Android 视图优化merge标签分析

左手的ㄟ右手 2022-08-06 11:12 259阅读 0赞

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 和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;
            }
        }