docs/html/guide/topics/ui/how-android-draws.jd

page.title=How Android Draws Views
parent.title=User Interface
parent.link=index.html
@jd:body


<p>When an {@link android.app.Activity} receives focus, it will be requested to 
draw its layout.
The Android framework will handle the procedure for drawing, but the 
{@link android.app.Activity} must provide
the root node of its layout hierarchy.</p>

<p>Drawing begins with the root node of the layout. It is requested to measure and 
draw the layout tree. Drawing is handled by walking the tree and rendering each 
{@link android.view.View} that intersects the invalid region. In turn, each 
{@link android.view.ViewGroup} is responsible for requesting
each of its children to be drawn 
(with the {@link android.view.View#draw(Canvas) draw()} method) 
and each {@link android.view.View} is responsible for drawing itself.
 Because the tree is traversed in-order,
   this means that parents will be drawn before (i.e., behind) their children, with
   siblings drawn in the order they appear in the tree.
   </p>

<div class="sidebox-wrapper">
<div class="sidebox">
  <p>The framework will not draw {@link android.view.View} objects that are not 
in the invalid region, and also 
   will take care of drawing the {@link android.view.View} background for you.</p>
   <p>You can force a {@link android.view.View} to draw, by calling 
{@link android.view.View#invalidate()}.
   </p>
</div>
</div>

<p>
   Drawing the layout is a two pass process: a measure pass and a layout pass. 
The measuring pass is implemented in {@link android.view.View#measure(int, int)} 
and is a top-down traversal of the {@link android.view.View} tree. Each {@link android.view.View} 
pushes dimension specifications down the tree
   during the recursion. At the end of the measure pass, every 
{@link android.view.View} has stored
   its measurements. The second pass happens in
   {@link android.view.View#layout(int,int,int,int)} and is also top-down. During
   this pass each parent is responsible for positioning all of its children
   using the sizes computed in the measure pass.
   </p>
   
   <p>
   When a {@link android.view.View} object's 
{@link android.view.View#measure(int, int) measure()} method 
returns, its {@link android.view.View#getMeasuredWidth()} and
   {@link android.view.View#getMeasuredHeight()} values must be set, along 
   with those for all of that {@link android.view.View} object's descendants. 
A {@link android.view.View} object's measured width and 
measured height values must respect the constraints imposed by the 
{@link android.view.View} object's parents. This guarantees
   that at the end of the measure pass, all parents accept all of their
   children's measurements. A parent {@link android.view.View} may call 
{@link android.view.View#measure(int, int) measure()} more than once on
   its children. For example, the parent may measure each child once with
   unspecified dimensions to find out how big they want to be, then call
   {@link android.view.View#measure(int, int) measure()} on them again with 
actual numbers if the sum of all the children's
   unconstrained sizes is too big or too small (that is, if the children 
don't agree among themselves
  as to how much space they each get, the parent will intervene and set 
the rules on the second pass).
   </p>
   
<div class="sidebox-wrapper">
<div class="sidebox"><p>
   To initiate a layout, call {@link android.view.View#requestLayout}. 
This method is typically
   called by a {@link android.view.View} on itself 
when it believes that is can no longer fit within
   its current bounds.</p>
</div>
</div>

   <p>
   The measure pass uses two classes to communicate dimensions. The
   {@link android.view.ViewGroup.LayoutParams} class is used by 
{@link android.view.View} objects to tell their parents how they
   want to be measured and positioned. The base 
{@link android.view.ViewGroup.LayoutParams}  class just
   describes how big the {@link android.view.View} wants to be for both 
width and height. For each
   dimension, it can specify one of:</p>
   <ul>
    <li> an exact number
    <li>{@link android.view.ViewGroup.LayoutParams#MATCH_PARENT MATCH_PARENT}, 
which means the {@link android.view.View} wants to be as big as its parent
    (minus padding)</li>
    <li>{@link android.view.ViewGroup.LayoutParams#WRAP_CONTENT WRAP_CONTENT}, 
which means that the {@link android.view.View} wants to be just big enough to
    enclose its content (plus padding).</li>
   </ul>
  <p>There are subclasses of {@link android.view.ViewGroup.LayoutParams} for 
different subclasses of {@link android.view.ViewGroup}.
   For example, {@link android.widget.RelativeLayout} has its own subclass of 
{@link android.view.ViewGroup.LayoutParams}, which includes
   the ability to center child {@link android.view.View} objects 
horizontally and vertically.
   </p>
   
   <p>
   {@link android.view.View.MeasureSpec MeasureSpec} objects are used to push 
requirements down the tree from parent to
   child. A {@link android.view.View.MeasureSpec MeasureSpec} can be in one of 
three modes:</p>
   <ul>
    <li>{@link android.view.View.MeasureSpec#UNSPECIFIED UNSPECIFIED}: This is 
used by a parent to determine the desired dimension
    of a child {@link android.view.View}. For example, a 
{@link android.widget.LinearLayout} may call 
{@link android.view.View#measure(int, int) measure()} on its child
    with the height set to {@link android.view.View.MeasureSpec#UNSPECIFIED UNSPECIFIED} 
and a width of {@link android.view.View.MeasureSpec#EXACTLY EXACTLY} 240 to 
find out how tall the child {@link android.view.View} wants to be given a 
width of 240 pixels.</li>
    <li>{@link android.view.View.MeasureSpec#EXACTLY EXACTLY}: This is used 
by the parent to impose an exact size on the
    child. The child must use this size, and guarantee that all of its
    descendants will fit within this size.</li>
    <li>{@link android.view.View.MeasureSpec#AT_MOST AT MOST}: This is used by 
the parent to impose a maximum size on the
    child. The child must guarantee that it and all of its descendants will fit
    within this size.</li>
   </ul>