基于 6.0.1_r10

作用

ViewStub 可以用來優(yōu)化布局，實(shí)現(xiàn)布局的懶加載。

ViewStub 不占用屏幕空間，大小為0

只有當(dāng)調(diào)用 inflate 或者 viewStub.setVisibility(View.VISIBLE||INVISIBLE) 時(shí)才會(huì)加載布局

構(gòu)造方法

ViewStub 5 個(gè)構(gòu)造方法，層層包裹，最終會(huì)調(diào)用下面方法

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

    final TypedArray a = context.obtainStyledAttributes(attrs,
            R.styleable.ViewStub, defStyleAttr, defStyleRes);
    mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
    mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);
    mID = a.getResourceId(R.styleable.ViewStub_id, NO_ID);
    a.recycle();

    setVisibility(GONE);
    setWillNotDraw(true);
}

可以看出 ViewStub 構(gòu)造方法

1. 組件本身 setVisibility(GONE) 
2. setWillNotDraw(true);
所以 ViewStub 不會(huì)繪制出來，不占用屏幕空間。

ViewStub 使用

1. setVisibility()
   ViewStub 重載了 View 的 setVisibility() 方法

     @Override
    @android.view.RemotableViewMethod
    public void setVisibility(int visibility) {
        if (mInflatedViewRef != null) {
            View view = mInflatedViewRef.get();
            if (view != null) {
                view.setVisibility(visibility);
            } else {
                throw new IllegalStateException("setVisibility called on un-referenced view");
            }
        } else {
            super.setVisibility(visibility);
            if (visibility == VISIBLE || visibility == INVISIBLE) {
                inflate();
            }
        }
    }
   

2. 直接調(diào)用 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);
            }
            final View view = factory.inflate(mLayoutResource, parent,
                    false);
   
            if (mInflatedId != NO_ID) {
                view.setId(mInflatedId);
            }
   
            final int index = parent.indexOfChild(this);
            parent.removeViewInLayout(this);
   
            final ViewGroup.LayoutParams layoutParams = getLayoutParams();
            if (layoutParams != null) {
                parent.addView(view, index, layoutParams);
            } else {
                parent.addView(view, index);
            }
   
            mInflatedViewRef = new WeakReference<View>(view);
   
            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");
    }
    }
   

分析以上代碼

1. ViewStub 必須有一個(gè)父控件
2. setVisibility 顯示布局的時(shí)候，會(huì)調(diào)用 inflate
3. inflate 只能調(diào)用一次，會(huì)把加載到的 View 放到弱引用 mInflatedViewRef 中
   兩次調(diào)用 inflate ，會(huì)因?yàn)榈谝淮伟?ViewStub 從 parentView 中移除，導(dǎo)致出現(xiàn) crash
4. inflate 調(diào)用時(shí)會(huì)用真正的布局，替換 ViewStub (加載到父布局同樣的位置)
5. inflate 調(diào)用以后，setVisibility 使用 mInflatedViewRef 控制布局顯示/隱藏

以上就是 ViewStub 的全部，代碼比較簡單注釋也比較全面。

總結(jié)一個(gè)簡單的流程圖如下

viewStub_01.png

ViewStub 用到 LayoutInflater 加載布局文件，LayoutInflater 的用法可以參考LayoutInflater 源碼閱讀筆記

參考資料

developer 官方文檔

ViewStub 源碼

LayoutInflater 源碼閱讀筆記