Android的布局方式有兩種，一種是通過xml布局，一種是通過java代碼布局，兩種布局方式各有各的好處，當(dāng)然也可以相互混合使用。很多人都習(xí)慣用xml布局，那xml布局是如何轉(zhuǎn)換成view的呢？本文從源碼的角度來簡單分析下整個過程。

首先，創(chuàng)建一個新的項(xiàng)目，默認(rèn)生成一個activity，其中xml布局很簡單，就一個RelativeLayout套了一個ImageView，代碼及效果如下：

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

界面1

其中關(guān)鍵之處在于調(diào)用了父類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實(shí)例，那我們直接來看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();  
    }  
}

我們知道每個activity實(shí)際都對應(yīng)一個PhoneWindow，擁有一個頂層的DecorView，DecorView繼承自FrameLayout，作為根View，其中包含了一個標(biāo)題區(qū)域和內(nèi)容區(qū)域，這里的mContentParent就是其內(nèi)容區(qū)域。關(guān)于PhoneWindow和DecorView的具體內(nèi)容，讀者可自行查閱。這段代碼的意思很簡單，如果DecorView的內(nèi)容區(qū)域?yàn)閚ull，就先初始化，否則就先把內(nèi)容區(qū)域的子View全部移除，最后再引入layout布局，所以，關(guān)鍵在于mLayoutInflater.inflate(layoutResID, mContentParent); 代碼繼續(xù)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();  
    }  
} 

這里首先根據(jù)layout布局文件的Id生成xml資源解析器，然后再調(diào)用inflate(parser, root, attachToRoot)生成具體的view。XmlResourceParser是繼承自XmlPullParser和AttributeSet的接口，這里的parser其實(shí)是XmlBlock的內(nèi)部類Parser的實(shí)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，獲取xml根節(jié)點(diǎn)名：

final String name = parser.getName(); 

第39行根據(jù)節(jié)點(diǎn)名創(chuàng)建臨時View（temp），這個臨時view（temp）也是xml布局的根view：

View temp = createViewFromTag(name, attrs);  

第61行，在臨時view（temp）的節(jié)點(diǎn)下創(chuàng)建所有子View，顯然這個方法里是通過遍歷xml所有子view節(jié)點(diǎn)，調(diào)用createViewFromTag方法生成子view并加載到根view中：

rInflate(parser, temp, attrs);  

第68到76行，則是判斷，如果inflate方法有父view，則把臨時view（temp）加載到父view中再返回，如果沒有，則直接返回臨時view（temp），我們這里調(diào)用inflate方法的時候顯然有父view，即mContentParent，也就是最頂層view DecorView的內(nèi)容區(qū)域。這里最關(guān)鍵有兩個方法，一個是createViewFromTag，另一個是rInflate，現(xiàn)在來逐一分析：createViewFromTag實(shí)際最終調(diào)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;  
    }  
}  

其實(shí)這個方法很簡單，就是通過xml節(jié)點(diǎn)名，通過反射獲取view的實(shí)例再返回，其中先去map中查詢構(gòu)造函數(shù)是否存在，如果存在則直接根據(jù)構(gòu)造函數(shù)創(chuàng)建實(shí)例，這樣做的好處是不用每次都通過class去獲取構(gòu)造函數(shù)再創(chuàng)建實(shí)例，我們看第18行通過類實(shí)例獲取構(gòu)造函數(shù)：

constructor = clazz.getConstructor(mConstructorSignature);

其中mConstructorSignature定義如下：

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

很顯然，這里用的是帶有Context和AttributeSet兩個參數(shù)的構(gòu)造函數(shù)，這也就是為什么，自定義view一定要重載這個構(gòu)造函數(shù)的原因。最后就是rInflate方法：

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();  
}  

實(shí)這個方法也很簡單，就是通過parser解析xml節(jié)點(diǎn)再生成對應(yīng)View的過程。
XML轉(zhuǎn)換成View的過程就是這樣了，如有錯誤之處，還望指正，回到本文開頭，其實(shí)我們還可以這樣寫：

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

界面2

大家發(fā)現(xiàn)問題沒，相較于本文開頭的寫法，后面的灰色布局變成全屏了，我們來看看xml代碼：

<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>

我明明設(shè)置了RelativeLayout的寬度和高度分別為300dip，但為什么全屏了？這是因?yàn)閘ayout_width和layout_height是相對于父布局而言的，我們這里inflate的時候設(shè)置的父布局為null，所以這個屬性設(shè)置也就無效了，指定一個父布局就可以了，例如：

@Override  
protected 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);  
}

現(xiàn)在，界面顯示效果就和“界面1”相同了。