Android布局优化ViewStub源码分析

Posted 2022-11-23 lxn_李小牛

前言

关于布局优化的内容之前总结过一篇，可以移步Android布局优化总结，今天我们从源码的角度来分析一下ViewStub是如何实现的。

源码分析

 

@RemoteView
public final class ViewStub extends View 
    private int mInflatedId; // 被填充View的id
    private int mLayoutResource; // layout布局资源id
    // 弱引用，引用填充的View，以便在inflate之后可以通过setVisibility方法设置可见性
    private WeakReference<View> mInflatedViewRef;

    private LayoutInflater mInflater;
    private OnInflateListener mInflateListener; // inflate监听

    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);
        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); // 默认不可见
        setWillNotDraw(true); // 如果View不绘制任何内容，设置这个标记可以优化性能，默认View没有设置这个标记，如果重写onDraw，就不要设置这个标记
    

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) 
        setMeasuredDimension(0, 0); // 测量时尺寸为0
    

    @Override
    public void draw(Canvas canvas)  // 不绘制内容
    

    @Override
    protected void dispatchDraw(Canvas canvas) 
    
..... 省去部分代码

    private View inflateViewNoAdd(ViewGroup parent) 
        final LayoutInflater factory;
        if (mInflater != null) 
            factory = mInflater;
         else 
            factory = LayoutInflater.from(mContext);
         // 通过inflate填充布局
        final View view = factory.inflate(mLayoutResource, parent, false);

        if (mInflatedId != NO_ID) 
            view.setId(mInflatedId);
        
        return view;
    

    private void replaceSelfWithView(View view, ViewGroup parent) 
        final int index = parent.indexOfChild(this);
        parent.removeViewInLayout(this); // 移除ViewStub，后面不能在inflate

        final ViewGroup.LayoutParams layoutParams = getLayoutParams(); // 获得ViewStub的布局参数
        if (layoutParams != null) 
            parent.addView(view, index, layoutParams); // 把ViewStub指定的布局添加到parent中
         else 
            parent.addView(view, index);
        
    

    /**
     * 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(); // 获取ViewStub的parent

        if (viewParent != null && viewParent instanceof ViewGroup) 
            if (mLayoutResource != 0) 
                final ViewGroup parent = (ViewGroup) viewParent;
                final View view = inflateViewNoAdd(parent);
                replaceSelfWithView(view, parent);

                mInflatedViewRef = new WeakReference<>(view);
                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");
        
    

    /**
     * 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;
    

    /**
     * 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); // ViewStub填充完毕的回调
    

    /** @hide **/
    public class ViewReplaceRunnable implements Runnable 
        public final View view;

        ViewReplaceRunnable(View view) 
            this.view = view;
        

        @Override
        public void run() 
            replaceSelfWithView(view, (ViewGroup) getParent());