Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)

本文原创， 转载请注明出处：http://blog.csdn.net/qinjuning

在之前一篇博文中<<Android中View绘制流程以及invalidate()等相关方法分析>>，简单的阐述 了Android View

绘制流程的三个步骤，即：

1、 measure过程 --- 测量过程

2、 layout 过程 --- 布局过程
3、 draw 过程 --- 绘制过程

要想对Android 中View这块深入理解，对这三个步骤地学习是必不可少的 。

今天，我着重讲解下如下三个内容：

1、measure过程

2、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT属性的原理说明

3、xml布局文件解析成View树的流程分析。

希望对大家能有帮助。- -分析版本基于Android 2.3 。

1、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT

初入Android殿堂的同学们，对这三个属性一定又爱又恨。爱的是使用起来挺爽地---照葫芦画瓢即可，恨的

却是时常混淆这几个属性地意义，需要三思而后行。在带着大家重温下这几个属性的用法吧(希望我没有啰嗦)。

这三个属性都用来适应视图的水平或垂直大小，一个以视图的内容或尺寸为基础的布局比精确地指定视图范围

更加方便。

① fill_parent

设置一个视图的布局为fill_parent将强制性地使视图扩展至父元素大小。

② match_parent

Android 中match_parent和fill_parent意思一样，但match_parent更贴切，于是从2.2开始两个词都可以

用，但2.3版本后建议使用match_parent。

③ wrap_content

自适应大小，强制性地使视图扩展以便显示其全部内容。以TextView和ImageView控件为例，设置为

wrap_content将完整显示其内部的文本和图像。布局元素将根据内容更改大小。

可不要重复造轮子，以上摘自<<Androidfill_parent、wrap_content和match_parent的区别>>。

当然，我们可以设置View的确切宽高，而不是由以上属性指定。

        android:layout_weight="wrap_content"   //自适应大小
        android:layout_weight="match_parent"   //与父视图等高
        android:layout_weight="fill_parent"	   //与父视图等高
        android:layout_weight="100dip"         //精确设置高度值为 100dip

接下来，我们需要转换下视角，看看ViewGroup.LayoutParams类及其派生类。

2、ViewGroup.LayoutParams类及其派生类

2.1、 ViewGroup.LayoutParams类说明

Android API中如下介绍：

LayoutParams are used by views to tell their parents how they want to be laid out.

意思大概是说：View通过LayoutParams类告诉其父视图它想要地大小(即，长度和宽度)。

因此，每个View都包含一个ViewGroup.LayoutParams类或者其派生类，View类依赖于ViewGroup.LayoutParams。

路径：frameworks\base\core\java\android\view\View.java

public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {
  ...
  /**
   * The layout parameters associated with this view and used by the parent
   * {@link android.view.ViewGroup} to determine how this view should be
   * laid out.
   * {@hide}
   */
  //该View拥有的 LayoutParams属性，父试图添加该View时，会为其赋值，特别注意，其类型为ViewGroup.LayoutParams。
  protected ViewGroup.LayoutParams mLayoutParams;  
  ...
}

