View视图框架源码分析之一:android是如何创建一个view的

View是所有控件的一个基类,无论是布局(Layout),还是控件(Widget)都是继承自View类。只不过layout是一个特殊的view,它里面创建一个view的数组可以包含其他的view而已。 

这一篇文章把所有的layout和widget都统称为view,那么android是如何创建一个view的呢? 

一。在代码中直接new出来。 
比如说你要创建一个TextView的实例,那么你可以这样写:``` TextView text = new TextView(c);  //c为context对象,表明textview是在此对象中运行的。


  
二。把控件写在xml文件中然后通过LayoutInflater初始化一个view。  
注意:下面的内容不是顺序的看的而是交替的看的。否则可能弄迷糊。  
可以通过  

//通过系统提供的实例获得一个LayoutInflater对象 LayoutInflater inflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); //第一个参数为xml文件中view的id,第二个参数为此view的父组件,可以为null,android会自动寻找它是否拥有父组件 View view = inflater.inflate(R.layout.resourceid, null);


这样也得到了一个view的实例,让我们一步一步来看,这个view是怎么new出来的。  
看类android.view.LayoutInflater  

public View inflate(int resource, ViewGroup root) {      return inflate(resource, root, root != null);  }   public View inflate(int resource, ViewGroup root, boolean attachToRoot) {      /可以看到通过resource id返回了一个XmlResourceParser,通过类名就可以猜测      这是一个xml的解析类。但点进去一看,发现它只是一个接口,它继承自 XmlPullParser用于pull方式解析xml的接口。和AttributeSet用于获取此view的所有属性。      那么需要能找到它的实现类。先看下面resource类。      /      XmlResourceParser parser = getContext().getResources().getLayout(resource);      try {          return inflate(parser, root, attachToRoot);      } finally {          parser.close();      }  }    /    * 终于到了重点,获取一个这个View的实例    /  public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {      synchronized (mConstructorArgs) {          /*            * 获取一个实现此AttributeSet的实例。因为此XmlPullParser是继承自AttributeSet            * 的,所以parser对象可以直接作为一个AttributeSet对象。也可以用组合的方式            * 把parser传递给另外一个实现自AttributeSet的对象,来获取一个AttributeSet实例。          /          final AttributeSet attrs = Xml.asAttributeSet(parser);          mConstructorArgs[0] = mContext;     //构造函数的参数,第一个值是此view运行所在的对象context          View result = root;            try {              // parser同时也继承了xmlPullParser,所以可以用pull解析来获取此view的根节点              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!");              }              //获得根节点标签的名字              final String name = parser.getName();                             //如果它的根节点是一个merge对象,则必须手动设置此view的父节点,否则抛出异常              //因为由merge创建的xml文件,常常被其他layout所包含              if (TAG_MERGE.equals(name)) {                  if (root == null || !attachToRoot) {                      throw new InflateException(" can be used only with a valid "                              + "ViewGroup root and attachToRoot=true");                  }                    rInflate(parser, root, attrs);              } else {                  // 此inflate的xml文件中的root view。即我们通过inflate返回得到的view                  View temp = createViewFromTag(name, attrs);                    ViewGroup.LayoutParams params = null;                    if (root != null) {                      // 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);                      }                  }                      // 加载temp下所有的子view                  rInflate(parser, temp, attrs);                      //如果给出了root,则把此view添加到root中去                  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;          }            return result;      }  }      /   *   * 有上至下递归的初始化所有子view和子view的子view。在此方法被调用完成后   * 会调用此view的parent view的onFinishInflate方法。表明其子view全部加载完毕   /  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(" cannot be the root element");              }              parseInclude(parser, parent, attrs);          } else if (TAG_MERGE.equals(name)) {              throw new InflateException(" must be the root element");          } else {            //看这里,创建view的方法。而且这里已经重新获得了它的              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();  }      View createViewFromTag(String name, AttributeSet attrs) {      if (name.equals("view")) {          name = attrs.getAttributeValue(null, "class");      }        if (DEBUG) System.out.println("******* Creating view: " + name);        try {          View view = (mFactory == null) ? null : mFactory.onCreateView(name,                  mContext, attrs);            if (view == null) {              if (-1 == name.indexOf('.')) {      //这里只是为了判断xml文件中tag的属性是否加了包名                  view = onCreateView(name, attrs);              } else {                  view = createView(name, null, attrs);              }          }            if (DEBUG) System.out.println("Created view is: " + view);          return view;        } catch (InflateException e) {          throw e;        } catch (ClassNotFoundException e) {          InflateException ie = new InflateException(attrs.getPositionDescription()                  + ": Error inflating class " + name);          ie.initCause(e);          throw ie;        } catch (Exception e) {          InflateException ie = new InflateException(attrs.getPositionDescription()                  + ": Error inflating class " + name);          ie.initCause(e);          throw ie;      }  }    /**  * 真正创建一个view的方法,  * 此方法是用反射获取构造器来实例对象而不是直接new出来这是为了处于性能优化考虑,  * 同一个类名的不同对象,可以直接得到缓存的构造器直接获取一个构造器对象实例。而不需要  * 重复进行new操作。  *  * @param name 此View的全名  * @param prefix 前缀,值为 "android.view."其实就是是否包含包名  * @param attrs 此view的属性值,传递给此view的构造函数  */   public final View createView(String name, String prefix, AttributeSet attrs)          throws ClassNotFoundException, InflateException {      Constructor constructor = sConstructorMap.get(name);    //缓存中是否已经有了一个构造函数      Class clazz = null;        try {          if (constructor == null) {              //通过类名获得一个class对象              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);                  }              }              //通过参数类型获得一个构造器,参数列表为context,attrs              constructor = clazz.getConstructor(mConstructorSignature);              sConstructorMap.put(name, constructor);     //把此构造器缓存起来          } 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;        //args[0]已经在前面初始好了。这里只要初始化args[1]          return (View) constructor.newInstance(args);        //通过反射new出一个对象。。大功告成        } 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 ? "" : clazz.getName()));          ie.initCause(e);          throw ie;      }  }


