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：

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

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 objectspublic 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 Constructortry {    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。