Android 开发之 CoordinatorLayout 使用详解一

官网描述为：CoordinatorLayout 是一个增强版的 FrameLayout（继承自 ViewGroup）

用途：

1、作为应用的顶层视图。

2、作为一个可以指定子 View 之间相互作用的容器，通过给 CoordinatorLayout 的子 View 指定 CoordinatorLayout.Behavior 来定义子 view 之间的相互作用。（你可以想象成：CoordinatorLayout 相当于在两个 View 之间充当中介,这样子的好处就是两个 view 之间的耦合度降低了，只需要跟 coordinatorLayout 打交到即可，而 CoordinatorLayout.Behavior 相当于两个 view 之间的协议，即通过怎样的规则来约束双方的行为。）

设计概念：

1. CoordinatorLayout：CoordinatorLayout 作为最顶层视图，将负责管理所有的子 view，使其内部的子 View 彼此间产生一种联系。这个联系通过 Behavior 来实现（包括了滑动状态的处理以及 View 状态的处理）。
2. AppBarLayout：AppBarLayout 继承自限性布局，作为增强版的线性布局，他增加了对滑动手势的处理。
3. Behavior：Behavior 是 google 新提出的，能够让你以非侵入式的方式去处理目标 View 和其他 View 的交互行为。Behavior 需要设置在触发事件（比如滚动）的 view 上，且这个 View_必须是 CoordinatorLayout 的第一层级下的子 view_，否则没有效果，因为 Behavior 的初始化是在 CoordinatorLayout 的 LayoutParams 中通过反射完成的。
   Behavior 实例化方式：1、通过 app:layout_behavior 声明 ；2、在你的自定义 View 类上添加 @DefaultBehavior(MyBehavior.class)；
4. Behavior 只是个接口，其调用是由_NestedScrollingParent 与 NestedScrollingChild 接口负责调用。_

接下来我们通过阅读部分源码进行学习：

首先，我们从两个 view 是如何通过 coordinatorlayout 产生关联来入手；看代码

 LayoutParams(Context context, AttributeSet attrs) {
            super(context, attrs);

            final TypedArray a = context.obtainStyledAttributes(attrs,
                    R.styleable.CoordinatorLayout_Layout);

            this.gravity = a.getInteger(
                    R.styleable.CoordinatorLayout_Layout_android_layout_gravity,
                    Gravity.NO_GRAVITY);
            mAnchorId = a.getResourceId(R.styleable.CoordinatorLayout_Layout_layout_anchor,
                    View.NO_ID);
            this.anchorGravity = a.getInteger(
                    R.styleable.CoordinatorLayout_Layout_layout_anchorGravity,
                    Gravity.NO_GRAVITY);

            this.keyline = a.getInteger(R.styleable.CoordinatorLayout_Layout_layout_keyline,
                    -1);

            insetEdge = a.getInt(R.styleable.CoordinatorLayout_Layout_layout_insetEdge, 0);
            dodgeInsetEdges = a.getInt(
                    R.styleable.CoordinatorLayout_Layout_layout_dodgeInsetEdges, 0);
            mBehaviorResolved = a.hasValue(
                    R.styleable.CoordinatorLayout_Layout_layout_behavior);
            if (mBehaviorResolved) {
                mBehavior = parseBehavior(context, attrs, a.getString(
                        R.styleable.CoordinatorLayout_Layout_layout_behavior));
            }
            a.recycle();

            if (mBehavior != null) {
                // If we have a Behavior, dispatch that it has been attached
                mBehavior.onAttachedToLayoutParams(this);
            }
        }
​

从

mBehaviorResolved = a.hasValue(
        R.styleable.CoordinatorLayout_Layout_layout_behavior);
if (mBehaviorResolved) {
    mBehavior = parseBehavior(context, attrs, a.getString(
            R.styleable.CoordinatorLayout_Layout_layout_behavior));
}
​

