_something
the android inflator will set the layout param for the child with the appropriate Layout params with the type matching the parent control, you could also do this in code and if you forget or set the wrong type you will get an exception in runtime.
the parent control needs this information to layout the child control correctly and to his liking.
Please see the following: Android Developer site - Layout Params
I think this picture says it all

LinearLayoutParams are causing a casting issue

I must be missing something really stupid.
I have a LinearLayout in my view:
_editTextViews = (LinearLayout)findViewById(R.id.editDesAndLocViews);
I am trying to dynamically change the height of that view by setting the LayoutParams
I have two LinearLayout.LayoutParams
LinearLayout.LayoutParams collapsedParams;
LinearLayout.LayoutParams openParams;
that are set in the constructor like so:
collapsedParams = new LinearLayout.LayoutParams(LayoutParams.MATCH_PARENT,0);
openParams = new LinearLayout.LayoutParams(LayoutParams.MATCH_PARENT,LayoutParams.WRAP_CONTENT);
But as soon as i try to apply one of those params to my LinearLayout, like so:
_editTextViews.setLayoutParams(collapsedParams);
I get the following FATAL EXCEPTION:
At first i thought my project needed to be cleaned as it seems to think i am trying to force a relative layout to use linear layout params, but everything is LinearLayout. The LinearLayout in question does contain a couple RelativeLayouts. I don't know if / why that would be the problem.
please help

Change your LayoutParamss' type to either ViewGroup.LayoutParams or RelativeLayout.LayoutParams.
The LayoutParams you apply have to match the parent layout of the view. LayoutParams define how a View is layed out in its parent view (layout). Because each layout type takes different parametres for laying out a view (e.g. only LinearLayout has weight, only RelativeLayout can use anchors), they all have their own LayoutParams.

edit layout of existing child in RelativeLayout

I have a RelativeLayout with a number of children. The RelativeLayout is the main layout of my activity. I need to add to one of the children the layout value layout_alignParentBottom="true". I need to do this programmatically. the link How to set the android property layout_alignParentBottom to a button dynamically is not working for me because, basically, I don't want to have to inflate the RelativeLayout as I already have it at the setContentView(...). Any ideas how to do this?

I figured it out:
RelativeLayout.LayoutParams layout = (LayoutParams) theChild.getLayoutParams();
layout.addRule(RelativeLayout.ALIGN_PARENT_BOTTOM);

Change LayoutParams of LinearLayout that is inside of RelativeLayout

I have LinearLayout inside of RelativeLayout. I would like to change height of LinearLayout programmatically. I have tried do this with LinearLayout.LayoutParams but I've got an exception. How to solve this problem? Thanks.

Try with RelativeLayout.LayoutParams as the parent is a RelativeLayout.
The type of the LayoutParams depends on the parent type.

How can i set margin to a view in another class?

I do a constructer and set some view inside programmatically and add some rule. so When i call constructer i see default view.
Now i want to reach someof this view and change set margin.It is basic i know but i dosent work for me.
Can anybody help?
Part of my constructer:
_photoImgVw = new ImageView(getContext());
_photoImgVw.setImageResource(R.drawable.pic);
_photoImgVw.setAdjustViewBounds(true);
_photoImgVw.setMaxWidth(100);
_photoImgVw.setId(2);
lp2=new LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT);
lp2.setMargins(0,0,0,0);
addView(_photoImgVw,lp2);
LayoutParams lp3=new LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT);
lp3.addRule(RelativeLayout.RIGHT_OF,_photoImgVw.getId());
lp3.setMargins(0,0,0,0);
addView(_nameTxt,lp3);
Call in the activity for change:
facebookPersonView.lp2.setMargins(10,300,100,20);

Make sure that the layout params lp2 are an instance of the class RelativeLayout.LayoutParams instead of MarginLayoutParams, or any other subclass of it named LayoutParams (there are quite a few, see the top of the documentation).
If the params are an instance of the latter, they won't contain any rules for the RelativeLayout. Which means the parent layout doesn't know where to place the view¹ and therefore can't make any sense out of the margins.
It's best to declare that explicitly in a way like this:
RelativeLayout.LayoutParams lp2
= new RelativeLayout.LayoutParams(LayoutParams.WRAP_CONTENT,
LayoutParams.WRAP_CONTENT);
lp2.setMargins(0,0,0,0);
addView(_photoImgVw,lp2);
¹ It will still make it visible in the top-left corner, but it's kind of an undefined state.