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View视图的创建

View视图的创建

作者: wethereornot | 来源:发表于2018-05-04 14:39 被阅读0次

    在撸代码的过程中经常遇到创建 View 视图,创建视图有四种方式,但是这四种创建方式到底有什么不同呢?以前只知道能创建,但是不是这四种创建方式的区别,今天让我们一起去解析一下这几种创建方式。

    • view 创建方式
      在android API中有下列几种创建方式


      view创建方法.jpg

      点进去看源码可以知道这四个方法最终调用同一个方法

         * @param parser xml解析器
         * @param root 父布局
         * @param attachToRoot Whether the inflated hierarchy should be attached to
         *        the root parameter? If false, root is only used to create the
         *        correct subclass of LayoutParams for the root view in the XML.
         */
    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
          //  最终都会调用这一个方法中
    }
    
    • 重点分析inflate()这个方法,上源码
      这个方法分为两个部分(标号为 1 ,2 处),下面会详细介绍
     public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
            synchronized (mConstructorArgs) {
                Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
    
                final Context inflaterContext = mContext;
                // 获取子布局的属性
                final AttributeSet attrs = Xml.asAttributeSet(parser);
                Context lastContext = (Context) mConstructorArgs[0];
                mConstructorArgs[0] = inflaterContext;
                //将父布局赋值给 result
                View result = root;
    
                try {
                     ....省略....
                    // 获取布局节点的名称
                    final String name = parser.getName();
    
                     // 1. 如果根布局标签是"merge"
                    if (TAG_MERGE.equals(name)) {
                        // 满足root!=null&&attachToRoot=true才行,也就是说"merge"无法独立存在,必须要添加到ViewGroup中
                        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 {
                        // 2.根布局标签不是"merge",调用createViewFromTag()把根布局的View创建出来
                        // Temp is the root view that was found in the xml
                        final View temp = createViewFromTag(root, name, inflaterContext, attrs);
    
                        ViewGroup.LayoutParams params = null;
    
                        // 2.1如果父布局不为空
                        if (root != null) {
                            if (DEBUG) {
                                System.out.println("Creating params from root: " +
                                        root);
                            }
                            // Create layout params that match root, if supplied
                            //从attrs中获取子布局的宽高
                            params = root.generateLayoutParams(attrs);
                            //如果attachToRoot ->false 把父布局的参数设置给新建的view
                            if (!attachToRoot) {
                                // Set the layout params for temp if we are not
                                // attaching. (If we are, we use addView, below)
                                temp.setLayoutParams(params);
                            }
                        }
    
                        // Inflate all children under temp against its context.
                        rInflateChildren(parser, temp, attrs, true);
                     
                        // 2.2如果父布局不为空 并且 attachToRoot true 时 将创建的temp 添加到父布局中
                        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.
                        // 2.3如果父布局为空 或者 attachToRoot ->false 创建的temp布局 作为结果返回
                        if (root == null || !attachToRoot) {
                            result = temp;
                        }
                    }
    
                } catch (XmlPullParserException e) {
                  ......省略
                } finally {
                  ......省略
                }
                // 3.返回结果
                return result;
            }
        }
    

    当跟布局标签是“merge”时 会走标号1,跟布局标签不是“merge”时走标号2。(“merge”标签到底起什么作用呢?请看 《性能优化之布局优化篇二 使用<merge>标签 》

    • 部分一( 如果根布局标签是"merge")
       // 如果根布局标签是"merge"
                    if (TAG_MERGE.equals(name)) {
                        // 满足root!=null&&attachToRoot=true才行,也就是说"merge"无法独立存在,必须要添加到ViewGroup中
                        if (root == null || !attachToRoot) {
                            throw new InflateException("<merge /> can be used only with a valid "
                                    + "ViewGroup root and attachToRoot=true");
                        }
                        //会执行这个方法
                        rInflate(parser, root, inflaterContext, attrs, false);
                    }
    
    //将子布局中的控件实例化 添加到父布局中
    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 {
                    //创建一个view
                    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);
                    //添加到父布局容器中
                    viewGroup.addView(view, params);
                }
            }
    
            if (pendingRequestFocus) {
                parent.restoreDefaultFocus();
            }
    
            if (finishInflate) {
                //这时候通知父控件执行onFinishInflate方法,而此时,也紧紧是将所有的子控件实例化到内存中,也就是可以通过getChildAt()来获取相应的子控件实例了。
                parent.onFinishInflate();
            }
        }
    
    

    是merge标签,则会创建子控件并且添加到parent 父布局中

    • 部分二( 如果根布局标签不是"merge")
    else {
                        // 根布局标签不是"merge",调用createViewFromTag()把根布局的View创建出来
                        // Temp is the root view that was found in the xml
                        final View temp = createViewFromTag(root, name, inflaterContext, attrs);
    
                        ViewGroup.LayoutParams params = null;
    
                        //如果父布局不为空
                        if (root != null) {
                            if (DEBUG) {
                                System.out.println("Creating params from root: " +
                                        root);
                            }
                            // Create layout params that match root, if supplied
                            //从attrs中获取子布局的宽高
                            params = root.generateLayoutParams(attrs);
                            //如果attachToRoot ->false 把父布局的参数设置给新建的view
                            if (!attachToRoot) {
                                // Set the layout params for temp if we are not
                                // attaching. (If we are, we use addView, below)
                                temp.setLayoutParams(params);
                            }
                        }
    
                      
                        // Inflate all children under temp against its context.
                        rInflateChildren(parser, temp, attrs, true);
    
                        // We are supposed to attach all the views we found (int temp)
                        // to root. Do that now.
                        //如果父布局不为空 并且 attachToRoot true 时 将创建的temp 添加到父布局中
                        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.
                        //如果父布局为空 或者 attachToRoot ->false 创建的temp布局 作为结果返回
                        if (root == null || !attachToRoot) {
                            result = temp;
                        }
                    }
    
                } catch (XmlPullParserException e) {
                    ......省略
                } finally {
                    ......省略
                }
                return result;
            }
        }
    

    如果根布局不是merage标签
    1.首先会将根布局创建出来 View temp = createViewFromTag(root, name, inflaterContext, attrs);
    2.判断父布局parent是否为空,不为空,会从attrs中获取根布局的宽高 parmas,然后再判断 attachToRoot,
    如果 attachToRoot==false ,会将 parmas 设置给创建的 temp;
    3.循环创建根布局里的子控件,添加到temp根布局;
    4.如果 parent != null 并且 attachToRoot ==true时,将temp 添加到父布局中;
    5.如果 parent == null 或者 attachToRoot ==false时, 将temp作为结果result返回;

    总结
    1.两个参数时,根据 (root != null) 值 给 attachToRoot 赋值
    2.一般情况来说,我们一般不会在布局中使用“merage"标签
    3.parent !=null 时,从attrs中获取宽高 根据 attachToRoot = false 时 给根布局设置parmas(宽高)
    4.parent != null 且 attachToRoot == true 时 将根布局 temp 添加到父布局 parent 中
    5.parent ==null 或者 attachToRoot == false 时 将根布局 temp 作为结果result返回

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