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POWhy do layout params work in Android Programming?
 

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  1. POWhy do layout params work in Android Programming?
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    <p>First off my background: I'm new to Java coming over from Ruby. If that helps.</p> <p>I'm confused about how layout params work. I'm following a basic Hello World introduction to creating an Android App. Step 1, extend the Activity class, and the onCreate() method to access the XML layout. Ok I get that. </p> <p>Then I create a layout (say a RelativeLayout) in the Main.XML. So this is making use of the RelativeLayout class which extends the ViewGroup class, ok so far. Then lets say I create a button inside this. This is where my question starts. If I look at the example I am following I see attributes being assigned to the button that belong to the RelativeLayout class (i.e: android:layout_alignParentRight="true"). These seem to be the layout params. But why does this work? The button class seems to inherit from the View class. Why can a button object accept attributes for the RelativeLayout object? Maybe my Ruby programming is confusing me..</p> <p>Thanks!</p> <p>Update: For posterity sake: thank you to Slothsberry for pointing out the <a href="http://developer.android.com/guide/topics/ui/declaring-layout.html" rel="nofollow">XML Layouts</a> link, which seems to describe the answer clearly in 2 sections the section on "Attributes" and on "Layout Paramters". The attributes section reads: </p> <blockquote> <p>Every View and ViewGroup object supports their own variety of XML attributes. Some attributes are specific to a View object (for example, TextView supports the textSize attribute), but these attributes are also inherited by any View objects that may extend this class. Some are common to all View objects, because they are inherited from the root View class (like the id attribute). And, other attributes are considered "layout parameters," which are attributes that describe certain layout orientations of the View object, as defined by that object's parent ViewGroup object.</p> </blockquote> <p>The layout parameters section though is perhaps the section that really answers this question. Where it states: </p> <blockquote> <p>Every ViewGroup class implements a nested class that extends ViewGroup.LayoutParams. This subclass contains property types that define the size and position for each child view, as appropriate for the view group. As you can see in figure 1, the parent view group defines layout parameters for each child view (including the child view group).</p> </blockquote> <p>They give a nice diagram as well. It seems that a beginning programmer needs to recognize that while Java classes are referenced, the XML acts more like a CSS sheet and that attributes are first computed in a nested fashion before being computed and carried over to their Java class counterparts. That's my current understanding anyways :)</p>
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    1. CONo, you're right to think this is weird. I've been programming in java for over 5 year, and this is something that when I first saw it made no sense to me. I'm still not sure why it works like this. Hopefully we'll get an answer.
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