2.2、 ViewGroup.LayoutParams源码分析

路径位于：frameworks\base\core\java\android\view\ViewGroup.java

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
	...
	 public static class LayoutParams {
        /**
         * Special value for the height or width requested by a View.
         * FILL_PARENT means that the view wants to be as big as its parent,
         * minus the parent's padding, if any. This value is deprecated
         * starting in API Level 8 and replaced by {@link #MATCH_PARENT}.
         */
        @Deprecated
        public static final int FILL_PARENT = -1;  // 注意值为-1，Android2.2版本不建议使用
        /**
         * Special value for the height or width requested by a View.
         * MATCH_PARENT means that the view wants to be as big as its parent,
         * minus the parent's padding, if any. Introduced in API Level 8.
         */
        public static final int MATCH_PARENT = -1; // 注意值为-1
        /**
         * Special value for the height or width requested by a View.
         * WRAP_CONTENT means that the view wants to be just large enough to fit
         * its own internal content, taking its own padding into account.
         */
        public static final int WRAP_CONTENT = -2; // 注意值为-2
        /**
         * Information about how wide the view wants to be. Can be one of the
         * constants FILL_PARENT (replaced by MATCH_PARENT ,
         * in API Level 8) or WRAP_CONTENT. or an exact size.
         */
        public int width;  //该View的宽度，可以为WRAP_CONTENT/MATCH_PARENT 或者一个具体值
        /**
         * Information about how tall the view wants to be. Can be one of the
         * constants FILL_PARENT (replaced by MATCH_PARENT ,
         * in API Level 8) or WRAP_CONTENT. or an exact size.
         */
        public int height; //该View的高度，可以为WRAP_CONTENT/MATCH_PARENT 或者一个具体值
        /**
         * Used to animate layouts.
         */
        public LayoutAnimationController.AnimationParameters layoutAnimationParameters;
        /**
         * Creates a new set of layout parameters. The values are extracted from
         * the supplied attributes set and context. The XML attributes mapped
         * to this set of layout parameters are:、
         */
        public LayoutParams(Context c, AttributeSet attrs) {
            TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
            setBaseAttributes(a,
                    R.styleable.ViewGroup_Layout_layout_width,
                    R.styleable.ViewGroup_Layout_layout_height);
            a.recycle();
        }

        /**
         * Creates a new set of layout parameters with the specified width
         * and height.
         */
        public LayoutParams(int width, int height) {
            this.width = width;
            this.height = height;
        }
        /**
         * Copy constructor. Clones the width and height values of the source.
         *
         * @param source The layout params to copy from.
         */
        public LayoutParams(LayoutParams source) {
            this.width = source.width;
            this.height = source.height;
        }
        /**
         * Used internally by MarginLayoutParams.
         * @hide
         */
        LayoutParams() {
        }
        /**
         * Extracts the layout parameters from the supplied attributes.
         *
         * @param a the style attributes to extract the parameters from
         * @param widthAttr the identifier of the width attribute
         * @param heightAttr the identifier of the height attribute
         */
        protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {
            width = a.getLayoutDimension(widthAttr, "layout_width");
            height = a.getLayoutDimension(heightAttr, "layout_height");
        }
}

我们发现FILL_PARENT/MATCH_PARENT值为 -1 ，WRAP_CONETENT值为-2，是不是有点诧异？ 将值

设置为负值的目的是为了区别View的具体值(an exact size) 总是大于0的。

ViewGroup子类可以实现自定义LayoutParams，自定义LayoutParams提供了更好地扩展性，例如LinearLayout

就有LinearLayout.LayoutParams自定义类(见下文)。整个LayoutParams类家族还是挺复杂的。

ViewGroup.LayoutParams及其常用派生类的类图(部分类图)如下：

该类图是在太庞大了，大家有兴趣的去看看Android API吧。

前面我们说过，每个View都包含一个ViewGroup.LayoutParams类或者其派生类，下面我们的疑问是Android框架

中时如何为View设置其LayoutParams属性的。

有两种方法会设置View的LayoutParams属性：

1、 直接添加子View时，常见于如下几种方法：ViewGroup.java

//Adds a child view.	
void addView(View child, int index)
//Adds a child view with this ViewGroup's default layout parameters 
//and the specified width and height.
void addView(View child, int width, int height)
//Adds a child view with the specified layout parameters.		
void addView(View child, ViewGroup.LayoutParams params)

