android UI布局优化之merge标签

merge标签可以降低UI层级

1.<merge />只可以作为xml layout的根节点。
2.当需要inflate的xml layout本身是由merge作为根节点的话，需要将被导入的xml layout置于viewGroup中，同时需要设置attachToRoot为True。

一个布局可以重复利用，可以抽成一个view，这个view继承的viewgroup是其xml的跟布局，这样xml的根布局就可以用merge替代，

// 继承RelativeLayout
public class RadioPalyerHistoryView extends RelativeLayout {

    public RadioPalyerHistoryView(@NonNull Context context) {
        this(context, null);
    }

    public RadioPalyerHistoryView(@NonNull Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public RadioPalyerHistoryView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mContext = context;
       // 必须attachToRoot为true
        View view = LayoutInflater.from(context).inflate(R.layout.radio_palyer_history_right_layout, this, true)；
// 跟布局是merge标签，在跟布局设置的padding不管用了，代码设置
 setPadding(CommonUtils.dip2px(context,12),CommonUtils.dip2px(context,10),CommonUtils.dip2px(context,12),CommonUtils.dip2px(context,10));
    }

其跟布局先用RelativeLayout，这样方便写布局看下子view的布局位置，等写好布局了，用merge替换

<?xml version="1.0" encoding="utf-8"?>
<!-- 跟布局是RelativeLayout-->
<merge xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:tools="http://schemas.android.com/tools">

    <TextView
        android:id="@+id/tv_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textColor="@color/black_27292B"
        android:textSize="15sp"
        tools:text="何以小 第二集"
        android:ellipsize="end"
        android:singleLine="true"/>

    <TextView
        android:id="@+id/tv_progress"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="13sp"
        android:textColor="@color/black_989A9B"
        android:layout_marginTop="5dp"
        tools:text="听至10%"
        android:layout_below="@id/tv_title"/>

    <TextView
        android:id="@+id/tv_time"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="13sp"
        android:textColor="@color/black_989A9B"
        android:layout_marginTop="5dp"
        tools:text="4个小时前"
        android:layout_below="@id/tv_title"
        android:layout_toRightOf="@id/tv_progress"
        android:layout_marginLeft="7dp"/>


</merge>

优点：减少布局层级；缺点：跟布局merge不能设置padding(设置的padding不起作用），可以把设置在跟布局的padding设置到代码；不可以设置固定值的宽高，例如：

<merge xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="280dp"
    android:paddingLeft="12dp"
    android:paddingRight="12dp"
    android:paddingTop="10dp"
    android:paddingBottom="10dp">

layout_height="280dp"和:paddingLeft="12dp"这些都不会起作用，需要在代码中设置；
原因是：在在LayoutInflater的inflate方法中，

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root;

            try {
            // 省略
                ...
                // merge标签必须有root，attachToRoot为true
                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, inflaterContext, attrs, false);
                } else {
                   ... 

            } catch (XmlPullParserException e) {
            ...

            return result;
        }
    }

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 {
                //。 没有用merge标签的宽高属性，padding属性，直接addview到parent中
                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 (pendingRequestFocus) {
            parent.restoreDefaultFocus();
        }

        if (finishInflate) {
            parent.onFinishInflate();
        }
    }