在类android.content.res.Resources类中获取XmlResourceParser对象;

    public XmlResourceParser getLayout(int id) throws NotFoundException {         return loadXmlResourceParser(id, "layout");     }   /package/ XmlResourceParser loadXmlResourceParser(int id, String type)             throws NotFoundException {         synchronized (mTmpValue) {             /TypedValue对象保存了一些有关resource 的数据值,比如说,对于一个view来说,在xml                 文件中可以定义许多属性,TypedValue保存了其中一个属性的相关信息,包括此属性的值的类型                 type,是boolean还是color还是reference还是String,这些在attr.xml文件下都有定义。                 它的值的字符串名称;一个属性有多个值时,它从xml文件中获取的值它的顺序data;如果此属性的值                 的类型是一个reference则保存它的resource id的等等。             /             TypedValue value = mTmpValue;             getValue(id, value, true);             if (value.type == TypedValue.TYPE_STRING) {                 return loadXmlResourceParser(value.string.toString(), id,                         value.assetCookie, type);             }             throw new NotFoundException(                     "Resource ID #0x" + Integer.toHexString(id) + " type #0x"                     + Integer.toHexString(value.type) + " is not valid");         }     }       /        *  getValue方法,id表示要查找的控件的 id,outValue是一个对象,用于保存一些属性相关信息        *  resolveRefs为true表明,当通过属性id找到xml文件中的标签时,比如是一个<Button  android:id="@+id/button"/>        * 它的值是一个引用reference,则继续解析获得这个id的值。这里看AssetManager类的实现*/     public void getValue(int id, TypedValue outValue, boolean resolveRefs)             throws NotFoundException {         boolean found = mAssets.getResourceValue(id, outValue, resolveRefs);         if (found) {             return;         }         throw new NotFoundException("Resource ID #0x"                                     + Integer.toHexString(id));     }           /package/ XmlResourceParser loadXmlResourceParser(String file, int id,             int assetCookie, String type) throws NotFoundException {         if (id != 0) {             try {                 //取缓存                 synchronized (mCachedXmlBlockIds) {                     // First see if this block is in our cache.                     final int num = mCachedXmlBlockIds.length;                     for (int i=0; i<num; i++) {                         if (mCachedXmlBlockIds[i] == id) {                             //System.out.println("**** REUSING XML BLOCK!  id="                             //                   + id + ", index=" + i);                             return mCachedXmlBlocks[i].newParser();                         }                     }                      //第一次加载时,会打开这个文件获取一个xml数据块对象。                    // 这里先看AssetManager类的实现                     XmlBlock block = mAssets.openXmlBlockAsset(                             assetCookie, file);                       //下面会把此xmlBlock对象缓存起来,保存id和block,                     //以后如果是同样的id,直接在缓存中取XmlBlock。                     //这样就不用再在本地方法中打开文件创建解析树了。                     if (block != null) {                          int pos = mLastCachedXmlBlockIndex+1;                         if (pos >= num) pos = 0;                         mLastCachedXmlBlockIndex = pos;                         XmlBlock oldBlock = mCachedXmlBlocks[pos];                         if (oldBlock != null) {                             oldBlock.close();                         }                         mCachedXmlBlockIds[pos] = id;                         mCachedXmlBlocks[pos] = block;                         //返回的内部类继承了XmlResourceParser,在APi中此类是隐藏的                         return block.newParser();                     }                 }             } catch (Exception e) {                 NotFoundException rnf = new NotFoundException(                         "File " + file + " from xml type " + type + " resource ID #0x"                         + Integer.toHexString(id));                 rnf.initCause(e);                 throw rnf;             }         }           throw new NotFoundException(                 "File " + file + " from xml type " + type + " resource ID #0x"                 + Integer.toHexString(id));     }


