ViewStub原理，解决约束布局（ constraintLayout）ViewStub约束失效问题，解决ViewStub约束失效约束不起作用的问题

解决ViewStub约束布局 约束失效的问题

1.背景

在项目中进行性能优化或包体积控制的时候，就要进行xml或布局优化。
通常我们进行布局优化或xml加载优化的时候，就不得不提到到的几个标签merge、include、ViewStub，这几个标签作用不做赘述，重点讲一下本期的ViewStub；

ViewStub使用场景是，需要复用layout但第一时间用不到，使用ViewStub做延迟加载。其原理特性就是：将目标layout引入后，替换自己，并将自己的属性LayoutParams设置给目标View/layout

ViewStub在使用的时候，有两种方式：

1. 在在xml中通过layout属性引入目标layout；
2. 或者在代码中动态使用setLayoutResource注入；

最后在代码调用ViewStub.inflate()或ViewStub.setVisibility(View.VISIBLE)使引入的布局替换ViewStub。

2.问题

上面使用方式没有任何问题，使用起来也非常快捷方便，但问题也随之而来。

• 一定要有一个layout
  ViewStub引入布局，一定要有一个额外的Layout，即使是Layout里面只有一个View，也需要单独为它提供一个Layout；

• 它只能引入Layout
  有时候我们在代码中，需要动态的添加一个View，如果ViewGroup比较简单例如线性布局或者FrameLayout还好收，但是复杂的View，相对布局就需要动态设置Rule，而约束布局就相对来说比较复杂了，不是太了解约束布局动态更改约束规则的朋友就头疼了（我是自己研究了一下午才完全搞明白的），需要动态的加好几行代码控制约束，碰到View动态变换位置的场景更是要写好多代码。

于是，我就在想，如何解决这两个问题呢？为什么不能让ViewStub 把这两者结合呢

为什么ViewStub不能用在动态添加View，结合自己的特性，把ViewStub本身的layoutParams设置给动态添加的View呢？

3.寻找可行性

带着上面的问题，我重新看了一下它的源码。

@RemoteView
public final class ViewStub extends View {
   
   
    private int mInflatedId;
    private int mLayoutResource;

    private WeakReference<View> mInflatedViewRef;

    private LayoutInflater mInflater;
    private OnInflateListener mInflateListener;

    public ViewStub(Context context) {
   
   
        this(context, 0);
    }

    /**
     * Creates a new ViewStub with the specified layout resource.
     *
     * @param context The application's environment.
     * @param layoutResource The reference to a layout resource that will be inflated.
     */
    public ViewStub(Context context, @LayoutRes int layoutResource) {
   
   
        this(context, null);

        mLayoutResource = layoutResource;
    }

    public ViewStub(Context context, AttributeSet attrs) {
   
   
        this(context, attrs, 0);
    }

    public ViewStub(Context context, AttributeSet attrs, int defStyleAttr) {
   
   
        this(context, attrs, defStyleAttr, 0);
    }

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

        final TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.ViewStub, defStyleAttr, defStyleRes);
        saveAttributeDataForStyleable(context, R.styleable.ViewStub, attrs, a, 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);
    }

    @IdRes
    public int getInflatedId() {
   
   
        return mInflatedId;
    }

    /**
     * Defines the id taken by the inflated view. If the inflated id is
     * {@link View#NO_ID}, the inflated view keeps its original id.
     * 为引入的layout设置一个id
     */
    @android.view.RemotableViewMethod(asyncImpl = "setInflatedIdAsync")
    public void setInflatedId(@IdRes int inflatedId) {
   
   
        mInflatedId = inflatedId;
    }

    /** @hide **/
    public Runnable setInflatedIdAsync(@IdRes int inflatedId) {
   
   
        mInflatedId = inflatedId;
        return null;
    }

    @LayoutRes
    public int getLayoutResource() {
   
   
        return mLayoutResource;
    }

    /**
     * Specifies the layout resource to inflate when this StubbedView becomes visible or invisible
     * or when {@link #inflate()} is invoked. The View created by inflating the layout resource is
     * used to replace this StubbedView in its parent.
     * 注入需要引入的layoutId
     */
    @android.view.RemotableViewMethod(asyncImpl = "setLayoutResourceAsync")
    public void setLayoutResource(@LayoutRes int layoutResource) {
   
   
        mLayoutResource = layoutResource;
    }

    /** @hide **/
    public Runnable setLayoutResourceAsync(@LayoutRes int layoutResource) {
   
   
        mLayoutResource = layoutResource;
        return null;
    }

    public void setLayoutInflater(LayoutInflater inflater) {
   
   
        mInflater = inflater;
    }

    /**
     * Get current {@link LayoutInflater} used in {@link #inflate()}.
     */
    public LayoutInflater getLayoutInflater() {
   
   
        return mInflater;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   
   
        setMeasuredDimension(0, 0);
    }

    @Override
    public void draw(Canvas canvas) {
   
   
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
   
   
    }

    /**
     * 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.
     * 设置visibility  为VISIBLE 或者INVISIBLE时，会直接引入布局
     */
    @Override
    @android.view.RemotableViewMethod(asyncImpl = "setVisibilityAsync")
    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();
            }
        }
    }

    /** @hide **/
    public Runnable setVisibilityAsync(int visibility) {
   
   
        if (visibility == VISIBLE || visibility == INVISIBLE) {
   
   
            ViewGroup parent = (ViewGroup) getParent();