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);
        }
    }

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。