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Android LayoutInflater深入分析及应用

2019-12-12 03:50:22
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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, root, 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, ViewGroup root, 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, ViewGroup root, 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, root, 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, attrs, 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, attrs, 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, root, attrs, false, false)和rInflate(parser, temp, attrs, true, 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, parent, attrs, 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, name, attrs, inheritContext);        final ViewGroup viewGroup = (ViewGroup) parent;        final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);        rInflate(parser, view, attrs, true, 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, String name, AttributeSet attrs, 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, name, viewContext, attrs);      } else if (mFactory != null) {        view = mFactory.onCreateView(name, viewContext, attrs);      } else {        view = null;      }      if (view == null && mPrivateFactory != null) {        view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs);      }      if (view == null) {        final Object lastContext = mConstructorArgs[0];        mConstructorArgs[0] = viewContext;        try {          if (-1 == name.indexOf('.')) {            view = onCreateView(parent, name, 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, prefix, attrs);            }          } else if (allowedState.equals(Boolean.FALSE)) {            failNotAllowed(name, prefix, 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

感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!

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