Home | History | Annotate | Download | only in ui
      1 page.title=How Android Draws Views
      2 parent.title=User Interface
      3 parent.link=index.html
      4 @jd:body
      5 
      6 
      7 <p>When an Activity receives focus, it will be requested to draw its layout.
      8 The Android framework will handle the procedure for drawing, but the Activity must provide
      9 the root node of its layout hierarchy.</p>
     10 
     11 <p>Drawing begins with the root node of the layout. It is requested to measure and 
     12 draw the layout tree. Drawing is handled by walking the tree and rendering each View that
     13    intersects the invalid region. In turn, each View group is responsible for requesting
     14 each of its children to be drawn (with the <code>{@link android.view.View#draw(Canvas) draw()}</code> method) 
     15 and each View is responsible for drawing itself.
     16  Because the tree is traversed in-order,
     17    this means that parents will be drawn before (i.e., behind) their children, with
     18    siblings drawn in the order they appear in the tree.
     19    </p>
     20 
     21 <div class="sidebox-wrapper">
     22 <div class="sidebox">
     23   <p>The framework will not draw Views that are not in the invalid region, and also 
     24    will take care of drawing the Views background for you.</p>
     25    <p>You can force a View to draw, by calling <code>{@link android.view.View#invalidate()}</code>.
     26    </p>
     27 </div>
     28 </div>
     29 
     30 <p>
     31    Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
     32    pass is implemented in <code>{@link android.view.View#measure(int, int)}</code> and is a top-down traversal
     33    of the View tree. Each View pushes dimension specifications down the tree
     34    during the recursion. At the end of the measure pass, every View has stored
     35    its measurements. The second pass happens in
     36    <code>{@link android.view.View#layout(int,int,int,int)}</code> and is also top-down. During
     37    this pass each parent is responsible for positioning all of its children
     38    using the sizes computed in the measure pass.
     39    </p>
     40    
     41    <p>
     42    When a View's <code>measure()</code> method returns, its <code>{@link android.view.View#getMeasuredWidth()}</code> and
     43    <code>{@link android.view.View#getMeasuredHeight()}</code> values must be set, along with those for all of
     44    that View's descendants. A View's measured width and measured height values
     45    must respect the constraints imposed by the View's parents. This guarantees
     46    that at the end of the measure pass, all parents accept all of their
     47    children's measurements. A parent View may call <code>measure()</code> more than once on
     48    its children. For example, the parent may measure each child once with
     49    unspecified dimensions to find out how big they want to be, then call
     50    <code>measure()</code> on them again with actual numbers if the sum of all the children's
     51    unconstrained sizes is too big or too small (i.e., if the children don't agree among themselves
     52   as to how much space they each get, the parent will intervene and set the rules on the second pass).
     53    </p>
     54    
     55 <div class="sidebox-wrapper">
     56 <div class="sidebox"><p>
     57    To initiate a layout, call <code>{@link android.view.View#requestLayout}</code>. This method is typically
     58    called by a View on itself when it believes that is can no longer fit within
     59    its current bounds.</p>
     60 </div>
     61 </div>
     62 
     63    <p>
     64    The measure pass uses two classes to communicate dimensions. The
     65    {@link android.view.View.MeasureSpec} class is used by Views to tell their parents how they
     66    want to be measured and positioned. The base LayoutParams class just
     67    describes how big the View wants to be for both width and height. For each
     68    dimension, it can specify one of:</p>
     69    <ul>
     70     <li> an exact number
     71     <li><var>FILL_PARENT</var>, which means the View wants to be as big as its parent
     72     (minus padding)</li>
     73     <li><var>WRAP_CONTENT</var>, which means that the View wants to be just big enough to
     74     enclose its content (plus padding).</li>
     75    </ul>
     76   <p>There are subclasses of LayoutParams for different subclasses of ViewGroup.
     77    For example, RelativeLayout has its own subclass of LayoutParams, which includes
     78    the ability to center child Views horizontally and vertically.
     79    </p>
     80    
     81    <p>
     82    MeasureSpecs are used to push requirements down the tree from parent to
     83    child. A MeasureSpec can be in one of three modes:</p>
     84    <ul>
     85     <li><var>UNSPECIFIED</var>: This is used by a parent to determine the desired dimension
     86     of a child View. For example, a LinearLayout may call <code>measure()</code> on its child
     87     with the height set to <var>UNSPECIFIED</var> and a width of <var>EXACTLY</var> 240 to find out how
     88     tall the child View wants to be given a width of 240 pixels.</li>
     89     <li><var>EXACTLY</var>: This is used by the parent to impose an exact size on the
     90     child. The child must use this size, and guarantee that all of its
     91     descendants will fit within this size.</li>
     92     <li><var>AT_MOST</var>: This is used by the parent to impose a maximum size on the
     93     child. The child must guarantee that it and all of its descendants will fit
     94     within this size.</li>
     95    </ul>
     96    
     97 
     98 
     99