Android LayoutInflater深入分析及应用

概述LayoutInflater解析前言：在Android中，如果是初级玩家，很可能对LayoutInflater不太熟悉，或许只是在Fragment的onCreateView()中模式化的使用过而已。但如果稍微有些工作经验的人就知道，这个类有多么重要，它是连

LayoutInflater解析

前言：

在AndroID中，如果是初级玩家，很可能对LayoutInflater不太熟悉，或许只是在Fragment的onCreateVIEw()中模式化的使用过而已。但如果稍微有些工作经验的人就知道，这个类有多么重要，它是连接布局XMl和Java代码的桥梁，我们常常疑惑，为什么AndroID支持在XML书写布局？

我们想到的必然是AndroID内部帮我们解析xml文件，LayoutInflater就是帮我们做了这个工作。
首先LayoutInflater是一个系统服务，这个我们可以从from方法看出来

 /**   * Obtains the LayoutInflater from the given context.   */  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;  }

通常我们拿到LayoutInflater对象之后就会调用其inflate方法进行加载布局，inflate是一个重载方法

public VIEw inflate(int resource,VIEwGroup root) {    return inflate(resource,root,root != null);  }

可以看到，我们调用2个参数的方法时候其默认是添加到父布局中的（父布局一般不为空）

public VIEw inflate(int resource,VIEwGroup root,boolean attachToRoot) {    final Resources res = getContext().getResources();    if (DEBUG) {      Log.d(TAG,"INFLATING from resource: \"" + res.getResourcename(resource) + "\" ("          + Integer.toHexString(resource) + ")");    }    final XmlResourceParser parser = res.getLayout(resource);    try {      return inflate(parser,attachToRoot);    } finally {      parser.close();    }  }

这个方法中，其实是使用Resources将资源ID还原为XMlResoourceParser对象，然后调用inflate(XmlPullParser parser,boolean attachToRoot)方法，解析布局的具体步骤都是在这个方法中实现

public VIEw inflate(XmlPullParser parser,boolean attachToRoot) {    synchronized (mConstructorArgs) {      Trace.traceBegin(Trace.TRACE_TAG_VIEW,"inflate");      final AttributeSet attrs = Xml.asAttributeSet(parser);      Context lastContext = (Context)mConstructorArgs[0];      mConstructorArgs[0] = mContext;      VIEw result = root;      try {        // Look for the root node.        //1.循环寻找根节点，其实就是节点指针遍历的过程        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!");        }          //2.得到节点的名字，用于判断该节点        final String name = parser.getname();        if (DEBUG) {          System.out.println("**************************");          System.out.println("Creating root vIEw: "              + name);          System.out.println("**************************");        }          //3.对节点名字进行判断，然后是merge就将其添加到父布局中（依据Merge的特性必须添加到父布局中）        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,attrs,false,false);        } else {        //4.创建根据节点创建VIEw          // Temp is the root vIEw that was found in the xml          final VIEw temp = createVIEwFromTag(root,name,false);          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            //5.根据attrs生成布局参数            params = root.generateLayoutParams(attrs);            if (!attachToRoot) {              // Set the layout params for temp if we are not              // attaching. (If we are,we use addVIEw,below)              //6.如果VIEw不添加到父布局中，那就给其本身设置布局参数              temp.setLayoutParams(params);            }          }          if (DEBUG) {            System.out.println("-----> start inflating children");          }          // Inflate all children under temp          // 7.将该节点下的子VIEw全部加载          rInflate(parser,temp,true,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.          //8.如果添加到父布局中，直接addVIEw          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.          //9.如果不添加到父布局，那么将自己返回          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;      }      Trace.traceEnd(Trace.TRACE_TAG_VIEW);      return result;    }  }

重点的步骤我已经加上注释了，核心

1.找到根布局标签
2.创建根节点对应的VIEw
3.创建其子VIEw

我们从这里面可以看出来，子VIEw的解析其实都是rInflate方法，如果xml中有根布局，那么就调用createVIEwFromTag创建布局中的根VIEw。我们也可以明白merge的原来，因为它直接调用rInflate添加到父VIEw中，看到rInflate(parser,false)和rInflate(parser,true)第二个参数区别我们就明白了。

接下来我们看下rInflate如何创建多个布局

voID rInflate(XmlPullParser parser,VIEw parent,final AttributeSet attrs,boolean finishInflate,boolean inheritContext) throws XmlPullParserException,IOException {    //获取当前解析器指针所在节点处于布局层次    final int depth = parser.getDepth();    int type;    //进行树的深度优先遍历（如果一个节点有子节点将会再次进入rInflate，否则继续循环）    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();      //如果其中有request_focus标签，那就给这个节点VIEw设置焦点      if (TAG_REQUEST_FOCUS.equals(name)) {        parseRequestFocus(parser,parent);      //如果其中有tag标签，那就给这个节点VIEw设置tag(key,value)      } else if (TAG_TAG.equals(name)) {        parseVIEwTag(parser,parent,attrs);      } else if (TAG_INCLUDE.equals(name)) {      //如果其中是include标签，如果include标签        if (parser.getDepth() == 0) {          throw new InflateException("<include /> cannot be the root element");        }        parseInclude(parser,inheritContext);      } else if (TAG_MERGE.equals(name)) {        throw new InflateException("<merge /> must be the root element");      } else {          //创建该节点代表的VIEw并添加到父vIEw中，此外遍历子节点        final VIEw vIEw = createVIEwFromTag(parent,inheritContext);        final VIEwGroup vIEwGroup = (VIEwGroup) parent;        final VIEwGroup.LayoutParams params = vIEwGroup.generateLayoutParams(attrs);        rInflate(parser,vIEw,true);        vIEwGroup.addVIEw(vIEw,params);      }    }      //代表着一个节点含其子节点遍历结束    if (finishInflate) parent.onFinishInflate();  }