三个重载方法的区别只是添加View时构造LayoutParams对象的方式不同而已，稍后我们探寻一下它们的源码。

2、 通过xml布局文件指定某个View的属性为：android:layout_heigth=””以及android:layout_weight=”” 时。

总的来说，这两种方式都会设定View的LayoutParams属性值----指定的或者Default值。

方式1流程分析：

直接添加子View时，比较容易理解，我们先来看看这种方式设置LayoutParams的过程：

路径：\frameworks\base\core\java\android\view\ViewGroup.java

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
	...
    /**
     * Adds a child view. If no layout parameters are already set on the child, the
     * default parameters for this ViewGroup are set on the child.
     *
     * @param child the child view to add
     *
     * @see #generateDefaultLayoutParams()
     */
    public void addView(View child) {
        addView(child, -1);
    }
    /**
     * Adds a child view. If no layout parameters are already set on the child, the
     * default parameters for this ViewGroup are set on the child.
     *
     * @param child the child view to add
     * @param index the position at which to add the child
     *
     * @see #generateDefaultLayoutParams()
     */
    public void addView(View child, int index) {
        LayoutParams params = child.getLayoutParams();
        if (params == null) {
            params = generateDefaultLayoutParams(); //返回默认地LayoutParams类，作为该View的属性值
            if (params == null) {//如果不能获取到LayoutParams对象，则抛出异常。
                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
            }
        }
        addView(child, index, params);
    }
    /**
     * Adds a child view with this ViewGroup's default layout parameters and the
     * specified width and height.
     *
     * @param child the child view to add
     */
    public void addView(View child, int width, int height) {
    	//返回默认地LayoutParams类，作为该View的属性值
    	final LayoutParams params = generateDefaultLayoutParams(); 
        params.width = width;   //重新设置width值
        params.height = height; //重新设置height值
        addView(child, -1, params); //这儿，我们有指定width、height的大小了。
    }
    /**
     * Adds a child view with the specified layout parameters.
     *
     * @param child the child view to add
     * @param params the layout parameters to set on the child
     */
    public void addView(View child, LayoutParams params) {
        addView(child, -1, params);
    }
    /**
     * Adds a child view with the specified layout parameters.
     *
     * @param child the child view to add
     * @param index the position at which to add the child
     * @param params the layout parameters to set on the child
     */
    public void addView(View child, int index, LayoutParams params) {
        ...
        // addViewInner() will call child.requestLayout() when setting the new LayoutParams
        // therefore, we call requestLayout() on ourselves before, so that the child's request
        // will be blocked at our level
        requestLayout();
        invalidate();
        addViewInner(child, index, params, false);
    }
    /**
     * Returns a set of default layout parameters. These parameters are requested
     * when the View passed to {@link #addView(View)} has no layout parameters
     * already set. If null is returned, an exception is thrown from addView.
     *
     * @return a set of default layout parameters or null
     */
    protected LayoutParams generateDefaultLayoutParams() {
        //width 为 WRAP_CONTENT大小 ， height 为WRAP_CONTENT 
    	//ViewGroup的子类可以重写该方法，达到其特定要求。稍后会以LinearLayout类为例说明。
    	return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }
    private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {

        if (!checkLayoutParams(params)) { //params对象是否为null
            params = generateLayoutParams(params); //如果params对象是为null，重新构造个LayoutParams对象
        }
        //preventRequestLayout值为false
        if (preventRequestLayout) {  
            child.mLayoutParams = params; //为View的mLayoutParams属性赋值
        } else {
            child.setLayoutParams(params);//为View的mLayoutParams属性赋值，但会调用requestLayout()请求重新布局
        }
        //if else 语句会设置View为mLayoutParams属性赋值
        ...
    }
	...
}

主要功能就是在添加子View时为其构建了一个LayoutParams对象。但更重要的是，ViewGroup的子类可以重载

上面的几个方法，返回特定的LayoutParams对象，例如：对于LinearLayout而言，则是LinearLayout.LayoutParams

对象。这么做地目的是，能在其他需要它的地方，可以将其强制转换成LinearLayout.LayoutParams对象。

LinearLayout重写函数地实现为：

public class LinearLayout extends ViewGroup {
	...
    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LinearLayout.LayoutParams(getContext(), attrs);
    }
    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        //该LinearLayout是水平方向还是垂直方向
    	if (mOrientation == HORIZONTAL) { 
            return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        } else if (mOrientation == VERTICAL) {
            return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
        }
        return null;
    }
    @Override
    protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }
	/**
     * Per-child layout information associated with ViewLinearLayout.
     * 
     * @attr ref android.R.styleable#LinearLayout_Layout_layout_weight
     * @attr ref android.R.styleable#LinearLayout_Layout_layout_gravity
     */ //自定义的LayoutParams类
    public static class LayoutParams extends ViewGroup.MarginLayoutParams {
        /**
         * Indicates how much of the extra space in the LinearLayout will be
         * allocated to the view associated with these LayoutParams. Specify
         * 0 if the view should not be stretched. Otherwise the extra pixels
         * will be pro-rated among all views whose weight is greater than 0.
         */
        @ViewDebug.ExportedProperty(category = "layout")
        public float weight;      //  见于属性，android:layout_weight=""  ;
        /**
         * Gravity for the view associated with these LayoutParams.
         *
         * @see android.view.Gravity
         */
        public int gravity = -1;  // 见于属性， android:layout_gravity=""  ; 
        /**
         * {@inheritDoc}
         */
        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
            TypedArray a =c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.LinearLayout_Layout);
            weight = a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight, 0);
            gravity = a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity, -1);

