RecyclerView.ItemDecoration

decoration 英文意思：

英[ˌdekəˈreɪʃn] 美[ˌdɛkəˈreʃən] n. 装饰品; 装饰，装潢; 装饰图案，装饰风格; 奖章;

[例句]The decoration and furnishings had to be practical enough for a family home 房子的装潢和家具都必须很实用，适合家居生活。 [

其他] 复数：decorations

RecyclerView.ItemDecoration 装饰类，都装饰啥？

RecyclerView.ItemDecoration装饰的目标当然是RecyclerView里面每个item.

之前ListView有divier属性可以修改分隔线的样式

RecyclerView则是提供了

recyclerView.addItemDecoration()

 看一下源码：

   public static abstract class ItemDecoration {
        /**
         * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
         * Any content drawn by this method will be drawn before the item views are drawn,
         * and will thus appear underneath the views.
         *
         * @param c Canvas to draw into
         * @param parent RecyclerView this ItemDecoration is drawing into
         * @param state The current state of RecyclerView
         */
        public void onDraw(Canvas c, RecyclerView parent, State state) {
            onDraw(c, parent);
        }

        /**
         * @deprecated
         * Override {@link #onDraw(Canvas, RecyclerView, RecyclerView.State)}
         */
        @Deprecated
        public void onDraw(Canvas c, RecyclerView parent) {
        }

        /**
         * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
         * Any content drawn by this method will be drawn after the item views are drawn
         * and will thus appear over the views.
         *
         * @param c Canvas to draw into
         * @param parent RecyclerView this ItemDecoration is drawing into
         * @param state The current state of RecyclerView.
         */
        public void onDrawOver(Canvas c, RecyclerView parent, State state) {
            onDrawOver(c, parent);
        }

        /**
         * @deprecated
         * Override {@link #onDrawOver(Canvas, RecyclerView, RecyclerView.State)}
         */
        @Deprecated
        public void onDrawOver(Canvas c, RecyclerView parent) {
        }


        /**
         * @deprecated
         * Use {@link #getItemOffsets(Rect, View, RecyclerView, State)}
         */
        @Deprecated
        public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
            outRect.set(0, 0, 0, 0);
        }

        /**
         * Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies
         * the number of pixels that the item view should be inset by, similar to padding or margin.
         * The default implementation sets the bounds of outRect to 0 and returns.
         *
         * <p>
         * If this ItemDecoration does not affect the positioning of item views, it should set
         * all four fields of <code>outRect</code> (left, top, right, bottom) to zero
         * before returning.
         *
         * <p>
         * If you need to access Adapter for additional data, you can call
         * {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the
         * View.
         *
         * @param outRect Rect to receive the output.
         * @param view    The child view to decorate
         * @param parent  RecyclerView this ItemDecoration is decorating
         * @param state   The current state of RecyclerView.
         */
        public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
            getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
                    parent);
        }
    }
View Code

会发现主要有下面三个方法：

public void onDraw(Canvas c, RecyclerView parent, State state)
public void onDrawOver(Canvas c, RecyclerView parent, State state)
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)
 
那来看看都什么意思干么用：
直接看注释吧
onDraw()：
/**
* Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
* Any content drawn by this method will be drawn before the item views are drawn,
* and will thus appear underneath the views.
*
* @param c Canvas to draw into
* @param parent RecyclerView this ItemDecoration is drawing into
* @param state The current state of RecyclerView
*/　　

大概是说会在item画之前画些东西，会现在Item下面。那相当于是一个背景。

 onDrawOver()

  /**
         * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
         * Any content drawn by this method will be drawn after the item views are drawn
         * and will thus appear over the views.
         *
         * @param c Canvas to draw into
         * @param parent RecyclerView this ItemDecoration is drawing into
         * @param state The current state of RecyclerView.
         */
        public void onDrawOver(Canvas c, RecyclerView parent, State state) {
            onDrawOver(c, parent);
        }
will be drawn after the item views are drawn and will thus appear over the views.
很明白与上面的相对，会有Item画完之后去画，在浮在item内容上面。

getItemOffsets()
 /**
         * Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies
         * the number of pixels that the item view should be inset by, similar to padding or margin.
         * The default implementation sets the bounds of outRect to 0 and returns.
         *
         * <p>
         * If this ItemDecoration does not affect the positioning of item views, it should set
         * all four fields of <code>outRect</code> (left, top, right, bottom) to zero
         * before returning.
         *
         * <p>
         * If you need to access Adapter for additional data, you can call
         * {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the
         * View.
         *
         * @param outRect Rect to receive the output.
         * @param view    The child view to decorate
         * @param parent  RecyclerView this ItemDecoration is decorating
         * @param state   The current state of RecyclerView.
         */
        public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
            getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
                    parent);
        }

