decoration 英文意思：
英[ˌdekəˈreɪʃn] 美[ˌdɛkəˈreʃən] n. 裝飾品; 裝飾，裝潢; 裝飾圖案，裝飾風格; 獎章;
[例句]The decoration and furnishings had to be practical enough for a family home 房子的裝潢和家具都必須很實用，適合家居生活。 [
其他] 復數：decorations
RecyclerView.ItemDecoration 裝飾類，都裝飾啥？
RecyclerView.ItemDecoration裝飾的目標當然是RecyclerView里面每個item.
之前ListView有divier屬性可以修改分隔線的樣式
RecyclerView則是提供了
recyclerView.addItemDecoration()
看一下源碼：
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); } }
會發現主要有下面三個方法：
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)
那來看看都什么意思干么用：
直接看注釋吧
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 */
大概是說會在item畫之前畫些東西，會現在Item下面。那相當於是一個背景。
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.
很明白與上面的相對，會有Item畫完之后去畫，在浮在item內容上面。
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); }
也說得很明白：similar to padding or margin.跟padding margin相似。給設置邊距的。
從getItemOffsets()來開始實踐
我們給recyclerview 加上背景以便觀察。
<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>
實現在自義定的decoration類
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); } }
馬上來用
recyclerView.addItemDecoration(new MyDecoration());
看看效果，嘿嘿，尼瑪不是那么回事，怎么不起作用呢？
蒙逼了？
仔細看一下代碼，方法調用了super.getItemOffsets(outRect, view, parent, state);再點進去看，
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
parent);
}
然后再調用了
@Deprecated
public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
outRect.set(0, 0, 0, 0);
}
看到沒
outRect.set(0, 0, 0, 0);
給設置成0了。
那我們
outRect.left = 5; outRect.right = 5; outRect.bottom = 10;
設置的值就成白做了。
所以得把這個賦值放到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); //切記要先調super. outRect.left = 5; outRect.right = 5; outRect.bottom = 10; } }
這回可以了。
看一下效果：
紅色底出來了。相當於padding .
只是底紅露出，不能當分隔線，你非要用也行。
主要的還是要用ondraw把分隔線畫出來。
下面來試ondraw().
原型：如下
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state)
這是一個回調的方法，給了一個畫布，RecyclerView ，和RecyclerView.State
RecyclerView.State：給了下面三個狀態
static final int STEP_START = 1;
static final int STEP_LAYOUT = 1 << 1;
static final int STEP_ANIMATIONS = 1 << 2;
意思是我們可以在這三個不同時期做不同的事，畫不同的東西。有需求可以安排。
RecyclerView parent
既然是parent 那他的child都是個個item.
那整個方法的意思就是用Canvas c給每個item都畫上裝飾（分隔線）。
來一段代碼：
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); //切記要先調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; //畫了一個矩型 c.drawRect(left, top, right, bottom, dividerPaint); } } }
outRect.bottom 給了 20的值，要是不給這個值，那將什么也看不到。因為是在item下層的;
最終效果：黑色線是onDraw結果，紅是底
接下來看：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); //切記要先調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; //畫了一個矩型 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; //畫了一個矩型 c.drawRect(left, top, right, bottom, dividerPaint); } } }
drawOver在上層畫，所以不需要getItemOffsets配合。
至此，ImtemDecoration這個類基本我們就會用了。
如果對Item位置做一些邏輯處理，還能做出一些有意思的東西。接下來再寫。告一段落。