            a.recycle();
        }
        /**
         * {@inheritDoc}
         */
        public LayoutParams(int width, int height) {
            super(width, height);
            weight = 0;
        }
        /**
         * Creates a new set of layout parameters with the specified width, height
         * and weight.
         *
         * @param width the width, either {@link #MATCH_PARENT},
         *        {@link #WRAP_CONTENT} or a fixed size in pixels
         * @param height the height, either {@link #MATCH_PARENT},
         *        {@link #WRAP_CONTENT} or a fixed size in pixels
         * @param weight the weight
         */
        public LayoutParams(int width, int height, float weight) {
            super(width, height);
            this.weight = weight;
        }
        public LayoutParams(ViewGroup.LayoutParams p) {
            super(p);
        }
        public LayoutParams(MarginLayoutParams source) {
            super(source);
        }
    }
	...
}

LinearLayout.LayoutParams类继承至ViewGroup.MarginLayoutParams类，添加了对android:layout_weight以及

android:layout_gravity这两个属性的获取和保存。而且它的重写函数返回的都是LinearLayout.LayoutParams

类型。这样，我们可以再对子View进行其他操作时，可以将将其强制转换成LinearLayout.LayoutParams对象进行

使用。

例如，LinearLayout进行measure过程，使用了LinearLayout.LayoutParam对象，有如下代码：

public class LinearLayout extends ViewGroup {
	...
	@Override  //onMeasure方法。
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	    //判断是垂直方向还是水平方向，这儿我们假设是VERTICAL垂直方向，
		if (mOrientation == VERTICAL) {
	        measureVertical(widthMeasureSpec, heightMeasureSpec);
	    } else {
	        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
	    }
	}
	 /**
     * Measures the children when the orientation of this LinearLayout is set
     * to {@link #VERTICAL}.
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the parent.
     * @param heightMeasureSpec Vertical space requirements as imposed by the parent.
     *
     * @see #getOrientation()
     * @see #setOrientation(int)
     * @see #onMeasure(int, int)
     */
	  void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
	        mTotalLength = 0;
	        ...
	        // See how tall everyone is. Also remember max width.
	        for (int i = 0; i < count; ++i) {
	            final View child = getVirtualChildAt(i); //获得索引处为i的子VIew   
                ...
                //注意，我们将类型为 ViewGroup.LayoutParams的实例对象强制转换为了LinearLayout.LayoutParams，
                //即父对象转换为了子对象，能这样做的原因就是LinearLayout的所有子View的LayoutParams类型都为
                //LinearLayout.LayoutParams
	            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
                ...
	    }
	...
}

超类ViewGroup.LayoutParams强制转换为了子类LinearLayout.LayoutParams，因为LinearLayout的每个

”直接“子View的LayoutParams属性都是LinearLayout.LayoutParams类型，因此可以安全转换。

PS : Android 2.3源码Launcher2中也实现了自定义的LayoutParams类，在IDLE界面的每个View至少包含如下

信息：所在X方向的单元格索引和高度、所在Y方向的单元格索引和高度等。

路径:packages\apps\Launcher2\src\com\android\launcher2\CellLayout.java

public class CellLayout extends ViewGroup {
	... 
	public static class LayoutParams extends ViewGroup.MarginLayoutParams {
	        /**
	         * Horizontal location of the item in the grid.
	         */
	        public int cellX;   //X方向的单元格索引
	        /**
	         * Vertical location of the item in the grid.
	         */
	        public int cellY;   //Y方向的单元格索引
	        /**
	         * Number of cells spanned horizontally by the item.
	         */
	        public int cellHSpan;  //水平方向所占高度
	        /**
	         * Number of cells spanned vertically by the item.
	         */
	        public int cellVSpan;  //垂直方向所占高度
            ...
	        public LayoutParams(Context c, AttributeSet attrs) {
	            super(c, attrs);
	            cellHSpan = 1;  //默认为高度 1
	            cellVSpan = 1;
	        }

