Android 视图优化merge标签分析


当在布局文件中复用另外的布局时, <merge />标签能够在布局层次消除多余的视图元素。


当我们使用了<merge/> 标签后系统会做哪些操作呢?


来看源码:

系统将xml布局转换成view对象经历以下方法:

当调用:View.inflate(context, resource, root)   时:

    public static View inflate(Context context, int resource, ViewGroup root) {
        LayoutInflater factory = LayoutInflater.from(context);
        return factory.inflate(resource, root);
    }


继续调用inflate:

public View inflate(int resource, ViewGroup root) {
        return inflate(resource, root, root != null);
    }
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();
        }
    }

已经出现 XmlResoureParser 了,继续inflate

 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.
                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!");
                }

                final String name = parser.getName();

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

                if (TAG_MERGE.equals(name)) { //!!!!注意这里是merge标签,
                    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);//执行merge
                } else {
                    // 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
                        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);
                        }
                    }

                    if (DEBUG) {
                        System.out.println("-----> start inflating children");
                    }
                    // Inflate all children under temp
                    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.
                    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;
            }

            Trace.traceEnd(Trace.TRACE_TAG_VIEW);

            return result;
        }
    }



当解析xml时,遇到了merge标签:调用  rInflate(parser, root, attrs, false, false);

同时也注意到 merge的必须是ViewGroup   attachToRoot=true


 void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
            boolean finishInflate, boolean inheritContext) 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_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, parent, attrs, inheritContext);
            } else if (TAG_MERGE.equals(name)) {//merge也不能是 非root元素
                throw new InflateException("<merge /> must be the root element");
            } else {
                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);
            }
        }

        if (finishInflate) parent.onFinishInflate();
    }

来到: final View view = createViewFromTag(parent, name, attrs, inheritContext);



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

        Context viewContext; //这里<span style="font-family: 'Comic Sans MS';">inheritContext为false,注意viewContext的值</span>
        if (parent != null && inheritContext) {
            viewContext = parent.getContext();
        } else {
            viewContext = mContext;
        }

        // Apply a theme wrapper, if requested. //获取xml里定义的属性
        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! 这是什么注释??
            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;
        }
    }