android.content.res.AssetManager类

/package/ final boolean getResourceValue(int ident,                                            TypedValue outValue,                                            boolean resolveRefs) {     int block = loadResourceValue(ident, outValue, resolveRefs);     if (block >= 0) {         if (outValue.type != TypedValue.TYPE_STRING) {             return true;         }         //mStringBlocks通过本地方法保存所有布局文件的文件名         outValue.string = mStringBlocks[block].get(outValue.data);         return true;     }     return false; }    //这是一个本地方法,是在本地方法中获取这个控件信息,返回通过此控件的id找到的文件名  //的位置,由于个人对c++不是很了解,只初略的解释本地方法的一些功能。  //对于的JNI文件位于:\frameworks\base\core\jni\android_util_AssetManager.cpp  private native final int loadResourceValue(int ident, TypedValue outValue,                                            boolean resolve);             /**      * 通过文件名,在本地方法中找到这个xml文件,并且在本地方法中生成一个xml解析对象。      * 返回一个id,这个id对应java中的xmlBlock对象。这样xml文件就被load进了内存。       * 也就是android所说的预编译,以后再访问只要直接去取数据即可      /     /package/ final XmlBlock openXmlBlockAsset(int cookie, String fileName)     throws IOException {     synchronized (this) {         if (!mOpen) {             throw new RuntimeException("Assetmanager has been closed");         }         int xmlBlock = openXmlAssetNative(cookie, fileName);         if (xmlBlock != 0) {             /             * 在XmlBlock对象中,终于到找了实现XmlResourceParser接口的类             * Parser,它是XmlBlock的一个内部类。这里面可以获取所有xml文件中的内容。             * 不管是属性还是Tag标签。这里xmlBlock是用来与本地类中的解析树对象对应的。             * 所有的解析方法,其实都是调用的本地xml解析树中的方法。所以此类中有大量的             * 本地方法。             */             XmlBlock res = new XmlBlock(this, xmlBlock);             incRefsLocked(res.hashCode());             return res;         }     }     throw new FileNotFoundException("Asset XML file: " + fileName); }


三 。通过view.findViewById(resourceid)获得一个view的实例  
android.View.View类中

//调用了通过id检索view的方法 public final View findViewById(int id) {     if (id < 0) {         return null;     }     return findViewTraversal(id); }   //不是吧,这不是坑爹吗?猜想肯定是被viewgroup重写了 protected View findViewTraversal(int id) {     if (id == mID) {         return this;     }     return null; }


android.View.ViewGroup类中

//哈哈,果然重写了此方法。其实就是在viewgroup包含的 //子view数组中进行遍历。那么view是什么时候被加入进 //viewgroup中的呢?如果是在代码中写,肯定是直接使用 //addView方法把view加入viewGroup。如果写在xml布局文件 //中,其实是在第二种方法中被加入view的。inflate加载父view //时会同时把其所有的子view加载完,同时addView到父view中  protected View findViewTraversal(int id) {         if (id == mID) {             return this;         }           final View[] where = mChildren;         final int len = mChildrenCount;           for (int i = 0; i < len; i++) {             View v = where[i];               if ((v.mPrivateFlags & IS_ROOT_NAMESPACE) == 0) {                 v = v.findViewById(id);                   if (v != null) {                     return v;                 }             }         }           return null;     }