这几句我们可以看到。
mBehaviorResolved 是个 boolean 变量，如果

R.styleable.CoordinatorLayout_Layout_layout_behavior CoordinatorLayout 的

layout_behavior 这个字段设置有值，
1、mBehaviorResolved = true -》调用 parseBehavior 方法，将所需参数传入通过 java 的反射技术返回一个 Behavior 实例。

static Behavior parseBehavior(Context context, AttributeSet attrs, String name) {
        if (TextUtils.isEmpty(name)) {
            return null;
        }

        final String fullName;
        if (name.startsWith(".")) {
            // Relative to the app package. Prepend the app package name.
            fullName = context.getPackageName() + name;
        } else if (name.indexOf('.') >= 0) {
            // Fully qualified package name.
            fullName = name;
        } else {
            // Assume stock behavior in this package (if we have one)
            fullName = !TextUtils.isEmpty(WIDGET_PACKAGE_NAME)
                    ? (WIDGET_PACKAGE_NAME + '.' + name)
                    : name;
        }

        try {
            Map<String, Constructor> constructors = sConstructors.get();
            if (constructors == null) {
                constructors = new HashMap<>();
                sConstructors.set(constructors);
            }
            Constructor c = constructors.get(fullName);
            if (c == null) {
                final Class clazz = (Class) Class.forName(fullName, true,
                        context.getClassLoader());
                c = clazz.getConstructor(CONSTRUCTOR_PARAMS);
                c.setAccessible(true);
                constructors.put(fullName, c);
            }
            return c.newInstance(context, attrs);
        } catch (Exception e) {
            throw new RuntimeException("Could not inflate Behavior subclass " + fullName, e);
        }
    }
​

通过这一段我们可以知道，最后是通过调用 Behavior 的参数为(context,attrs)的构造函数进行实例化。
实例化出 Behavior 之后我们会调用 behavior 的 onAttachedToLayoutParams 方法 将 LayoutParams 的实例对象传进去 mBehavior.onAttachedToLayoutParams(this);

mBehavior.onAttachedToLayoutParams 是一个当 LayoutParams 被实例化后的回调方法。

通过这里，我们的

CoordinatorLayout 就能够跟用 layout_behavior 标识的子 View 产生联系。

当子 View 发生变化时，CoordinatorLayout 又是如何处理的的，请看下面代码：

 final void onChildViewsChanged(@DispatchChangeEvent final int type) {
        final int layoutDirection = ViewCompat.getLayoutDirection(this);
        final int childCount = mDependencySortedChildren.size();
        final Rect inset = acquireTempRect();
        final Rect drawRect = acquireTempRect();
        final Rect lastDrawRect = acquireTempRect();

        for (int i = 0; i < childCount; i++) {
            final View child = mDependencySortedChildren.get(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (type == EVENT_PRE_DRAW && child.getVisibility() == View.GONE) {
                // Do not try to update GONE child views in pre draw updates.
                continue;
            }

            // Check child views before for anchor
            for (int j = 0; j < i; j++) {
                final View checkChild = mDependencySortedChildren.get(j);

                if (lp.mAnchorDirectChild == checkChild) {
                    offsetChildToAnchor(child, layoutDirection);
                }
            }

            // Get the current draw rect of the view
            getChildRect(child, true, drawRect);

            // Accumulate inset sizes
            if (lp.insetEdge != Gravity.NO_GRAVITY && !drawRect.isEmpty()) {
                final int absInsetEdge = GravityCompat.getAbsoluteGravity(
                        lp.insetEdge, layoutDirection);
                switch (absInsetEdge & Gravity.VERTICAL_GRAVITY_MASK) {
                    case Gravity.TOP:
                        inset.top = Math.max(inset.top, drawRect.bottom);
                        break;
                    case Gravity.BOTTOM:
                        inset.bottom = Math.max(inset.bottom, getHeight() - drawRect.top);
                        break;
                }
                switch (absInsetEdge & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.LEFT:
                        inset.left = Math.max(inset.left, drawRect.right);
                        break;
                    case Gravity.RIGHT:
                        inset.right = Math.max(inset.right, getWidth() - drawRect.left);
                        break;
                }
            }

            // Dodge inset edges if necessary
            if (lp.dodgeInsetEdges != Gravity.NO_GRAVITY && child.getVisibility() == View.VISIBLE) {
                offsetChildByInset(child, inset, layoutDirection);
            }

            if (type == EVENT_PRE_DRAW) {
                // Did it change? if not continue
                getLastChildRect(child, lastDrawRect);
                if (lastDrawRect.equals(drawRect)) {
                    continue;
                }
                recordLastChildRect(child, drawRect);
            }

