6AppBarLayout與scrollFlags

AppBarLayout分組

在這里說過AppBarLayout可以分為可滑出和不可滑出上下2部分，其實(shí)細(xì)致一點(diǎn)可以分三部分，如下圖所示，下滑最后出現(xiàn)（part 1），下滑立刻出現(xiàn)(part2)，無法滑出（part3），其中part1和2合起來就是可以滑出的部分。

xml代碼如下

   <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:theme="@style/AppTheme.AppBarOverlay"
        android:layout_height="wrap_content">


        <TextView
            android:gravity="center"
            app:layout_scrollFlags="scroll"
            android:textSize="20sp"
            android:text="下滑最后出現(xiàn)"
            android:background="#447700"
            android:layout_width="match_parent"
            android:layout_height="70dp" />


        <TextView
            android:gravity="center"
            app:layout_scrollFlags="scroll|enterAlways"
            android:textSize="20sp"
            android:text="下滑立刻出現(xiàn)"
            android:background="#004477"
            android:layout_width="match_parent"
            android:layout_height="100dp" />


        <TextView
            android:gravity="center"
            android:textSize="20sp"
            android:text="無法滑出去"
            android:background="#ff0000"
            android:layout_width="match_parent"
            android:layout_height="100dp" />

    </android.support.design.widget.AppBarLayout>

主要關(guān)注layout_scrollFlags，可以看到part3無scroll標(biāo)志，代表無法滾出；part2是scroll|enterAlways代表下滑立刻出現(xiàn)；part1是scroll下滑的時(shí)候最后出現(xiàn)。
為什么會(huì)這樣，主要和mDownPreScrollRange、mDownScrollRange有關(guān)，可以看下邊代碼。mDownPreScrollRange控制著嵌套滑動(dòng)的父view的onNestedPreScroll部分可滑距離，mDownScrollRange控制著嵌套滑動(dòng)的父view的onNestedScroll部分。

//AppBarLayout
    /**
     * Return the scroll range when scrolling down from a nested pre-scroll.
     */
    private int getDownNestedPreScrollRange() {
        if (mDownPreScrollRange != INVALID_SCROLL_RANGE) {
            // If we already have a valid value, return it
            return mDownPreScrollRange;
        }

        int range = 0;
        for (int i = getChildCount() - 1; i >= 0; i--) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            final int childHeight = child.getMeasuredHeight();
            final int flags = lp.mScrollFlags;

            if ((flags & LayoutParams.FLAG_QUICK_RETURN) == LayoutParams.FLAG_QUICK_RETURN) {
                // First take the margin into account
                range += lp.topMargin + lp.bottomMargin;
                // The view has the quick return flag combination...
                if ((flags & LayoutParams.SCROLL_FLAG_ENTER_ALWAYS_COLLAPSED) != 0) {
                    // If they're set to enter collapsed, use the minimum height
                    range += ViewCompat.getMinimumHeight(child);
                } else if ((flags & LayoutParams.SCROLL_FLAG_EXIT_UNTIL_COLLAPSED) != 0) {
                    // Only enter by the amount of the collapsed height
                    range += childHeight - ViewCompat.getMinimumHeight(child);
                } else {
                    // Else use the full height
                    range += childHeight;
                }
            } else if (range > 0) {
                // If we've hit an non-quick return scrollable view, and we've already hit a
                // quick return view, return now
                break;
            }
        }
        return mDownPreScrollRange = Math.max(0, range - getTopInset());
    }

    /**
     * Return the scroll range when scrolling down from a nested scroll.
     */
    private int getDownNestedScrollRange() {
        if (mDownScrollRange != INVALID_SCROLL_RANGE) {
            // If we already have a valid value, return it
            return mDownScrollRange;
        }

        int range = 0;
        for (int i = 0, z = getChildCount(); i < z; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            int childHeight = child.getMeasuredHeight();
            childHeight += lp.topMargin + lp.bottomMargin;

            final int flags = lp.mScrollFlags;

            if ((flags & LayoutParams.SCROLL_FLAG_SCROLL) != 0) {
                // We're set to scroll so add the child's height
                range += childHeight;

                if ((flags & LayoutParams.SCROLL_FLAG_EXIT_UNTIL_COLLAPSED) != 0) {
                    // For a collapsing exit scroll, we to take the collapsed height into account.
                    // We also break the range straight away since later views can't scroll
                    // beneath us
                    range -= ViewCompat.getMinimumHeight(child) + getTopInset();
                    break;
                }
            } else {
                // As soon as a view doesn't have the scroll flag, we end the range calculation.
                // This is because views below can not scroll under a fixed view.
                break;
            }
        }
        return mDownScrollRange = Math.max(0, range);
    }

實(shí)際效果如下所示

此時(shí)
mTotalScrollRange=56dp+70dp+100dp （part1+part2，其實(shí)toolbar也是屬于part1的）
mDownPreScrollRange=100dp（part2）
mDownScrollRange=56dp+70dp+100dp

scrollFlags

-scroll代表可滾動(dòng)，被標(biāo)注后算到mTotalScrollRange里，要寫其他flag必須先寫scroll才有效
-enterAlways下滑，這個(gè)view立刻跑出來，算在mDownPreScrollRange內(nèi)
-enterAlwaysCollapsed下滑的時(shí)候在onNestedPreScroll階段先滑出一個(gè)最小高度，這個(gè)參數(shù)我試了下都存在一定問題，沒找到一個(gè)合適的場(chǎng)景。用enterAlwaysCollapsed必須先寫 scroll和enterAlways
-exitUntilCollapsed 向上滾動(dòng)直到折疊，往往用于CollapsingToolbarLayout內(nèi)，后邊會(huì)有介紹