Android布局优化：ViewStub标签实现延迟加载（源码解析原理）

摘要：好处官方对的解析一个不可见大小为的试图下面会分析这两点实现好处显示优酷视频加载评论列表的，当没有数据或者网络加载失败时，如果空列表的会占用资源当有数据时，才会列表的，延迟加载了布局使用步骤文件每一个必须有属性，其中的值就是被的的可以通过这

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.

 
        
stub_list.xml文件

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.

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);
                // 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);
}

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);
}