LayoutInflater源码解析

Android使用LayoutInflater来进行布局加载，通常获取方式有两种：

第一种：

LayoutInflater layoutInflater = LayoutInflater.from(context); 

第二种：

LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 

从源码中可以看出第一种是第二种的封装简化，便于使用：

 public static LayoutInflater from(Context context) {
         LayoutInflater LayoutInflater =
                 (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
         if (LayoutInflater == null) {
             throw new AssertionError("LayoutInflater not found.");
         }
         return LayoutInflater;
     }

我们通过调用inflate方法便可以完成对布局的加载：

layoutInflater.inflate(resource, root, true);  

LayoutInflater中的inflate方法有若干种重载方式，最终都调用了如下代码：

 public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
         synchronized (mConstructorArgs) {
             //获取xml中属性信息
             final AttributeSet attrs = Xml.asAttributeSet(parser);
             Context lastContext = (Context)mConstructorArgs[0];
             mConstructorArgs[0] = mContext;
             View result = root;

             try {
                 // 查找根节点.
                 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!");
                 }
                 //获取根节点名称
                 final String name = parser.getName();

                 if (DEBUG) {
                     System.out.println("**************************");
                     System.out.println("Creating root view: "
                             + name);
                     System.out.println("**************************");
                 }
                 //如果是merge标签，必须保证父节点不为null且attachToRoot为true
                 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, false);
                 } else {
                     //代表布局文件中根节点的view
                     View temp;
                     if (TAG_1995.equals(name)) {
                         temp = new BlinkLayout(mContext, attrs);
                     } else {
                         //利用反射，通过root名称创建view
                         temp = createViewFromTag(root, 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
                         //当提供了父容器时，由父容器根据属性值创建布局参数
                         params = root.generateLayoutParams(attrs);
                         if (!attachToRoot) {
                             // Set the layout params for temp if we are not
                             // attaching. (If we are, we use addView, below)
                             //当不把当前view附加到父容器中，则设置获取到的布局参数
                            //否则使用下面的addView方法设置
                             temp.setLayoutParams(params);
                         }
                     }

                     if (DEBUG) {
                         System.out.println("-----> start inflating children");
                     }
                     // Inflate all children under temp
                    //递归调用此方法加载子布局
                     rInflate(parser, temp, attrs, true);
                     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;
         }
     }

这里，Android使用了PULL来解析xml布局文件，并通过反射来创建出当前view：

temp = createViewFromTag(root, name, attrs);

我们查看一下源码：

 View createViewFromTag(View parent, String name, AttributeSet attrs) {
         if (name.equals("view")) {
             name = attrs.getAttributeValue(null, "class");
         }

         if (DEBUG) System.out.println("******** Creating view: " + name);

         try {
             View view;
             if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);
             else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);
             else view = null;

             if (view == null && mPrivateFactory != null) {
                 view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);
             }

             if (view == null) {
                 if (-1 == name.indexOf('.')) {
                     view = onCreateView(parent, 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;
         }
     }

里面根据不同情况，调用了onCreateView方法，利用反射来创建view。其中可以使用指定的factory来创建view，这样的钩子设计使得inflate方法变得十分灵活。

然后调用rInflate(parser, temp, attrs, true)方法来递归查找temp中的子view，并添加到上层view中：

 void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
             boolean finishInflate) 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 if (TAG_1995.equals(name)) {
                 final View view = new BlinkLayout(mContext, attrs);
                 final ViewGroup viewGroup = (ViewGroup) parent;
                 final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                 rInflate(parser, view, attrs, true);
                 viewGroup.addView(view, params);
             } else {
                 final View view = createViewFromTag(parent, name, attrs);
                 final ViewGroup viewGroup = (ViewGroup) parent;
                 final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                 rInflate(parser, view, attrs, true);
                 viewGroup.addView(view, params);
             }
         }

         if (finishInflate) parent.onFinishInflate();
     }

里面也用到onCreateView方法创建子view，然后将其加入到父view中返回。

通过查看上面的源码，我们可以发现inflate方法中的三个参数int resource, ViewGroup root, boolean attachToRoot的作用如下：

resource指定了要加载的view，root作为view外面一层的父容器，attachToRoot表示是否将view加入到父容器。

当指定了父容器，并且attachToRoot为true，则将view加入到父容器中。

如果指定了父容器，却将attachToRoot设置为false，那么只是从父容器中生成了view布局的参数并设置给view

当未指定父容器时，直接返回view本身。

总结

通过研究LayoutInflater源码的设计，我们了解到代码的执行细节的同时，也可以发现：

LayoutInflater创建view对象时候使用了简单工厂模式，并通过加入钩子方法，利用抽象工厂模式让coder可以使用自定义的工厂方法来创建view。