ViewStub 源码实现

 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。