	        public LayoutParams(ViewGroup.LayoutParams source) {
	            super(source); //默认为高度 1
	            cellHSpan = 1;
	            cellVSpan = 1;
	        }
	        
	        public LayoutParams(int cellX, int cellY, int cellHSpan, int cellVSpan) {
	            super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
	            this.cellX = cellX;
	            this.cellY = cellY;
	            this.cellHSpan = cellHSpan;
	            this.cellVSpan = cellVSpan;
	        }
	        ...
	    }
	...
}

对该自定义CellLayout.LayoutParams类的使用可以参考LinearLayout.LayoutParams类，我也不再赘述了。

方法2流程分析：

使用属性android:layout_heigth=””以及android:layout_weight=”” 时，为某个View设置LayoutParams值。

其实这种赋值方法其实也如同前面那种，只不过它需要一个前期孵化过程---需要利用XML解析将布局文件

解析成一个完整的View树，可别小看它了，所有Xxx.xml的布局文件都需要解析成一个完整的View树。下面，

我们就来仔细走这个过程，重点关注如下两个方面

①、xml布局是如何解析成View树的 ;

②、android:layout_heigth=””和android:layout_weight=””的解析。

PS： 一直以来，我都想当然android:layout_heigth以及android:layout_weight这两个属性的解析过程是在

View.java内部完成的，但当我真正去找寻时，却一直没有在View.java类或者ViewGroup.java类找到。直到一位

网友的一次提问，才发现它们的藏身之地。

3、布局文件解析流程分析

解析布局文件时，使用的类为LayoutInflater。 关于该类的使用请参考如下博客：

<android中LayoutInflater的使用>>

主要有如下API方法：

publicViewinflate(XmlPullParserparser,ViewGrouproot, boolean attachToRoot)

publicViewinflate(int resource,ViewGrouproot)

publicViewinflate(int resource,ViewGrouproot, boolean attachToRoot)

这三个类主要迷惑之处在于地三个参数attachToRoot，即是否将该View树添加到root中去。具体可看这篇博客：

<<关于inflate的第3个参数>>

当然还有LayoutInflater的inflate()的其他重载方法，大家可以自行了解下。

我利用下面的例子给大家走走这个流程 ：

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //1、该方法最终也会调用到 LayoutInflater的inflate()方法中去解析。
        setContentView(R.layout.main);
        
        //2、使用常见的API方法去解析xml布局文件，
        LayoutInflater layoutInflater = (LayoutInflater)getSystemService();
        View root = layoutInflater.inflate(R.layout.main, null);
    }
}

Step 1、获得LayoutInflater的引用。

路径：\frameworks\base\core\java\android\app\ContextImpl.java

/**
 * Common implementation of Context API, which provides the base
 * context object for Activity and other application components.
 */
class ContextImpl extends Context {
	if (WINDOW_SERVICE.equals(name)) {
        return WindowManagerImpl.getDefault();
    } else if (LAYOUT_INFLATER_SERVICE.equals(name)) {
        synchronized (mSync) {
            LayoutInflater inflater = mLayoutInflater;
            //是否已经赋值，如果是，直接返回引用
            if (inflater != null) {
                return inflater;
            }
            //返回一个LayoutInflater对象，getOuterContext()指的是我们的Activity、Service或者Application引用
            mLayoutInflater = inflater = PolicyManager.makeNewLayoutInflater(getOuterContext());
            return inflater;
        }
    } else if (ACTIVITY_SERVICE.equals(name)) {
        return getActivityManager();
    }...
}

继续去PolicyManager查询对应函数，看看内部实现。

路径：frameworks\base\core\java\com\android\internal\policy\PolicyManager.java

public final class PolicyManager {
	private static final String POLICY_IMPL_CLASS_NAME = "com.android.internal.policy.impl.Policy";
	private static final IPolicy sPolicy;   // 这可不是Binder机制额，这只是是一个接口，别想多啦
	static {
        // Pull in the actual implementation of the policy at run-time
        try {
            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
            sPolicy = (IPolicy)policyClass.newInstance();
        }
        ...
	}
	...
	public static LayoutInflater makeNewLayoutInflater(Context context) {
        return sPolicy.makeNewLayoutInflater(context); //继续去实现类中去查找
    }
}