            // Update any behavior-dependent views for the change
            for (int j = i + 1; j < childCount; j++) {
                final View checkChild = mDependencySortedChildren.get(j);
                final LayoutParams checkLp = (LayoutParams) checkChild.getLayoutParams();
                final Behavior b = checkLp.getBehavior();

                if (b != null && b.layoutDependsOn(this, checkChild, child)) {
                    if (type == EVENT_PRE_DRAW && checkLp.getChangedAfterNestedScroll()) {
                        // If this is from a pre-draw and we have already been changed
                        // from a nested scroll, skip the dispatch and reset the flag
                        checkLp.resetChangedAfterNestedScroll();
                        continue;
                    }

                    final boolean handled;
                    switch (type) {
                        case EVENT_VIEW_REMOVED:
                            // EVENT_VIEW_REMOVED means that we need to dispatch
                            // onDependentViewRemoved() instead
                            b.onDependentViewRemoved(this, checkChild, child);
                            handled = true;
                            break;
                        default:
                            // Otherwise we dispatch onDependentViewChanged()
                            handled = b.onDependentViewChanged(this, checkChild, child);
                            break;
                    }

                    if (type == EVENT_NESTED_SCROLL) {
                        // If this is from a nested scroll, set the flag so that we may skip
                        // any resulting onPreDraw dispatch (if needed)
                        checkLp.setChangedAfterNestedScroll(handled);
                    }
                }
            }
        }

        releaseTempRect(inset);
        releaseTempRect(drawRect);
        releaseTempRect(lastDrawRect);
    }
​

我们可以看到文档说明，大概意思是当子 view 发生变化会调用该方法。该方法会遍历所有的子 view，
然后调用如下代码，layoutDependsOn（）这个方法是做什么的呢？我们接下来看下该方法。

if (b != null && b.layoutDependsOn(this, checkChild, child)) {
                    if (type == EVENT_PRE_DRAW && checkLp.getChangedAfterNestedScroll()) {
                        // If this is from a pre-draw and we have already been changed
                        // from a nested scroll, skip the dispatch and reset the flag
                        checkLp.resetChangedAfterNestedScroll();
                        continue;
                    }

                    final boolean handled;
                    switch (type) {
                        case EVENT_VIEW_REMOVED:
                            // EVENT_VIEW_REMOVED means that we need to dispatch
                            // onDependentViewRemoved() instead
                            b.onDependentViewRemoved(this, checkChild, child);
                            handled = true;
                            break;
                        default:
                            // Otherwise we dispatch onDependentViewChanged()
                            handled = b.onDependentViewChanged(this, checkChild, child);
                            break;
                    }

                    if (type == EVENT_NESTED_SCROLL) {
                        // If this is from a nested scroll, set the flag so that we may skip
                        // any resulting onPreDraw dispatch (if needed)
                        checkLp.setChangedAfterNestedScroll(handled);
                    }
                }
​

 /**
         * Determine whether the supplied child view has another specific sibling view as a
         * layout dependency.
         *
         *
​

This method will be called at least once in response to a layout request. If it * returns true for a given child and dependency view pair, the parent CoordinatorLayout * will:

         *
​

2. Always lay out this child after the dependent child is laid out, regardless * of child order.

3. Call {@link #onDependentViewChanged} when the dependency view's layout or * position changes.

         *
         * @param parent the parent view of the given child
         * @param child the child view to test
         * @param dependency the proposed dependency of child
         * @return true if child's layout depends on the proposed dependency's layout,
         *         false otherwise
         *
         * @see #onDependentViewChanged(CoordinatorLayout, android.view.View, android.view.View)
         */
        public boolean layoutDependsOn(CoordinatorLayout parent, V child, View dependency) {
            return false;
        }
​

这个方法，大概意思是如果我们返回 true，说明当前发生变化的子 view 发生变化时。也就是该方法决定我们用

layout_behavior 标识的 view 是否应该做出相应的变化。默认返回 false，该方法需要我们在创建自己的 Behavior 时重写。
当返回 true 的话，可以看到会调用

b.onDependentViewRemoved(this, checkChild, child);

handled = b.onDependentViewChanged(this, checkChild, child);

为了更好理解这两句代码，我们举个例子，假设有 ViewA 和 ViewB ，当 ViewB 发生移动时，ViewA 要向反方向移动。
1、当 ViewB 被移除时会调用

b.onDependentViewRemoved(this, checkChild, child);
2、当 ViewB 发生变化时，会调用

handled = b.onDependentViewChanged(this, checkChild, child);
那么我们就可以在

b.onDependentViewChanged 里面写我们的功能代码了。

通过以上的分析，希望能帮到大家对 CoordinatorLayout 的协作调用过程有一些些的帮助。