ViewStub源码分析

<ViewStub 
    android:id="@+id/stub"
    android:inflatedId="@+id/realView" // 可设置，不过一般意义不大
    android:layout="@layout/my_real_view"
    android:visibility="GONE" // really no need, don‘t write useless code
    android:layout_width="match_parent"
    android:layout_height="40dip" />

ViewStub stub = (ViewStub) findViewById(R.id.stub);
View inflated = stub.inflate();

    public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        TypedArray a = context.obtainStyledAttributes(
                attrs, com.android.internal.R.styleable.ViewStub, defStyleAttr, defStyleRes);

        mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID); // 从layout文件中读取inflateId的值，如果有的话
        mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0); // 同样的，读取真正的layout文件

        a.recycle();

        a = context.obtainStyledAttributes(
                attrs, com.android.internal.R.styleable.View, defStyleAttr, defStyleRes);
        mID = a.getResourceId(R.styleable.View_id, NO_ID); // 读取View_id属性
        a.recycle();

        initialize(context);
    }

    private void initialize(Context context) {
        mContext = context;
        setVisibility(GONE); // 看到这里，你就明白了我前面xml文件中提到的没必要手动指定android:visibility="GONE"的原因
        setWillNotDraw(true); // 因为ViewStub只是个临时占位的，它不做任何绘制操作，所以打开这个标志位以优化绘制过程，提高性能
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(0, 0); // 实现很简单，写死的大小为0，0
    }

    @Override
    public void draw(Canvas canvas) { // 不做任何绘制
    }

    @Override
    protected void dispatchDraw(Canvas canvas) { // 没啥需要dispatch的
    }

    /**
     * When visibility is set to {@link #VISIBLE} or {@link #INVISIBLE},
     * {@link #inflate()} is invoked and this StubbedView is replaced in its parent
     * by the inflated layout resource. After that calls to this function are passed
     * through to the inflated view.
     *
     * @param visibility One of {@link #VISIBLE}, {@link #INVISIBLE}, or {@link #GONE}.
     *
     * @see #inflate() 
     */
    @Override
    @android.view.RemotableViewMethod
    public void setVisibility(int visibility) { // setVisibility做了些特殊处理
        if (mInflatedViewRef != null) { // 真正View的弱引用
            View view = mInflatedViewRef.get();
            if (view != null) {
                view.setVisibility(visibility);
            } else {
                throw new IllegalStateException("setVisibility called on un-referenced view");
            }
        } else { // 当还没inflate的时候，如果调了setVisibility方法，如果不是GONE的情况会自动在这种情况下调inflate的
            super.setVisibility(visibility);
            if (visibility == VISIBLE || visibility == INVISIBLE) {
                inflate(); // 非GONE的情况，会多调用inflate()方法，所以你拿到ViewStub的实例后既可以主动调
            }              // inflate()方法也可以通过setVisiblity(VISIBLE/INVISIBLE)来触发对inflate的间接调用
        }
    }

    /**
     * Inflates the layout resource identified by {@link #getLayoutResource()}
     * and replaces this StubbedView in its parent by the inflated layout resource.
     *
     * @return The inflated layout resource.
     *
     */
    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); // 注意这行，实际上还是调用了LayoutInflater的inflate方法，注意最后一个参数是false，所以这里返回的
                                // view就是真正layout文件里的root view而不是parent了。
                if (mInflatedId != NO_ID) {
                    view.setId(mInflatedId); // 如果layout文件里有给真正的view指定id就在这里设置上
                }

                final int index = parent.indexOfChild(this);
                parent.removeViewInLayout(this); // 找到ViewStub在parent中的位置并删除ViewStub，看到了，在inflate的过程中
                                                 // ViewStub真的是被删掉了！！！
                final ViewGroup.LayoutParams layoutParams = getLayoutParams(); // 获取LayoutParams，就是你在xml文件中写的
                                            // layout_xxx这样的属性，虽然实际上是属于ViewStub的，
                if (layoutParams != null) { // 但下面紧接着就用这个LayoutParams来添加真正的view了
                    parent.addView(view, index, layoutParams); // 注意下添加的位置就是原先ViewStub待的位置，
                                                               // 所以等于就是完完全全替换了
                } else {
                    parent.addView(view, index); // 如果layoutParams是null的case
                }

                mInflatedViewRef = new WeakReference<View>(view); // 用view初始化弱引用

                if (mInflateListener != null) { // 通过mInflateListener将inflate完成事件通知出去。。。
                    mInflateListener.onInflate(this, view);
                }

                return view; // 返回真正的view
            } else { // ViewStub必须指定真正的layout文件！！！
                throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
            }
        } else { // ViewStub也必须有个parent view的！！！
            throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
        }
    }

    /**
     * Specifies the inflate listener to be notified after this ViewStub successfully
     * inflated its layout resource.
     *
     * @param inflateListener The OnInflateListener to notify of successful inflation.
     *
     * @see android.view.ViewStub.OnInflateListener
     */
    public void setOnInflateListener(OnInflateListener inflateListener) {
        mInflateListener = inflateListener; // 技术上要想响应inflate完成事件，有2种途径，这里通过设置listener是一种方式，
    }                                       // 也可以通过更generic的方式：override真正view的onFinishInflate方法。

    /**
     * Listener used to receive a notification after a ViewStub has successfully
     * inflated its layout resource.
     *
     * @see android.view.ViewStub#setOnInflateListener(android.view.ViewStub.OnInflateListener) 
     */
    public static interface OnInflateListener {
        /**
         * Invoked after a ViewStub successfully inflated its layout resource.
         * This method is invoked after the inflated view was added to the
         * hierarchy but before the layout pass.
         *
         * @param stub The ViewStub that initiated the inflation.
         * @param inflated The inflated View.
         */
        void onInflate(ViewStub stub, View inflated);
    }
}