四。通过activity的setContentView方法和findViewById获取一个view的实例。  
它是通过  
getWindow().setContentView(layoutResID);设置window对象的view  
再来看看window对象是在哪里获得到的,在类Activity中找到  
mWindow = PolicyManager.makeNewWindow(this);  
它是由PolicyManager生成的。  
找到com.android.internal.policy.PolicyManager,找到方法

//window是由sPolicy对象创建的 public static Window makeNewWindow(Context context) {     return sPolicy.makeNewWindow(context); }   //sPolicy对象是通过反射,获取的一个实例 //此类的实现在com.android.internal.policy.impl.Policy中 private static final String POLICY_IMPL_CLASS_NAME =     "com.android.internal.policy.impl.Policy";   private static final IPolicy sPolicy;   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();     } catch (ClassNotFoundException ex) {         throw new RuntimeException(                 POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);     } catch (InstantiationException ex) {         throw new RuntimeException(                 POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);     } catch (IllegalAccessException ex) {         throw new RuntimeException(                 POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);     } }


找到com.android.internal.policy.impl.Policy类

public PhoneWindow makeNewWindow(Context context) {     return new PhoneWindow(context); }


它其实是一个phoneWindow对象,继承自window对象  
找到com.android.internal.policy.impl.phoneWindow 看它内部是如何把resourceid加载成一个view的

private ViewGroup mContentParent;  //这是window的顶层视图,它包含一些窗口的装饰,比图title bar,状态栏等等  private DecorView mDecor;    //这里的layoutResID也是由mLayoutInflater进行加载的,加载的方式与第二种方法一样。  //只不过这里把的到的view变成了mContentParent的子view @Override  public void setContentView(int layoutResID) {      if (mContentParent == null) {          installDecor();      } else {          mContentParent.removeAllViews();      }      mLayoutInflater.inflate(layoutResID, mContentParent);      final Callback cb = getCallback();      if (cb != null) {       //这是回调方法,表明mContentParent的子view已经发生改变          cb.onContentChanged();      }  }    //再来看看mContentParent究竟是何物,它肯定是一个viewGroup  private void installDecor() {      if (mDecor == null) {          mDecor = generateDecor();          mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);          mDecor.setIsRootNamespace(true);      }      if (mContentParent == null) {          mContentParent = generateLayout(mDecor);            mTitleView = (TextView)findViewById(com.android.internal.R.id.title);          if (mTitleView != null) {       //这里设置的是是否隐藏titleContainer,即头部titlebar              if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {                  View titleContainer = findViewById(com.android.internal.R.id.title_container);                  if (titleContainer != null) {                      titleContainer.setVisibility(View.GONE);                  } else {                      mTitleView.setVisibility(View.GONE);                  }                  if (mContentParent instanceof FrameLayout) {                      ((FrameLayout)mContentParent).setForeground(null);                  }              } else {                  mTitleView.setText(mTitle);              }          }      }  }       //当顶层view为null是,new了一个DecorView   protected DecorView generateDecor() {      return new DecorView(getContext(), -1);  }    //这里生成了mContentParent。  protected ViewGroup generateLayout(DecorView decor) {            mDecor.startChanging();      //根据window的不同参数选择layoutResource      View in = mLayoutInflater.inflate(layoutResource, null);      decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);      if (contentParent == null) {          throw new RuntimeException("Window couldn't find content container view");      }      return contentParent;  }    //顶层view是一个framelayout  private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {           public DecorView(Context context, int featureId) {              super(context);              mFeatureId = featureId;      }  }        //下面说明findVIewById    //首先是获取顶层view,即继承自FrameLayout的viewgorup  @Override  public final View getDecorView() {      if (mDecor == null) {          installDecor();      }      return mDecor;  }    //然后mDecor.findViewById根据id获取它的子view  //这里就是通过第三种方法获取它的子view