从上面可以看到，所以创建VIEw都将会交给createVIEwFromTag(VIEw parent,String name,AttributeSet attrs,boolean inheritContext)中，我们可以看下该方法如何创建VIEw

VIEw createVIEwFromTag(VIEw parent,boolean inheritContext) {    if (name.equals("vIEw")) {      name = attrs.getAttributeValue(null,"class");    }    Context vIEwContext;    if (parent != null && inheritContext) {      vIEwContext = parent.getContext();    } else {      vIEwContext = mContext;    }    // Apply a theme wrapper,if requested.    final TypedArray ta = vIEwContext.obtainStyledAttributes(attrs,ATTRS_theme);    final int themeResID = ta.getResourceID(0,0);    if (themeResID != 0) {      vIEwContext = new ContextthemeWrapper(vIEwContext,themeResID);    }    ta.recycle();    if (name.equals(TAG_1995)) {      // Let's party like it's 1995!      return new BlinkLayout(vIEwContext,attrs);    }    if (DEBUG) System.out.println("******** Creating vIEw: " + name);    try {      VIEw vIEw;      if (mFactory2 != null) {        vIEw = mFactory2.onCreateVIEw(parent,vIEwContext,attrs);      } else if (mFactory != null) {        vIEw = mFactory.onCreateVIEw(name,attrs);      } else {        vIEw = null;      }      if (vIEw == null && mPrivateFactory != null) {        vIEw = mPrivateFactory.onCreateVIEw(parent,attrs);      }      if (vIEw == null) {        final Object lastContext = mConstructorArgs[0];        mConstructorArgs[0] = vIEwContext;        try {          if (-1 == name.indexOf('.')) {            vIEw = onCreateVIEw(parent,attrs);          } else {            vIEw = createVIEw(name,null,attrs);          }        } finally {          mConstructorArgs[0] = lastContext;        }      }      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;    }  }

其实很简单，就是4个降级处理

if(factory2!=null){ factory2.onCreateVIEw(); }else if(factory!=null){ factory.onCreateVIEw(); }else if(mPrivateFactory!=null){ mPrivateFactory.onCreateVIEw(); }else{ onCreateVIEw() }

其他的onCreateVIEw我们不去设置的话为null，我们看下自己的onCreateVIEw(),其实这个方法会调用createVIEw()

public final VIEw createVIEw(String name,String prefix,AttributeSet attrs)      throws ClassNotFoundException,InflateException {      //从构造器Map（缓存）中获取需要的构造器    Constructor<? extends VIEw> constructor = sConstructorMap.get(name);    Class<? extends VIEw> clazz = null;    try {      Trace.traceBegin(Trace.TRACE_TAG_VIEW,name);      if (constructor == null) {        // Class not found in the cache,see if it's real,and try to add it        //如果缓存中没有需要的构造器，那就通过ClassLoader加载需要的类        clazz = mContext.getClassLoader().loadClass(            prefix != null ? (prefix + name) : name).asSubclass(VIEw.class);        if (mFilter != null && clazz != null) {          boolean allowed = mFilter.onLoadClass(clazz);          if (!allowed) {            failNotAllowed(name,prefix,attrs);          }        }        //将使用过的构造器缓存        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).asSubclass(VIEw.class);            boolean allowed = clazz != null && mFilter.onLoadClass(clazz);            mFilterMap.put(name,allowed);            if (!allowed) {              failNotAllowed(name,attrs);            }          } else if (allowedState.equals(Boolean.FALSE)) {            failNotAllowed(name,attrs);          }        }      }      Object[] args = mConstructorArgs;      args[1] = attrs;      constructor.setAccessible(true);      //通过反射获取需要的实例对象      final VIEw vIEw = constructor.newInstance(args);      if (vIEw instanceof VIEwStub) {        // Use the same context when inflating VIEwStub later.        final VIEwStub vIEwStub = (VIEwStub) vIEw;        //VIEwStub将创建一个属于自己的LayoutInflater，因为它需要在不同的时机去inflate        vIEwStub.setLayoutInflater(cloneInContext((Context) args[0]));      }      return vIEw;    } catch (NoSuchMethodException e) {      InflateException IE = new InflateException(attrs.getpositionDescription()          + ": Error inflating class "          + (prefix != null ? (prefix + name) : name));      IE.initCause(e);      throw IE;    } catch (ClassCastException e) {      // If loaded class is not a VIEw subclass      InflateException IE = new InflateException(attrs.getpositionDescription()          + ": Class is not a VIEw "          + (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;    } finally {      Trace.traceEnd(Trace.TRACE_TAG_VIEW);    }  }

大体步骤就是，

1.从缓存中获取特定VIEw构造器，如果没有，则加载对应的类，并缓存该构造器，
2.利用构造器反射构造对应的VIEw
3.如果是VIEwStub则复制一个LayoutInflater对象传递给它

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