Android布局优化：ViewStub标签实现延迟加载（源码解析原理）

1.ViewStub好处

• ViewStub is a lightweight view with no dimension that doesn’t draw anything or participate in the layout. it's an invisible, zero-sized View that can be used to lazily inflate layout resources at runtime.

  Android官方对ViewStub的解析:1.ViewStub一个不可见2.大小为0的试图. (下面会分析这两点实现)
  
  ViewStub好处：显示优酷视频加载评论列表的ListView，当没有数据或者网络加载失败时，如果inflate空列表的ListView会占用资源；当有数据时，才会inflate列表的ListView，延迟加载了布局.

2.ViewStub使用步骤

<ViewStub
        android:id="@+id/list_stub"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout="@layout/stub_list"
        android:inflatedId="@+id/stub_list"/>
        
stub_list.xml文件
<ListView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/list"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

• 每一个ViewStub必须有android:layout属性，其中android:inflatedId的值就是被inflate的View的ID.
• findViewById(R.id.list_stub)).setVisibility(View.VISIBLE);
  // or
  ListView listView = (ListView)((ViewStub) findViewById(R.id.list_stub)).inflate();

可以通过这两种方式inflate布局，第二种方式inflate布局不需要findViewById()找ListView.查看源码发现，可以通过设置setOnInflateListener()回调监听获取inflate的View.

3.ViewStub源码分析

public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context);

        final TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.ViewStub, defStyleAttr, defStyleRes);
        // 解析android:inflatedId属性，其值是被inflate的View的ID
        mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
        // 解析android:layout属性，其值是被inflate的View布局
        mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);
        // 解析android:id属性，可以通过findViewByID找到该ViewStub
        mID = a.getResourceId(R.styleable.ViewStub_id, NO_ID);
        a.recycle();
        // 最重要的两点
        setVisibility(GONE); // 设置不可见
        setWillNotDraw(true); // 本View不会调用onDraw()方法绘制内容
    }

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // 设置试图大小为0
    setMeasuredDimension(0, 0);
}

public void setVisibility(int visibility) {
        if (mInflatedViewRef != null) {
            // 已经inflate()已经调用，直接获取该inflate的View控制可见性
            View view = mInflatedViewRef.get();
            if (view != null) {
                view.setVisibility(visibility);
            } else {
                throw new IllegalStateException("setVisibility called on un-referenced view");
            }
        } else {
            // 如果没有调用过inflate()方法，并且设置了试图可见，就会调用inflate()加载布局
            super.setVisibility(visibility);
            if (visibility == VISIBLE || visibility == INVISIBLE) {
                inflate();
            }
        }
    }

public View inflate() {
        final ViewParent viewParent = getParent();

        if (viewParent != null && viewParent instanceof ViewGroup) {
            if (mLayoutResource != 0) {
                final ViewGroup parent = (ViewGroup) viewParent;
                final LayoutInflater factory;
                if (mInflater != null) {
                    factory = mInflater;
                } else {
                    factory = LayoutInflater.from(mContext);
                }
                // 加载真正的去加载布局试图
                final View view = factory.inflate(mLayoutResource, parent,
                        false);
                // 设置mInflatedId给inflate的View
                if (mInflatedId != NO_ID) {
                    view.setId(mInflatedId);
                }

                // 获取ViewStub在视图中的位置
                final int index = parent.indexOfChild(this);
                // 从视图移除ViewStub
                parent.removeViewInLayout(this);

                final ViewGroup.LayoutParams layoutParams = getLayoutParams();
                // 将inflate的试图加载父布局中，位置是被移除的ViewStub位置(也就是该ViewStub被替换成layoutView)
                if (layoutParams != null) {
                    parent.addView(view, index, layoutParams);
                } else {
                    parent.addView(view, index);
                }

                mInflatedViewRef = new WeakReference<View>(view);
                // 试图加载完成回调
                if (mInflateListener != null) {
                    mInflateListener.onInflate(this, view);
                }

                return view;
            } else {
                throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
            }
        } else {
            throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
        }

}

// 设置回调监听
public void setOnInflateListener(OnInflateListener inflateListener) {
        mInflateListener = inflateListener;
    }

public static interface OnInflateListener {
     // inflated：被inflate的View
    void onInflate(ViewStub stub, View inflated);
}

4.ViewStub使用中注意事项

• 一旦调用setVisibility(View.VISIBLE)或者inflate()方法之后，该ViewStub将会从试图中被移除（此时调用findViewById()是找不到该ViewStub对象).
• 如果指定了mInflatedId , 被inflate的layoutView的id就是mInflatedId.
• 被inflate的layoutView的layoutParams与ViewStub的layoutParams相同.

以上三点注意的事项可以从源码中得知:

//注意事项第二点
if (mInflatedId != NO_ID) {
    view.setId(mInflatedId);
}
//注意事项第一点
final int index = parent.indexOfChild(this);
parent.removeViewInLayout(this);

final ViewGroup.LayoutParams layoutParams = getLayoutParams();
if (layoutParams != null) {
     //注意事项第三点
    parent.addView(view, index, layoutParams);
} else {
    parent.addView(view, index);
}