也说得很明白：similar to padding or margin.跟padding margin相似。给设置边距的。

从getItemOffsets()来开始实践

我们给recyclerview 加上背景以便观察。

<android.support.v7.widget.RecyclerView
    android:id="@+id/rcv_qulitry"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#f00">
</android.support.v7.widget.RecyclerView>

实现在自义定的decoration类

package com.lechang.fragment.decoration;

import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Created by hongtao on 2017/11/17.
 */

public class MyDecoration extends RecyclerView.ItemDecoration {

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        outRect.left = 5;
        outRect.right = 5;
        outRect.bottom = 10;
        super.getItemOffsets(outRect, view, parent, state);
    }
}

马上来用

recyclerView.addItemDecoration(new MyDecoration());

看看效果，嘿嘿，尼玛不是那么回事，怎么不起作用呢？

蒙逼了？

仔细看一下代码，方法调用了super.getItemOffsets(outRect, view, parent, state);再点进去看，

public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
    getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
            parent);
}
然后再调用了
@Deprecated
public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
    outRect.set(0, 0, 0, 0);
}

看到没

outRect.set(0, 0, 0, 0);

给设置成0了。

那我们

       outRect.left = 5;
        outRect.right = 5;
        outRect.bottom = 10;

设置的值就成白做了。

所以得把这个赋值放到super之后。

package com.lechang.fragment.decoration;

import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Created by hongtao on 2017/11/17.
 */

public class MyDecoration extends RecyclerView.ItemDecoration {

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        //切记要先调super.
        outRect.left = 5;
        outRect.right = 5;
        outRect.bottom = 10;
    }
}

这回可以了。
看一下效果：

红色底出来了。相当于padding .

只是底红露出，不能当分隔线，你非要用也行。

主要的还是要用ondraw把分隔线画出来。

下面来试ondraw().

原型：如下 

public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state)

这是一个回调的方法，给了一个画布，RecyclerView ，和RecyclerView.State

RecyclerView.State：给了下面三个状态
static final int STEP_START = 1;
static final int STEP_LAYOUT = 1 << 1;
static final int STEP_ANIMATIONS = 1 << 2;

意思是我们可以在这三个不同时期做不同的事，画不同的东西。有需求可以安排。

 RecyclerView parent

既然是parent 那他的child都是个个item.

那整个方法的意思就是用Canvas c给每个item都画上装饰（分隔线）。

来一段代码：

package com.lechang.fragment.decoration;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Created by hongtao on 2017/11/17.
 */

public class MyDecoration extends RecyclerView.ItemDecoration {

    Paint dividerPaint;
    int dividerHeight = 3;

    public MyDecoration(Context context, int dividerHeight) {
        dividerPaint = new Paint();
        dividerPaint.setColor(Color.BLACK);
        this.dividerHeight = dividerHeight;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        //切记要先调super.
//        outRect.left = 5;
//        outRect.right = 5;
        outRect.bottom = 20;
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        for (int i = 0; i < childCount - 1; i++) {
            View view = parent.getChildAt(i);
            float top = view.getBottom();
            float bottom = view.getBottom() + dividerHeight;
            //画了一个矩型
            c.drawRect(left, top, right, bottom, dividerPaint);
        }
    }
}
outRect.bottom 给了 20的值，要是不给这个值，那将什么也看不到。因为是在item下层的;
最终效果：黑色线是onDraw结果，红是底



接下来看：
onDrawOver（）方法效果
package com.lechang.fragment.decoration;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Created by hongtao on 2017/11/17.
 */

public class MyDecoration extends RecyclerView.ItemDecoration {

    Paint dividerPaint;
    int dividerHeight = 3;

    public MyDecoration(Context context, int dividerHeight) {
        dividerPaint = new Paint();
        dividerPaint.setColor(Color.BLACK);
        this.dividerHeight = dividerHeight;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        //切记要先调super.
//        outRect.left = 5;
//        outRect.right = 5;
//        outRect.bottom = 20;
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        for (int i = 0; i < childCount - 1; i++) {
            View view = parent.getChildAt(i);
            float top = view.getBottom();
            float bottom = view.getBottom() + dividerHeight;
            //画了一个矩型
            c.drawRect(left, top, right, bottom, dividerPaint);
        }
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        for (int i = 0; i < childCount - 1; i++) {
            View view = parent.getChildAt(i);
            float top = view.getBottom();
            float bottom = view.getBottom() + dividerHeight;
            //画了一个矩型
            c.drawRect(left, top, right, bottom, dividerPaint);
        }
    }
}

 

drawOver在上层画，所以不需要getItemOffsets配合。

 至此，ImtemDecoration这个类基本我们就会用了。

如果对Item位置做一些逻辑处理，还能做出一些有意思的东西。接下来再写。告一段落。




        

	      

	    

	  

	  

	    
      
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          原文地址：https://www.cnblogs.com/mamamia/p/7850405.html