99 lines
4.9 KiB
Plaintext
99 lines
4.9 KiB
Plaintext
page.title=How Android Draws Views
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parent.title=User Interface
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parent.link=index.html
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@jd:body
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<p>When an Activity receives focus, it will be requested to draw its layout.
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The Android framework will handle the procedure for drawing, but the Activity must provide
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the root node of its layout hierarchy.</p>
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<p>Drawing begins with the root node of the layout. It is requested to measure and
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draw the layout tree. Drawing is handled by walking the tree and rendering each View that
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intersects the invalid region. In turn, each View group is responsible for requesting
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each of its children to be drawn (with the <code>{@link android.view.View#draw(Canvas) draw()}</code> method)
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and each View is responsible for drawing itself.
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Because the tree is traversed in-order,
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this means that parents will be drawn before (i.e., behind) their children, with
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siblings drawn in the order they appear in the tree.
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</p>
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<div class="sidebox-wrapper">
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<div class="sidebox">
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<p>The framework will not draw Views that are not in the invalid region, and also
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will take care of drawing the Views background for you.</p>
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<p>You can force a View to draw, by calling <code>{@link android.view.View#invalidate()}</code>.
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</p>
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</div>
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</div>
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<p>
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Drawing the layout is a two pass process: a measure pass and a layout pass. The measuring
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pass is implemented in <code>{@link android.view.View#measure(int, int)}</code> and is a top-down traversal
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of the View tree. Each View pushes dimension specifications down the tree
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during the recursion. At the end of the measure pass, every View has stored
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its measurements. The second pass happens in
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<code>{@link android.view.View#layout(int,int,int,int)}</code> and is also top-down. During
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this pass each parent is responsible for positioning all of its children
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using the sizes computed in the measure pass.
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</p>
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<p>
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When a View's <code>measure()</code> method returns, its <code>{@link android.view.View#getMeasuredWidth()}</code> and
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<code>{@link android.view.View#getMeasuredHeight()}</code> values must be set, along with those for all of
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that View's descendants. A View's measured width and measured height values
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must respect the constraints imposed by the View's parents. This guarantees
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that at the end of the measure pass, all parents accept all of their
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children's measurements. A parent View may call <code>measure()</code> more than once on
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its children. For example, the parent may measure each child once with
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unspecified dimensions to find out how big they want to be, then call
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<code>measure()</code> on them again with actual numbers if the sum of all the children's
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unconstrained sizes is too big or too small (i.e., if the children don't agree among themselves
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as to how much space they each get, the parent will intervene and set the rules on the second pass).
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</p>
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<div class="sidebox-wrapper">
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<div class="sidebox"><p>
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To initiate a layout, call <code>{@link android.view.View#requestLayout}</code>. This method is typically
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called by a View on itself when it believes that is can no longer fit within
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its current bounds.</p>
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</div>
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</div>
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<p>
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The measure pass uses two classes to communicate dimensions. The
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{@link android.view.View.MeasureSpec} class is used by Views to tell their parents how they
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want to be measured and positioned. The base LayoutParams class just
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describes how big the View wants to be for both width and height. For each
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dimension, it can specify one of:</p>
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<ul>
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<li> an exact number
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<li><var>FILL_PARENT</var>, which means the View wants to be as big as its parent
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(minus padding)</li>
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<li><var>WRAP_CONTENT</var>, which means that the View wants to be just big enough to
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enclose its content (plus padding).</li>
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</ul>
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<p>There are subclasses of LayoutParams for different subclasses of ViewGroup.
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For example, RelativeLayout has its own subclass of LayoutParams, which includes
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the ability to center child Views horizontally and vertically.
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</p>
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<p>
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MeasureSpecs are used to push requirements down the tree from parent to
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child. A MeasureSpec can be in one of three modes:</p>
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<ul>
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<li><var>UNSPECIFIED</var>: This is used by a parent to determine the desired dimension
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of a child View. For example, a LinearLayout may call <code>measure()</code> on its child
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with the height set to <var>UNSPECIFIED</var> and a width of <var>EXACTLY</var> 240 to find out how
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tall the child View wants to be given a width of 240 pixels.</li>
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<li><var>EXACTLY</var>: This is used by the parent to impose an exact size on the
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child. The child must use this size, and guarantee that all of its
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descendants will fit within this size.</li>
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<li><var>AT_MOST</var>: This is used by the parent to impose a maximum size on the
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child. The child must guarantee that it and all of its descendants will fit
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within this size.</li>
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</ul>
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