LayoutPrams

A layout defines the visual structure for a user interface, such as an activity or widget.

A layout is declared in XML, including screen elements that will appear in it. Code can be added to the application to modify the state of screen objects at runtime, including those declared in XML.

LayoutParams

Every single ViewGroup (e.g. LinearLayout, RelativeLayout, CoordinatorLayout, etc.) needs to store information about its children’s properties. About the way its children are being laid out in the ViewGroup. This information is stored in objects of a wrapper class ViewGroup.LayoutParams.

To include parameters specific to a particular layout type, ViewGroups use subclasses of ViewGroup.LayoutParams class.

E.g. for

• LinearLayout it’s LinearLayout.LayoutParams
• RelativeLayout it’s RelativeLayout.LayoutParams
• CoordinatorLayout it’s CoordinatorLayout.LayoutParams

Most of ViewGroups reutilize the ability to set margins for their children, so they do not subclass ViewGroup.LayoutParams directly, but they subclass ViewGroup.MarginLayoutParams instead (which itself is a subclass of ViewGroup.LayoutParams).

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LayoutParams in xml

LayoutParams objects are created based on the inflated layout xml file.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical">

<TextView
android:layout_width="wrap_content"
android:layout_height="50dp"
android:layout_gravity="right"
android:gravity="bottom"
android:text="Example text"
android:textColor="@android:color/holo_green_dark"/>

<ImageView
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:background="@android:color/holo_green_dark"
android:scaleType="centerInside"
android:src="@drawable/example"/>

</LinearLayout>

All parameters that begin with layout_ specify how the enclosing layout should work. When the layout is inflated, those parameters are wrapped in a proper LayoutParams object, that later will be used by the Layout to properly position a particular View within the ViewGroup. Other attributes of a View are directly View-related and are processed by the View itself.

For TextView:

• layout_width, layout_height and layout_gravity will be stored in a LinearLayout.LayoutParams object and used by the LinearLayout
• gravity, text and textColor will be used by the TextView itself

For ImageView:

• layout_width, layout_height and layout_weight will be stored in a LinearLayout.LayoutParams object and used by the LinearLayout
• background, scaleType and src will be used by the ImageView itself

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Getting LayoutParams object

getLayoutParams is a View’s method that allows to retrieve a current LayoutParams object.

Because the LayoutParams object is directly related to the enclosing ViewGroup, this method will return a non-null value only when View is attached to the ViewGroup. You need to bare in mind that this object might not be present at all times. Especially you should not depend on having it inside View’s constructor.

public class ExampleView extends View {

public ExampleView(Context context) {
super(context);
setupView(context);
}

public ExampleView(Context context, AttributeSet attrs) {
super(context, attrs);
setupView(context);
}

public ExampleView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
setupView(context);
}

private void setupView(Context context) {
if (getLayoutParams().height == 50){  // DO NOT DO THIS!
  // This might produce NullPointerException
doSomething();
}
}

//...
}

If you want to depend on having LayoutParams object, you should use onAttachedToWindow method instead.

public class ExampleView extends View {

public ExampleView(Context context) {
super(context);
}

public ExampleView(Context context, AttributeSet attrs) {
super(context, attrs);
}

public ExampleView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}

@Override
protected void onAttachedToWindow() {
super.onAttachedToWindow();
if (getLayoutParams().height == 50) { // getLayoutParams() will NOT return null here
doSomething();
}
}

//...
}

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Casting LayoutParams object

You might need to use features that are specific to a particular ViewGroup (e.g. you might want to programmatically change rules of a RelativeLayout). For that purpose you will need to know how to properly cast the ViewGroup.LayoutParams object.

This might be a bit confusing when getting a LayoutParams object for a child View that actually is another ViewGroup.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/outer_layout"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical">

<FrameLayout
android:id="@+id/inner_layout"
android:layout_width="match_parent"
android:layout_height="50dp"
android:layout_gravity="right"/>

</LinearLayout>

IMPORTANT: The type of LayoutParams object is directly related to the type of the ENCLOSING ViewGroup.

Incorrect casting:

FrameLayout innerLayout = (FrameLayout)findViewById(R.id.inner_layout);
FrameLayout.LayoutParams par = (FrameLayout.LayoutParams) innerLayout.getLayoutParams();
  // INCORRECT! This will produce ClassCastException

Correct casting:

FrameLayout innerLayout = (FrameLayout)findViewById(R.id.inner_layout);
LinearLayout.LayoutParams par = (LinearLayout.LayoutParams) innerLayout.getLayoutParams();
 // CORRECT! the enclosing layout is a LinearLayout