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带着问题去看源码——LayoutInflater

序言:最近在做一个类似商城的app(电商类),然后我就用了阿里新开源的VLayout布局来做,简直不要太方便啊,有空的话我会写一篇关于VLayout的文章,它的底层也是RecyclerView,只不过对LayoutManager和Adapter进行了封装而已,但是我们用的时候也需要对它进行再一次封装,因为像addHeader和addFooter这些都是没有的,在这个过程中我偶然发现用LayoutInflater实例的header和footer宽高竟然没用,这让我很捉急(因为项目很赶),然后百度才发现LayoutInflater通过XML加载布局的时候会产生不同的现象。

1、使用方法

(1)、获取LayoutInflater实例有两种方法:

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

(2)、inflate方法:

layoutInflater.inflate(int layoutId, ViewGroup root);  /**  * layoutId      布局文件  * root          在该布局的最外层嵌套一层父布局,如果不需要就传null  * attachToRoot  true表示将layout的view添加到root中  */ layoutInflater.inflate(int layoutId, ViewGroup root, attachToRoot);  layoutInflater.inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 

注意:这里重点解释一下第三个参数attachToRoot,有以下几种情况:

(1)、如果root为null,无论attachToRoot为true或者false,效果都是一样的

(2)、如果root不为null,attachToRoot为true,表示将layout布局添加到root布局中

(3)、如果root不为null,attachToRoot为false,表示不将layout布局添加到root布局,若要添加则需要手动addView

(4)、如果root不为null,不设置attachToRoot(即调用两个参数的方法),情况和(2)中一样

前两种方法是最常用的加载布局的方法,最终都会调用最后一种方法来加载布局:

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {     ...          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, inflaterContext, attrs, false);     }else {         // Temp is the root view that was found in the xml         final View temp = createViewFromTag(root, name, inflaterContext, 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);             }               }                  ...                  // 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);         }                              ...              }          ... } 

LayoutInflater是通过Pull解析(XmlPullParser解析器)方式来解析XML布局文件,解析出节点名之后,然后会调用rInflate方法,这个方法里面会遍历这个根布局下的子元素。然后会调用createViewFromTag方法,这个方法是干嘛的呢?顾名思义创建View的呗,创建View 之后会判断root是否为null,若不为null会为其生成一个params,然后设置给该View,如果root为null,则不会生成params,所以就没有宽高这些数据了。最后若root不为null会调用addView方法,将该生成的View添加到root布局中。

    void rInflate(XmlPullParser parser, View parent, Context context, AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {         final int depth = parser.getDepth();         int type;         boolean pendingRequestFocus = false;          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)) {                 pendingRequestFocus = true;                 consumeChildElements(parser);             } else if (TAG_TAG.equals(name)) {                 parseViewTag(parser, parent, attrs);             } else if (TAG_INCLUDE.equals(name)) {                 if (parser.getDepth() == 0) {                     throw new InflateException("<include /> cannot be the root element");                 }                 parseInclude(parser, context, parent, attrs);             } else if (TAG_MERGE.equals(name)) {                 throw new InflateException("<merge /> must be the root element");             } else {                 final View view = createViewFromTag(parent, name, context, attrs);                 final ViewGroup viewGroup = (ViewGroup) parent;                 final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                 rInflateChildren(parser, view, attrs, true);                 viewGroup.addView(view, params);             }         }          if (pendingRequestFocus) {             parent.restoreDefaultFocus();         }          if (finishInflate) {             parent.onFinishInflate();         }     } 

细心的朋友可能会发现,上面两段代码都调用了同一个方法createViewFromTag,每次都会递归调用这个方法来创建这个View下的子元素并且添加到根布局中。那我们来看一下这个方法里面都干了些什么:

    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs, boolean ignoreThemeAttr) {         if (name.equals("view")) {             name = attrs.getAttributeValue(null, "class");         }          // Apply a theme wrapper, if allowed and one is specified.         if (!ignoreThemeAttr) {             final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);             final int themeResId = ta.getResourceId(0, 0);             if (themeResId != 0) {                 context = new ContextThemeWrapper(context, themeResId);             }             ta.recycle();         }          if (name.equals(TAG_1995)) {             // Let's party like it's 1995!             return new BlinkLayout(context, attrs);         }          try {             View view;             if (mFactory2 != null) {                 view = mFactory2.onCreateView(parent, name, context, attrs);             } else if (mFactory != null) {                 view = mFactory.onCreateView(name, context, attrs);             } else {                 view = null;             }              if (view == null && mPrivateFactory != null) {                 view = mPrivateFactory.onCreateView(parent, name, context, attrs);             }              if (view == null) {                 final Object lastContext = mConstructorArgs[0];                 mConstructorArgs[0] = context;                 try {                     if (-1 == name.indexOf('.')) {                         view = onCreateView(parent, name, attrs);                     } else {                         view = createView(name, null, attrs);                     }                 } finally {                     mConstructorArgs[0] = lastContext;                 }             }              return view;         } catch (InflateException e) {             throw e;          } catch (ClassNotFoundException e) {             final InflateException ie = new InflateException(attrs.getPositionDescription()                     + ": Error inflating class " + name, e);             ie.setStackTrace(EMPTY_STACK_TRACE);             throw ie;          } catch (Exception e) {             final InflateException ie = new InflateException(attrs.getPositionDescription()                     + ": Error inflating class " + name, e);             ie.setStackTrace(EMPTY_STACK_TRACE);             throw ie;         }     } 

代码不是很多,这个方法把我们传进去的xml的节点名来生成一个View对象,如何生成的呢,调用了一个createView方法(onCreateView方法中也会调用createView方法):

public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException {     Constructor<? extends View> constructor = sConstructorMap.get(name);     if (constructor != null && !verifyClassLoader(constructor)) {         constructor = null;         sConstructorMap.remove(name);     }     ...     final View view = constructor.newInstance(args);     ... } 

看到这里大家都知道了吧,通过反射的方法来创建出最终的View。

好了,相信看到这里大家应该都能明白为什么有时候LayoutInflater出来的View宽高会失效了吧!

总结:LayoutInflater有两个加载布局的方法,分别是两个参数和三个参数的,这里有以下几种情况:

(1)、如果root为null,无论attachToRoot为true或者false,返回的都是一个不带LayoutParams的View。

(2)、如果root不为null,attachToRoot为true,会返回一个带有LayoutParams的View,并且该View会添加到root布局。

(3)、如果root不为null,attachToRoot为false,也会返回一个带有LayoutParams的View,但不会添加到root布局。

(4)、如果root不为null,不设置attachToRoot(即调用两个参数的方法),情况和(2)中一样。

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