写在前面

本次分析的源码的SDK的版本为25.3.1.

ViewStub的简单使用

可以查看官方文档的相关说明.

在xml中使用ViewStub

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    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:orientation="vertical"
    tools:context="com.houtrry.viewstubdemo.MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="button"
        android:textAllCaps="false"
        android:textSize="18sp"/>

    <ViewStub
        android:id="@+id/viewStub"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inflatedId="@+id/ll_root"
        android:layout="@layout/layout_stub"/>

</LinearLayout>

其中, android:layout=”@layout/layout_stub”是关联布局layout_stub, 即是使用@layout/layout_stub来填充布局.android:inflatedId=”@+id/ll_root”的作用稍后说明.

layout/layout_stub如下 

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:gravity="center"
              android:orientation="vertical">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@mipmap/ic_launcher_round"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="hello world!"
        android:textAllCaps="false"
        android:textColor="@color/colorAccent"
        android:textSize="15sp"/>
    
</LinearLayout>

在Activity中控制ViewStub

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private ViewStub mViewStub;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mViewStub = (ViewStub) findViewById(R.id.viewStub);

    LinearLayout ll_root = (LinearLayout) findViewById(R.id.ll_root);
    Log.d(TAG, "onCreate: ll_root: "+ll_root);

    findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            mViewStub.inflate();

            LinearLayout ll_root = (LinearLayout) findViewById(R.id.ll_root);
            Log.d(TAG, "onCreate: ll_root: "+ll_root);
        }
    });
}

ViewStub#inflate调用前, ViewStub没有任何显示.ViewStub#inflate调用后, 显示出layout/layout_stub的内容.
关于android:inflatedId=”@+id/ll_root”, ll_root就是layout_stub的id, 可以使用inflatedId的值来获取android:layout=”@layout/layout_stub”布局的根控件.
点击按钮后, 日志如下:

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D/MainActivity: onCreate: ll_root: null
D/MainActivity: onCreate: ll_root: android.widget.LinearLayout{180892 V.E..... ......I. 0,0-0,0 #7f0b0060 app:id/ll_root}

说明调用ViewStub#inflate前, 布局layout/layout_stub并没有填充, 也没法获取layout/layout_stub中的控件.ViewStub#inflate后才可以.
再次点击按钮, 程序崩溃, 崩溃信息如下

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E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.houtrry.viewstubdemo, PID: 4841
                  java.lang.IllegalStateException: ViewStub must have a non-null ViewGroup viewParent
                      at android.view.ViewStub.inflate(ViewStub.java:292)
                      at com.houtrry.viewstubdemo.MainActivity$1.onClick(MainActivity.java:28)
                      at android.view.View.performClick(View.java:4785)
                      at android.view.View$PerformClick.run(View.java:19884)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:135)
                      at android.app.ActivityThread.main(ActivityThread.java:5343)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:372)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)

提示ViewStub没有父控件. 说明ViewStub#inflate后, 不能再次调用ViewStub#inflate方法. 至于原因, 后面说明.

源码分析

源码内容

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@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);
    }

    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);
        setWillNotDraw(true);
    }

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

    @android.view.RemotableViewMethod
    public void setInflatedId(@IdRes int inflatedId) {
        mInflatedId = inflatedId;
    }

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

    @android.view.RemotableViewMethod
    public void setLayoutResource(@LayoutRes int layoutResource) {
        mLayoutResource = layoutResource;
    }

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

    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) {
    }

    @Override
    @android.view.RemotableViewMethod
    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();
            }
        }
    }

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

        if (viewParent != null && viewParent instanceof ViewGroup) {
            if (mLayoutResource != 0) {
                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) {
                    view.setId(mInflatedId);
                }

                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);

                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");
        }
    }

    public void setOnInflateListener(OnInflateListener inflateListener) {
        mInflateListener = inflateListener;
    }

    public static interface OnInflateListener {
        void onInflate(ViewStub stub, View inflated);
    }
}

构造方法

构造方法中, 我们获取了属性android:inflatedId 和android:layout的值, 并将ViewStub设置为隐藏.

ViewStub#inflate

首先, 获取ViewStub的parent, 如果parent为空或者parent不是ViewGroup, 则抛出异常”ViewStub must have a non-null ViewGroup viewParent”, 我们先把问题放在这里, 继续向下看.
mLayoutResource就是android:layout的值, 如果没有设置android:layout, 则抛出异常”ViewStub must have a valid layoutResource”.
接下来就是ViewStub的核心了.

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//找到ViewStub的父控件
final ViewGroup parent = (ViewGroup) viewParent;
final LayoutInflater factory;
if (mInflater != null) {
    factory = mInflater;
} else {
    factory = LayoutInflater.from(mContext);
}
//通过LayoutInflater获取android:layout的View
final View view = factory.inflate(mLayoutResource, parent,
        false);
//如果有设置android:inflatedId, 将android:inflatedId的值设置给填充的View, 这也是为什么我们能通过
//LinearLayout ll_root = (LinearLayout) findViewById(R.id.ll_root)获取layout/layout_stub的根控件
//LinearLayout的原因
if (mInflatedId != NO_ID) {
    view.setId(mInflatedId);
}
//获取ViewStub在父控件中的位置
final int index = parent.indexOfChild(this);
//将ViewStub从父控件中移除!!!.
//这里就是ViewStub#inflate调用第二次的时候出现"ViewStub must have a non-null ViewGroup viewParent"  
//异常的原因. 因为ViewStub#inflate执行后, ViewStub就被移除了, parent就是null.
parent.removeViewInLayout(this);
//在父控件的相同位置添加填充的View
final ViewGroup.LayoutParams layoutParams = getLayoutParams();
if (layoutParams != null) {
    parent.addView(view, index, layoutParams);
} else {
    parent.addView(view, index);
}

再往后, 就是用弱引用保存填充的View了, 以及执行InflateListener的监听回调.

ViewStub#setVisibility

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@Override
@android.view.RemotableViewMethod
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();
        }
    }
}

这就很简单了, 从弱引用中取出View, 调用View的setVisibility方法.

ViewStub#setLayoutResource和ViewStub#setInflatedId

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@android.view.RemotableViewMethod
public void setInflatedId(@IdRes int inflatedId) {
    mInflatedId = inflatedId;
}

@android.view.RemotableViewMethod
public void setLayoutResource(@LayoutRes int layoutResource) {
    mLayoutResource = layoutResource;
}

这个很简单了, 在调用ViewStub#inflate前, 我们可以在代码中设置android:inflatedId 和android:layout的值.
应该注意的是, ViewStub#inflate只能调用一次, 在ViewStub#inflate后调用ViewStub#setLayoutResource和ViewStub#setInflatedId将不会有任何效果.

总结

  1. ViewStub的原理是初始化的时候隐藏. ViewStub#inflate后, 将自己从父控件中移除, 并将填充View添加到ViewStub所在的位置进行显示.
  2. ViewStub#inflate只能调用一次. 调用一次后再调用的话, 会抛出异常”ViewStub must have a non-null ViewGroup viewParent”.
  3. ViewStub#inflate后, 使用ViewStub#setVisibility控制控件的显示与隐藏(实际调用的是填充View的显示与隐藏)