动态加载布局ViewStub

在实际开发中，有时候我们会遇到这样一种情况:就是有些布局或者view 不怎么经常用到，但是它的布局非常复杂，为了加载这种不经常用到的布局需要消耗很大的内存，而且对于配置低的手机来说，渲染也是比较慢的。碰到这样一种情况怎么办呢？有些人说直接设置View.gone。但是这仅仅是显示上处于隐藏而已，还是会占用内存空间。这时，ViewStub 就派上用场了。它可以实现动态的加载，也就是inflate()。先给出一个布局文件:

 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:paddingBottom="@dimen/activity_vertical_margin"
     android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     tools:context="com.example.main_hf.MainActivity" >

     <ImageView
         android:id="@+id/imageView"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content" />

     <Button
         android:id="@+id/submit"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignLeft="@id/imageView"
         android:layout_alignRight="@id/imageView"
         android:layout_below="@id/imageView"
         android:text="" />

     <ViewStub
         android:id="@+id/stub_import"
         android:layout_width="fill_parent"
         android:layout_height="wrap_content"
         android:layout_gravity="bottom"
         android:inflatedId="@+id/panel_import"
         android:layout="@layout/linear_item" />

 </RelativeLayout>

生成的布局如下：

接下来我们来显示ViewStub的内容吧：

 final ViewStub viewStub=(ViewStub) findViewById(R.id.stub_import);
             viewStub.inflate();
             viewStub.setVisibility(View.VISIBLE);

接下来我们再来看一下效果图，由于图较大，我只贴出ViewStub所指向的layout.

可以看到已经动态加载完成。

下面跟着源码走一遍：

 首先来看ViewStub的初始化：

 public ViewStub(Context context, AttributeSet attrs, int defStyle) {
         TypedArray a = context.obtainStyledAttributes(attrs, com.android.internal.R.styleable.ViewStub,
                 defStyle, 0);

         mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
         mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);

         a.recycle();

         a = context.obtainStyledAttributes(attrs, com.android.internal.R.styleable.View, defStyle, 0);
         mID = a.getResourceId(R.styleable.View_id, NO_ID);
         a.recycle();

         initialize(context);
     }

这里有两个变量需要注意一下，一个是mInflateId,它的作用是作为加载layout后，layout的id，,还有一个就是mLayoutResource，它的作用是需要加载资源的id.

 private void initialize(Context context) {
        mContext = context;
        setVisibility(GONE);
        setWillNotDraw(true);
    }

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

看到这个函数，也就明白了到底是怎么回事。如果说作为正常的view或者viewgroup肯定要设置mMeasuredWidth,mMeasuredHeight.

接下来看看最重要的inflate()函数

 public View inflate() {
         final ViewParent viewParent = getParent();

         if (viewParent != null && viewParent instanceof ViewGroup) {
             if (mLayoutResource != 0) {

 //得到父视图
 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) {

             //设置mInflatedId 为自己的Id
             view.setId(mInflatedId);
         }
         //在父视图中删除自己
         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);//view 的弱引用。

         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");
 }<span style="white-space:pre">                                                                           </span>

 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();
         }
     }
 }

如果不用弱引用可能会造成内存泄露。