从源码角度分析ViewStub 疑问与原理

res/layout/main.xml
<LinearLayout >
    <ViewStub
        android:id="@+id/viewstub"
        android:layout_width="100dip"
          android:layout_marginTop="100dip"
        android:layout_height="wrap_content"
        android:layout="@layout/sub_layout"
        />
   
</LinearLayout>

res/layout/sub_layout.xml
<TextView
     android:layout_width="50dip"
     android:layout_marginTop="50dip"
     android:layout_height="wrap_content"
     android:text="ViewStub中包含的TextVeiw"/>

    @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();
            }
        }
    }

public final class ViewStub extends View {
     ......

    public View inflate() {
        final ViewParent viewParent = getParent(); // 1 为什么可以直接获取父视图？

          // ViewStub的父视图必须是ViewGroup的子类
        if (viewParent != null && viewParent instanceof ViewGroup) {
            if (mLayoutResource != 0) { // ViewStub必须设置android:layout属性
                final ViewGroup parent = (ViewGroup) viewParent;
                final LayoutInflater factory;
                if (mInflater != null) {
                    factory = mInflater;
                } else {
                    factory = LayoutInflater.from(mContext);
                }
                // 2 inflate被加载视图 
                final View view = factory.inflate(mLayoutResource, parent,
                        false);

                if (mInflatedId != NO_ID) {
                    view.setId(mInflatedId);
                }

                // 从父视图中获取当前ViewStub在父视图中的位置
                final int index = parent.indexOfChild(this);
                // 当前ViewStub也是个View仅仅只是用来占位，所以先把占位的ViewStub视图删除
                parent.removeViewInLayout(this);

                // 3 此处获取的是ViewStub上面设置的参数
                final ViewGroup.LayoutParams layoutParams = getLayoutParams();
                if (layoutParams != null) {
                    parent.addView(view, index, layoutParams);
                } else {
                    parent.addView(view, index);
                }

                // 目的是在复写的setVisibility方法中使用
                // 因为ViewStub.setVisibility操作的是被加载视图并非当前ViewStub视图
                mInflatedViewRef = new WeakReference<View>(view);

                // 调用监听
                if (mInflateListener != null) {
                    mInflateListener.onInflate(this, view);
                }

                // 返回被加载视图，如果不需要当前可以忽略此返回对象
                return view;
            } else {
                throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
            }
        } else {
            throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
        }
    }
     ......
}

public class View {    
    public final ViewParent getParent() {
        return mParent;
    }

    void assignParent(ViewParent parent) {
        if (mParent == null) {
            mParent = parent;
        } else if (parent == null) {
            mParent = null;
        } else {
            throw new RuntimeException("view " + this + " being added, but"
                    + " it already has a parent");
        }
    }
}

public class ViewGroup {    
    public void addView(View child, int index, LayoutParams params) {
    
        ......
         
        addViewInner(child, index, params, false);
    }

    private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {
              
        ......           
         
        // tell our children
        if (preventRequestLayout) {
            child.assignParent(this);
        } else {
            child.mParent = this;
        }
         
          ......
     }    
}

public final class ViewStub extends View {

    public ViewStub(Context context) {
        initialize(context);
    }
     
     private void initialize(Context context) {
        mContext = context;
        setVisibility(GONE); // 初始化时把自己设置为隐藏
        setWillNotDraw(true);
    }
     
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(0, 0); // 所有子视图都设置为宽高为0
    }

    @Override
    public void draw(Canvas canvas) { // 不对自身与子视图进行绘制
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
    }
}