ViewStub详解

大家好，又见面了，我是你们的朋友全栈君。

源码：

package android.view;

import android.annotation.IdRes;
import android.annotation.LayoutRes;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.widget.RemoteViews.RemoteView;

import com.android.internal.R;

import java.lang.ref.WeakReference;

/** * ViewStub是一个用户不可见的，大小为0的view，用于在app运行时懒加载一些布局。 *当ViewStub设置用户可见(setVisibility)时，或者当调用inflate()方法时，布局会就被初始化。ViewStub会用初始化后的布局文件替换自己放入其父布局中(参考inflate中的replaceSelfWithView方法)。 * * 因此ViewStub会在view布局中存在一直到setVisibility或inflate被调用了。 * * 当被初始化的View添加到ViewStub的父布局时会使用ViewStub的布局属性，因此你可以使用ViewStub的inflatedId属性定义／重写View的id。例如： * * * <ViewStub android:id="@+id/stub" * android:inflatedId="@+id/subTree" * android:layout="@layout/mySubTree" * android:layout_width="120dip" * android:layout_height="40dip" /> * * * 上面例子里边定义了一个id为stub的ViewStub。在初始化布局subTree之后，这个ViewStub就会从它父布局中移除。新创建的mySubTree布局的View的id就会使用布局中定义的subTree，最后这个新创建的view的宽为120dip，高为40dip. * * * 如下方式是最标准的初始化布局View的方式： * * * ViewStub stub = findViewById(R.id.stub); * View inflated = stub.inflate(); * * * 当inflate()方法被调用后，ViewStub会被替换为初始化后的布局View，并且View会被返回。从而让app获取到view的引用而不用在调用一次findViewById(). * * @attr ref android.R.styleable#ViewStub_inflatedId * @attr ref android.R.styleable#ViewStub_layout */
@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);
    }

    /** * 通过传入布局id创建ViewStub（猜测是用于代码中动态初始化时调用的）。 * * @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);
    }

	/** * 解析布局文件创建ViewStub，并获取mInflatedId及mLayoutResource等属性。最后设置不可见及不绘制自身。 * */
    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);
    }

    /** * Returns the id taken by the inflated view. If the inflated id is * {@link View#NO_ID}, the inflated view keeps its original id. * * @return A positive integer used to identify the inflated view or * {@link #NO_ID} if the inflated view should keep its id. * * @see #setInflatedId(int) * @attr ref android.R.styleable#ViewStub_inflatedId */
    @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. * * @param inflatedId A positive integer used to identify the inflated view or * {@link #NO_ID} if the inflated view should keep its id. * * @see #getInflatedId() * @attr ref android.R.styleable#ViewStub_inflatedId */
    @android.view.RemotableViewMethod(asyncImpl = "setInflatedIdAsync")
    public void setInflatedId(@IdRes int inflatedId) { 
   
        mInflatedId = inflatedId;
    }

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

    /** * Returns the layout resource that will be used by {@link #setVisibility(int)} or * {@link #inflate()} to replace this StubbedView * in its parent by another view. * * @return The layout resource identifier used to inflate the new View. * * @see #setLayoutResource(int) * @see #setVisibility(int) * @see #inflate() * @attr ref android.R.styleable#ViewStub_layout */
    @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. * * @param layoutResource A valid layout resource identifier (different from 0.) * * @see #getLayoutResource() * @see #setVisibility(int) * @see #inflate() * @attr ref android.R.styleable#ViewStub_layout */
    @android.view.RemotableViewMethod(asyncImpl = "setLayoutResourceAsync")
    public void setLayoutResource(@LayoutRes int layoutResource) { 
   
        mLayoutResource = layoutResource;
    }

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

    /** * Set {@link LayoutInflater} to use in {@link #inflate()}, or {@code null} * to use the default. */
    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);
    }

	/** * 不做任何绘制，所以比普通view节省了时间与内存。 */
    @Override
    public void draw(Canvas canvas) { 
   
    }

    @Override
    protected void dispatchDraw(Canvas canvas) { 
   
    }

    /** * 当设置visibility为VISIBLE或INVISIBLE时，inflate方法就会被调用 * ，且初始化出来的view对象会替换StubView在其父布局中的位置。 * * @param visibility One of {@link #VISIBLE}, {@link #INVISIBLE}, or {@link #GONE}. * * @see #inflate() */
    @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();
            return new ViewReplaceRunnable(inflateViewNoAdd(parent));
        } else { 
   
            return null;
        }
    }

    private View inflateViewNoAdd(ViewGroup parent) { 
   
        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) { 
   
            view.setId(mInflatedId);
        }
        return view;
    }

    private void replaceSelfWithView(View view, ViewGroup parent) { 
   
        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);
        }
    }

    /** * 初始化布局view，替换stub在父布局中的位置（注：stub被替换之后getparent会为空，因此会抛出IllegalArgumentException异常）。 * * * @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 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;
    }

    /** * 接收布局view被创建的观察者，如果设置了会在布局view被初始化之后回调 listener的onInflate方法。 * * * @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);
    }

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

        ViewReplaceRunnable(View view) { 
   
            this.view = view;
        }

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

使用方法：
1.在布局中定义ViewStub:

<ViewStub android:id="@+id/stub" android:inflatedId="@+id/subTree" android:layout="@layout/mySubTree" android:layout_width="120dip" android:layout_height="40dip" />

2.布局中使用：

	 ViewStub stub = findViewById(R.id.stub);
	 //inflated就是初始化后的布局view对象
     View inflated = stub.inflate();
     //对inflated操作，设置文字，图片等。
     

抛砖引玉，不当之处敬请之处，万分感谢?。

注：其实也可以也可以用动态添加的方法添加View：在java/kotlin代码中动态初始化View，然后添加到对应的viewgroup中。

这样的话在布局中就不用多了个ViewStub的view了,性能也更好些。