IPolicy接口的实现对为Policy类。路径：/frameworks/base/policy/src/com/android/internal/policy/impl/Policy.java

//Simple implementation of the policy interface that spawns the right
//set of objects
public class Policy implements IPolicy{
	...
    public PhoneLayoutInflater makeNewLayoutInflater(Context context) {
        //实际上返回的是PhoneLayoutInflater类。
		return new PhoneLayoutInflater(context);
    }
}
//PhoneLayoutInflater继承至LayoutInflater类
public class PhoneLayoutInflater extends LayoutInflater {
	...
	/**
     * Instead of instantiating directly, you should retrieve an instance
     * through {@link Context#getSystemService}
     * 
     * @param context The Context in which in which to find resources and other
     *                application-specific things.
     * 
     * @see Context#getSystemService
     */
    public PhoneLayoutInflater(Context context) {
        super(context);
    }
	...
}

LayoutInflater是个抽象类，实际上我们返回的是PhoneLayoutInflater类，但解析过程的操作基本上是在

LayoutInflater中完成地。

Step 2、调用inflate()方法去解析布局文件。

public abstract class LayoutInflater {
    ...
    public View inflate(int resource, ViewGroup root) {
    	//继续看下个函数，注意root为null
        return inflate(resource, root, root != null); 
    }
    
    public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
    	//获取一个XmlResourceParser来解析XML文件---布局文件。
    	//XmlResourceParser类以及xml是如何解析的，大家自己有兴趣找找。
        XmlResourceParser parser = getContext().getResources().getLayout(resource);
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }
}
/**
 * The XML parsing interface returned for an XML resource.  This is a standard
 * XmlPullParser interface, as well as an extended AttributeSet interface and
 * an additional close() method on this interface for the client to indicate
 * when it is done reading the resource.
 */
public interface XmlResourceParser extends XmlPullParser, AttributeSet {
    /**
     * Close this interface to the resource.  Calls on the interface are no
     * longer value after this call.
     */
    public void close();
}

我们获得了一个当前应用程序环境的XmlResourceParser对象，该对象的主要作用就是来解析xml布局文件的。

XmlResourceParser类是个接口类，更多关于XML解析的，大家可以参考下面博客：

<<android之XmlResourceParser类使用实例>>

<<android解析xml文件的方式(其一)>>

<<android解析xml文件的方式(其二)>>

<<android解析xml文件的方式(其三)>>

Step 3 、真正地开始解析工作 。

public abstract class LayoutInflater {
    ...
    /**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     */
    //我们传递过来的参数如下： root 为null ， attachToRoot为false 。
    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context)mConstructorArgs[0];
            mConstructorArgs[0] = mContext;  //该mConstructorArgs属性最后会作为参数传递给View的构造函数
            View result = root;  //根View

            try {
                // Look for the root node.
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }
                ...
                final String name = parser.getName();  //节点名，即API中的控件或者自定义View完整限定名。
                if (TAG_MERGE.equals(name)) { // 处理<merge />标签
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }
                    //将<merge />标签的View树添加至root中，该函数稍后讲到。
                    rInflate(parser, root, attrs);
                } else {
                    // Temp is the root view that was found in the xml
                	//创建该xml布局文件所对应的根View。
                    View temp = createViewFromTag(name, attrs); 

                    ViewGroup.LayoutParams params = null;

                    if (root != null) {
                        // Create layout params that match root, if supplied
                    	//根据AttributeSet属性获得一个LayoutParams实例，记住调用者为root。
                        params = root.generateLayoutParams(attrs); 
                        if (!attachToRoot) { //重新设置temp的LayoutParams
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }
                    // Inflate all children under temp
                    //添加所有其子节点，即添加所有字View
                    rInflate(parser, temp, attrs);
                    
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
            } 
            ...
            return result;
        }
    }
    
