Android XML布局与View之间的转换

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Android的布局方式有两种，一种是通过xml布局，一种是通过java代码布局，两种布局方式各有各的好处，当然也可以相互混合使用。很多人都习惯用xml布局，那xml布局是如何转换成view的呢？本文从源码的角度来简单分析下整个过程。

首先，创建一个新的项目，默认生成一个activity，其中xml布局很简单，就一个RelativeLayout套了一个ImageView，代码及效果如下：

public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);    }}

其中关键之处就在于调用了父类Activity的setContentView方法：

    /**     * Set the activity content from a layout resource.  The resource will be     * inflated, adding all top-level views to the activity.     *      * @param layoutResID Resource ID to be inflated.     */    public void setContentView(int layoutResID) {        getWindow().setContentView(layoutResID);    }

getWindow返回的是PhoneWindow实例，那我们直接来看PhoneWindow中的setContentView方法：

    @Override    public void setContentView(int layoutResID) {        if (mContentParent == null) {            installDecor();        } else {            mContentParent.removeAllViews();        }        mLayoutInflater.inflate(layoutResID, mContentParent);        final Callback cb = getCallback();        if (cb != null) {            cb.onContentChanged();        }    }

我们知道每个activity实际都对应一个PhoneWindow，拥有一个顶层的DecorView，DecorView继承自FrameLayout，作为根View，其中包含了一个标题区域和内容区域，这里的mContentParent就是其内容区域。关于PhoneWindow和DecorView的具体内容，读者可自行查阅。这段代码的意思很简单，如果DecorView的内容区域为null，就先初始化，否则就先把内容区域的子View全部移除，最后再引入layout布局，所以，关键在于mLayoutInflater.inflate(layoutResID, mContentParent); 代码继续往下看：

    public View inflate(int resource, ViewGroup root) {        return inflate(resource, root, root != null);    }

    public View inflate(int resource, ViewGroup root, boolean attachToRoot) {        if (DEBUG) System.out.println("INFLATING from resource: " + resource);        XmlResourceParser parser = getContext().getResources().getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

这里首先根据layout布局文件的Id生成xml资源解析器，然后再调用inflate(parser, root, attachToRoot)生成具体的view。XmlResourceParser是继承自XmlPullParser和AttributeSet的接口，这里的parser其实是XmlBlock的内部类Parser的实例。

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;            View result = root;            try {                // Look for the root node.                int type;                while ((type = parser.next()) != XmlPullParser.START_TAG &&                        type != XmlPullParser.END_DOCUMENT) {                    // Empty                }                if (type != XmlPullParser.START_TAG) {                    throw new InflateException(parser.getPositionDescription()                            + ": No start tag found!");                }                final String name = parser.getName();                                if (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }                if (TAG_MERGE.equals(name)) {                    if (root == null || !attachToRoot) {                        throw new InflateException("<merge /> can be used only with a valid "                                + "ViewGroup root and attachToRoot=true");                    }                    rInflate(parser, root, attrs);                } else {                    // Temp is the root view that was found in the xml                    View temp = createViewFromTag(name, attrs);                    ViewGroup.LayoutParams params = null;                    if (root != null) {                        if (DEBUG) {                            System.out.println("Creating params from root: " +                                    root);                        }                        // Create layout params that match root, if supplied                        params = root.generateLayoutParams(attrs);                        if (!attachToRoot) {                            // Set the layout params for temp if we are not                            // attaching. (If we are, we use addView, below)                            temp.setLayoutParams(params);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }                    // Inflate all children under temp                    rInflate(parser, temp, attrs);                    if (DEBUG) {                        System.out.println("-----> done inflating children");                    }                    // 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;                    }                }            } catch (XmlPullParserException e) {                InflateException ex = new InflateException(e.getMessage());                ex.initCause(e);                throw ex;            } catch (IOException e) {                InflateException ex = new InflateException(                        parser.getPositionDescription()                        + ": " + e.getMessage());                ex.initCause(e);                throw ex;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;            }            return result;        }    }

第21行，获取xml根节点名：

final String name = parser.getName();

第39行根据节点名创建临时View（temp），这个临时view（temp）也是xml布局的根view：

View temp = createViewFromTag(name, attrs);

第61行，在临时view（temp）的节点下创建所有子View，显然这个方法里是通过遍历xml所有子view节点，调用createViewFromTag方法生成子view并加载到根view中：

rInflate(parser, temp, attrs);

