Android优化笔记-布局优化

Android布局优化

          布局是我们在应用开发中必不可少的环节，一般也相对容易，但是当你的应用变得越来越复杂时，布局上的优化就会成为性能优化的首要处理。因为它并不复杂，相对其他优化来讲。
   1.1 include布局
            当你的页面逐渐增多的时候，可能会出现多个页面含有同一个布局的情况，也就是多个页面共用同一个UI组件。，比如咱们常见自定义的标题栏，绝大部分应用都需要标题栏。 如果我们每个页面重复定义这个标题布局，那么当要修改标题的样式时我们必须对每个页面都进行修改，这会浪费不少时间，而且也会引发其他问题。这时候我们就可以使用include标签。它能够将子布局引入到一个布局文件，这样能够将子布局复用到其他布局文件中。

main_activity.xml


    

loading.xml


    

         loading.xml可被其他布局共用。提高代码的复用率，include标签实现的原理不难，在解析xml布局时，如果检测到include标签，那么就把该布局下的根视图添加到include所在的父布局视图中。对于布局xml的解析最终会调用到LayouInflater的inflate方法，该方法最终会调用rinflate方法如下:

LayoutInflater.java

void rInflate(XmlPullParser parser, View parent, Context context,
            AttributeSet attrs, boolean finishInflate) 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)) {  //如果xml中的节点是include节点 则调用parseInclude方法
                if (parser.getDepth() == 0) {
                    throw new InflateException(" cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException(" must be the root element");
            } else {
                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);
                viewGroup.addView(view, params);
            }
        }
        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

private void parseInclude(XmlPullParser parser, Context context, View parent,
            AttributeSet attrs) throws XmlPullParserException, IOException {
        int type;
        if (parent instanceof ViewGroup) {         
            final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
            final int themeResId = ta.getResourceId(0, 0);
            final boolean hasThemeOverride = themeResId != 0;
            if (hasThemeOverride) {
                context = new ContextThemeWrapper(context, themeResId);
            }
            ta.recycle();
            int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
            if (layout == 0) {//include中的布局没设置layout就会抛出异常
                final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
                if (value == null || value.length() <= 0) {
                    throw new InflateException("You must specify a layout in the"
                            + " include tag: ");
                }
                // Attempt to resolve the "?attr/name" string to an identifier.
                layout = context.getResources().getIdentifier(value.substring(1), null, null);
            }
            // The layout might be referencing a theme attribute.
            if (mTempValue == null) {
                mTempValue = new TypedValue();
            }
            if (layout != 0 && context.getTheme().resolveAttribute(layout, mTempValue, true)) {
                layout = mTempValue.resourceId;
            }
            if (layout == 0) {
                final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
                throw new InflateException("You must specify a valid layout "
                        + "reference. The layout ID " + value + " is not valid.");
            } else {
                final XmlResourceParser childParser = context.getResources().getLayout(layout);

                try {
					//获取属性集，即在include标签中设置的属性
                    final AttributeSet childAttrs = Xml.asAttributeSet(childParser);

                    while ((type = childParser.next()) != XmlPullParser.START_TAG &&
                            type != XmlPullParser.END_DOCUMENT) {
                        // Empty.
                    }
					...	
                    final String childName = childParser.getName();//解析布局中的元素
                    if (TAG_MERGE.equals(childName)) { //如果第一个元素是merge标签，那么调用rInflate函数解析。
                        // The  tag doesn't support android:theme, so
                        // nothing special to do here.
                        rInflate(childParser, parent, context, childAttrs, false);
                    } else {
						//栗子中情况会走这里，创建include进来的xml布局的view
                        final View view = createViewFromTag(parent, childName,
                                context, childAttrs, hasThemeOverride);
                        final ViewGroup group = (ViewGroup) parent;

                        final TypedArray a = context.obtainStyledAttributes(
                                attrs, R.styleable.Include);
                        final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
                        final int visibility = a.getInt(R.styleable.Include_visibility, -1);
                        a.recycle();
              
                        ViewGroup.LayoutParams params = null;
                        try {
						//获取布局属性
                            params = group.generateLayoutParams(attrs);
                        } catch (RuntimeException e) {
                            // Ignore, just fail over to child attrs.
                        }
                        if (params == null) {
                            params = group.generateLayoutParams(childAttrs);
                        }
                        view.setLayoutParams(params);

                        // 解析所有子控件
                        rInflateChildren(childParser, view, childAttrs, true);
						//include中设置ID,则会将ID设置给loading.xml中的根view
                        if (id != View.NO_ID) {
                            view.setId(id);
                        }
                        switch (visibility) {
                            case 0:
                                view.setVisibility(View.VISIBLE);
                                break;
                            case 1:
                                view.setVisibility(View.INVISIBLE);
                                break;
                            case 2:
                                view.setVisibility(View.GONE);
                                break;
                        }
						//将loading.xml的根view添加到它的上一层父控件中。
                        group.addView(view);
                    }
                } finally {
                    childParser.close();
                }
            }
        } else {
            throw new InflateException(" can only be used inside of a ViewGroup");
        }
        LayoutInflater.consumeChildElements(parser);
    }