    /*
     * default visibility so the BridgeInflater can override it.
     */
    View createViewFromTag(String name, AttributeSet attrs) {
    	//节点是否为View，如果是将其重新赋值，形如 <View class="com.qin.xxxView"></View>
        if (name.equals("view")) {  
            name = attrs.getAttributeValue(null, "class");
        }
        try {
            View view = (mFactory == null) ? null : mFactory.onCreateView(name,
                    mContext, attrs);  //没有设置工厂方法

            if (view == null) {
                //通过这个判断是Android API的View，还是自定义View
            	if (-1 == name.indexOf('.')) {
                    view = onCreateView(name, attrs); //创建Android API的View实例
                } else {
                    view = createView(name, null, attrs);//创建一个自定义View实例
                }
            }
            return view;
        } 
        ...
    }
    //获得具体视图的实例对象
    public final View createView(String name, String prefix, AttributeSet attrs) {
		Constructor constructor = sConstructorMap.get(name);
		Class clazz = null;
		//以下功能主要是获取如下三个类对象：
		//1、类加载器  ClassLoader
		//2、Class对象
		//3、类的构造方法句柄 Constructor
		try {
		    if (constructor == null) {
		    // Class not found in the cache, see if it's real, and try to add it
		    clazz = mContext.getClassLoader().loadClass(prefix != null ? (prefix + name) : name);
		    ...
		    constructor = clazz.getConstructor(mConstructorSignature);
		    sConstructorMap.put(name, constructor);
		} else {
		    // If we have a filter, apply it to cached constructor
		    if (mFilter != null) {
		        ...   
		    }
		}
		    //传递参数获得该View实例对象
		    Object[] args = mConstructorArgs;
		    args[1] = attrs;
		    return (View) constructor.newInstance(args);
		} 
		...
	}

}

这段代码的作用是获取xml布局文件的root View，做了如下两件事情

1、获取xml布局的View实例，通过createViewFromTag()方法获取，该方法会判断节点名是API 控件

还是自定义控件，继而调用合适的方法去实例化View。

2、判断root以及attachToRoot参数，重新设置root View值以及temp变量的LayoutParams值。

如果仔细看着段代码，不知大家心里有没有疑惑：当root为null时，我们的temp变量的LayoutParams值是为

null的，即它不会被赋值？有个View的LayoutParams值为空，那么，在系统中不会报异常吗？见下面部分

代码：

    //我们传递过来的参数如下： root 为null ， attachToRoot为false 。
    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            ...
            try {
                
                ...
                if (TAG_MERGE.equals(name)) { // 处理<merge />标签
                    ...
                } else {
                    // Temp is the root view that was found in the xml
                	//创建该xml布局文件所对应的根View。
                    View temp = createViewFromTag(name, attrs); 
                    ViewGroup.LayoutParams params = null;

                    //注意！！！ root为null时，temp变量的LayoutParams属性不会被赋值的。
                    if (root != null) {
                        // Create layout params that match root, if supplied
                    	//根据AttributeSet属性获得一个LayoutParams实例，记住调用者为root。
                        params = root.generateLayoutParams(attrs); 
                        if (!attachToRoot) { //重新设置temp的LayoutParams
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }
                    ...
                }
            } 
            ...
        }
    }

关于这个问题的详细答案，我会在后面讲到。这儿我简单说下，任何View树的顶层View被添加至窗口时，

一般调用WindowManager.addView()添加至窗口时，在这个方法中去做进一步处理。即使，LayoutParams

值为空，UI框架每次measure()时都忽略该View的LayoutParams值，而是直接传递MeasureSpec值至View树。

接下来，我们关注另外一个函数，rInflate()，该方法会递归调用每个View下的子节点，以当前View作为根View

形成一个View树。

    /**
     * Recursive method used to descend down the xml hierarchy and instantiate
     * views, instantiate their children, and then call onFinishInflate().
     */
    //递归调用每个字节点
    private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)
            throws XmlPullParserException, IOException {

        final int depth = parser.getDepth();
        int type;