第68到76行，则是判断，如果inflate方法有父view，则把临时view（temp）加载到父view中再返回，如果没有，则直接返回临时view（temp），我们这里调用inflate方法的时候显然有父view，即mContentParent，也就是最顶层view DecorView的内容区域。这里最关键有两个方法，一个是createViewFromTag，另一个是rInflate，现在来逐一分析：createViewFromTag实际最终调用的是createView方法：

public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {        Constructor constructor = sConstructorMap.get(name);        Class clazz = null;        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);                                if (mFilter != null && clazz != null) {                    boolean allowed = mFilter.onLoadClass(clazz);                    if (!allowed) {                        failNotAllowed(name, prefix, attrs);                    }                }                constructor = clazz.getConstructor(mConstructorSignature);                sConstructorMap.put(name, constructor);            } else {                // If we have a filter, apply it to cached constructor                if (mFilter != null) {                    // Have we seen this name before?                    Boolean allowedState = mFilterMap.get(name);                    if (allowedState == null) {                        // New class -- remember whether it is allowed                        clazz = mContext.getClassLoader().loadClass(                                prefix != null ? (prefix + name) : name);                                                boolean allowed = clazz != null && mFilter.onLoadClass(clazz);                        mFilterMap.put(name, allowed);                        if (!allowed) {                            failNotAllowed(name, prefix, attrs);                        }                    } else if (allowedState.equals(Boolean.FALSE)) {                        failNotAllowed(name, prefix, attrs);                    }                }            }            Object[] args = mConstructorArgs;            args[1] = attrs;            return (View) constructor.newInstance(args);        } catch (NoSuchMethodException e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (prefix != null ? (prefix + name) : name));            ie.initCause(e);            throw ie;        } catch (ClassNotFoundException e) {            // If loadClass fails, we should propagate the exception.            throw e;        } catch (Exception e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (clazz == null ? "<unknown>" : clazz.getName()));            ie.initCause(e);            throw ie;        }    }

其实这个方法很简单，就是通过xml节点名，通过反射获取view的实例再返回，其中先去map中查询构造函数是否存在，如果存在则直接根据构造函数创建实例，这样做的好处是不用每次都通过class去获取构造函数再创建实例，我们看第18通过类实例获取构造函数：

constructor = clazz.getConstructor(mConstructorSignature);

其中mConstructorSignature定义如下：

    private static final Class[] mConstructorSignature = new Class[] {            Context.class, AttributeSet.class};

很显然，这里用的是带有Context和AttributeSet两个参数的构造函数，这也就是为什么，自定义view一定要重载这个构造函数的原因。最后就是rInflate方法：

<pre name="code" class="html">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)) {                parseRequestFocus(parser, parent);            } else if (TAG_INCLUDE.equals(name)) {                if (parser.getDepth() == 0) {                    throw new InflateException("<include /> cannot be the root element");                }                parseInclude(parser, parent, attrs);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException("<merge /> must be the root element");            } else {                final View view = createViewFromTag(name, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflate(parser, view, attrs);                viewGroup.addView(view, params);            }        }        parent.onFinishInflate();    }

实这个方法也很简单，就是通过parser解析xml节点再生成对应View的过程。

XML转换成View的过程就是这样了，如有错误之处，还望指正，回到本文开头，其实我们还可以这样写：

@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);View content = LayoutInflater.from(this).inflate(R.layout.activity_main, null);setContentView(content);}

大家发现问题没，相较于本文开头的写法，后面的灰色布局变成全屏了，我们来看看xml代码：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="300dip"    android:layout_height="300dip"    android:background="#888888"    tools:context=".MainActivity" >    <ImageView        android:layout_width="200dip"        android:layout_height="200dip"        android:background="#238712"        android:contentDescription="@null" /></RelativeLayout>

我明明设置了RelativeLayout的宽度和高度分别为300dip，但为什么全屏了？这是因为layout_width和layout_height是相对于父布局而言的，我们这里inflate的时候设置的父布局为null，所以这个属性设置也就无效了，我们加一个父布局再试试：

@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);RelativeLayout rootView = new RelativeLayout(this);View content = LayoutInflater.from(this).inflate(R.layout.activity_main, rootView);setContentView(content);}

OK，我们白色的背景又有了~

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