        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {

            if (type != XmlPullParser.START_TAG) {
                continue;
            }
            final String name = parser.getName();
            
            if (TAG_REQUEST_FOCUS.equals(name)) { //处理<requestFocus />标签
                parseRequestFocus(parser, parent);
            } else if (TAG_INCLUDE.equals(name)) { //处理<include />标签
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);//解析<include />节点
            } else if (TAG_MERGE.equals(name)) { //处理<merge />标签
                throw new InflateException("<merge /> must be the root element");
            } else {
            	//根据节点名构建一个View实例对象
                final View view = createViewFromTag(name, attrs); 
                final ViewGroup viewGroup = (ViewGroup) parent;
                //调用generateLayoutParams()方法返回一个LayoutParams实例对象，
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflate(parser, view, attrs); //继续递归调用
                viewGroup.addView(view, params); //OK，将该View以特定LayoutParams值添加至父View中
            }
        }
        parent.onFinishInflate();  //完成了解析过程，通知....
    }

值得注意的是，每次addView前都调用了viewGroup.generateLayoutParams(attrs)去构建一个LayoutParams

实例，然后在addView()方法中为其赋值。参见如下代码：ViewGroup.java

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
	...
	
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }
	public static class LayoutParams {
        ... //会调用这个构造函数
        public LayoutParams(Context c, AttributeSet attrs) {
            TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
            setBaseAttributes(a,
                    R.styleable.ViewGroup_Layout_layout_width,
                    R.styleable.ViewGroup_Layout_layout_height);
            a.recycle();
        }
        protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {
            width = a.getLayoutDimension(widthAttr, "layout_width");
            height = a.getLayoutDimension(heightAttr, "layout_height");
        }
	
}

好吧 ~~ 我们还是探寻根底，去TypeArray类的getLayoutDimension()看看。

路径：/frameworks/base/core/java/android/content/res/TypedArray.java

public class TypedArray {
	...
	/**
     * Special version of {@link #getDimensionPixelSize} for retrieving
     * {@link android.view.ViewGroup}'s layout_width and layout_height
     * attributes.  This is only here for performance reasons; applications
     * should use {@link #getDimensionPixelSize}.
     * 
     * @param index Index of the attribute to retrieve.
     * @param name Textual name of attribute for error reporting.
     * 
     * @return Attribute dimension value multiplied by the appropriate 
     * metric and truncated to integer pixels.
     */
    public int getLayoutDimension(int index, String name) {
        index *= AssetManager.STYLE_NUM_ENTRIES;
        final int[] data = mData;
        //获得属性对应的标识符 ， Identifies，目前还没有仔细研究相关类。
        final int type = data[index+AssetManager.STYLE_TYPE];
        if (type >= TypedValue.TYPE_FIRST_INT
                && type <= TypedValue.TYPE_LAST_INT) {
            return data[index+AssetManager.STYLE_DATA];
        } else if (type == TypedValue.TYPE_DIMENSION) { //类型为dimension类型
            return TypedValue.complexToDimensionPixelSize(
                data[index+AssetManager.STYLE_DATA], mResources.mMetrics);
        }
        //没有提供layout_weight和layout_height会来到此处 ，这儿会报异常！
        //因此布局文件中的View包括自定义View必须加上属性layout_weight和layout_height。
        throw new RuntimeException(getPositionDescription()
                + ": You must supply a " + name + " attribute.");
    }
	...
}

从上面得知， 我们将View的AttributeSet属性传递给generateLayoutParams()方法，让其构建合适地

LayoutParams对象，并且初始化属性值weight和height。同时我们也得知布局文件中的View包括自定义View

必须加上属性layout_weight和layout_height，否则会报异常。

Step 3 主要做了如下事情：
首先，获得了了布局文件地root View，即布局文件中最顶层的View。

其次，通过递归调用，我们形成了整个View树以及设置了每个View的LayoutParams对象。

总结：通过对布局文件的解析流程的学习，也就是转换为View树的过程，我们明白了解析过程的个中奥妙，以及

设置ViewLayoutParams对象的过程。但是，我们这儿只是简单的浮光掠影，更深层次的内容希望大家能深入学习。

本来是准备接下去往下写的，但无奈贴出来的代码太多，文章有点长而且自己也有点凌乱了，因此决定做两篇

博客发表吧。下篇内容包括如下方面：

1、MeasureSpec类说明 ;

2、measure过程中如何正确设置每个View的长宽 ;

3、UI框架正确设置顶层View的LayoutParams对象，对Activity而言，顶层View则是DecorView，

其他的皆是普通View了。