Snackbar源码解析

引言

2015年5月，Google发布了Design Support Library，添加了很多组件用于支持Material Design。至今过去已经两年了，版本也由当初的22.2.0到现在的26.0.0 Alpha 1。想要了解其中控件的实现原理，当然是从最简单的开始，那就是这篇文章的主角——Snackbar。

基本使用

1. 只有文本提示

Snackbar.make(view, "This is a message", Snackbar.LENGTH_LONG).show();

1. 有点击按钮

Snackbar.make(view, "This is a message", Snackbar.LENGTH_LONG)        .setAction("UNDO", new View.OnClickListener() {            @Override            public void onClick(View v) {                //TODO do something            }        })        .show();

当然还有其他的属性及方法，具体的可参考Google官方文档。

带着问题去阅读

1. Snackbar是如何添加到界面上的？
2. Snackbar的显示位置如何修改？
3. Snackbar的布局是否可以修改？
4. 多个连续的Snackbar是如何管理显示的？
5. 在CoordinatorLayout中使用FloatingActionButton和SnackBar时，为什么Snackbar不会遮挡FloatingActionButton？

源码解析

源码基于25.3.0
解读源码，应该从什么地方下手呢？当然是从我们使用SnackBar最常用的方法下手了，第一个使用到的那就是make方法了。

make方法

public static Snackbar make(@NonNull View view, @NonNull CharSequence text,            @Duration int duration) {        final ViewGroup parent = findSuitableParent(view);        if (parent == null) {            throw new IllegalArgumentException("No suitable parent found from the given view. "                    + "Please provide a valid view.");        }     ...      // 后面代码省略    }

SnackBar中有两个make方法，区别是提示文字传递的类型，一个是CharSequence，一个是Resouse id。传Resouse id最终也会走到上述方法中。
先来看看方法内第一行代码，调用了findSuitableParent(View view)方法，代码如下：

private static ViewGroup findSuitableParent(View view) {    ViewGroup fallback = null;    do {        if (view instanceof CoordinatorLayout) {            // We've found a CoordinatorLayout, use it            return (ViewGroup) view;        } else if (view instanceof FrameLayout) {            if (view.getId() == android.R.id.content) {                // If we've hit the decor content view, then we didn't find a CoL in the                // hierarchy, so use it.                return (ViewGroup) view;            } else {                // It's not the content view but we'll use it as our fallback                fallback = (ViewGroup) view;            }        }        if (view != null) {            // Else, we will loop and crawl up the view hierarchy and try to find a parent            final ViewParent parent = view.getParent();            view = parent instanceof View ? (View) parent : null;        }    } while (view != null);    // If we reach here then we didn't find a CoL or a suitable content view so we'll fallback    return fallback;}

代码量不大，而且注释也很清楚。此方法的作用就是循环查找view的上层ViewGroup，直到找到CoordinatorLayout或到根布局结束，返回找到的ViewGroup。
根布局：id为android.R.id.content的布局实际上就是我们setContentView设置自己写的布局的父ViewGroup，类型是FrameLayout，具体的可以去了解DecorView。
再回过头来看看Snackbar的make方法：

public static Snackbar make(@NonNull View view, @NonNull CharSequence text,         @Duration int duration) {     ....//前面代码省略     final LayoutInflater inflater = LayoutInflater.from(parent.getContext());     final SnackbarContentLayout content =             (SnackbarContentLayout) inflater.inflate(                     R.layout.design_layout_snackbar_include, parent, false);     final Snackbar snackbar = new Snackbar(parent, content, content);     snackbar.setText(text);     snackbar.setDuration(duration);     return snackbar; }

通过inflate获取到SnackBarContentLayout布局，SnackBarContentLayout实际上是一个LinearLayout，再来看看R.layout.design_layout_snackbar_include：

<merge xmlns:android="http://schemas.android.com/apk/res/android">    <TextView            android:id="@+id/snackbar_text"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:layout_weight="1"            android:paddingTop="@dimen/design_snackbar_padding_vertical"            android:paddingBottom="@dimen/design_snackbar_padding_vertical"            android:paddingLeft="@dimen/design_snackbar_padding_horizontal"            android:paddingRight="@dimen/design_snackbar_padding_horizontal"            android:textAppearance="@style/TextAppearance.Design.Snackbar.Message"            android:maxLines="@integer/design_snackbar_text_max_lines"            android:layout_gravity="center_vertical|left|start"            android:ellipsize="end"            android:textAlignment="viewStart"/>    <Button            android:id="@+id/snackbar_action"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:layout_marginLeft="@dimen/design_snackbar_extra_spacing_horizontal"            android:layout_marginStart="@dimen/design_snackbar_extra_spacing_horizontal"            android:layout_gravity="center_vertical|right|end"            android:paddingTop="@dimen/design_snackbar_padding_vertical"            android:paddingBottom="@dimen/design_snackbar_padding_vertical"            android:paddingLeft="@dimen/design_snackbar_padding_horizontal"            android:paddingRight="@dimen/design_snackbar_padding_horizontal"            android:visibility="gone"            android:textColor="?attr/colorAccent"            style="?attr/borderlessButtonStyle"/></merge>

没错，这就是Snackbar的主要布局了，一个TextView和一个Button。
获取到的SnackbarContentLayout，通过实例化Snackbar，传进了Snackbar的构造方法中，最后到了Snackbar的父类BaseTransientBottomBar的构造方法中：

    protected BaseTransientBottomBar(@NonNull ViewGroup parent, @NonNull View content,            @NonNull ContentViewCallback contentViewCallback) {        ...        //省略不重要代码        mTargetParent = parent; //之前findSuitableParent方法找到的ViewGroup        //callback传进来的SnackbarContentLayout，其实现了ContentViewCallback接口        mContentViewCallback = contentViewCallback;        mContext = parent.getContext();        ThemeUtils.checkAppCompatTheme(mContext);        LayoutInflater inflater = LayoutInflater.from(mContext);        // Note that for backwards compatibility reasons we inflate a layout that is defined        // in the extending Snackbar class. This is to prevent breakage of apps that have custom        // coordinator layout behaviors that depend on that layout.        mView = (SnackbarBaseLayout) inflater.inflate(                R.layout.design_layout_snackbar, mTargetParent, false);        mView.addView(content);//将SnackbarContentLayout添加到SnackbarLayout中        ...//省略剩余代码    }    /**     * Returns the {@link BaseTransientBottomBar}'s view.     */    @NonNull    public View getView() {        return mView;    }

已经备注很详细了，接着来看看R.layout.design_layout_snackbar:

<view xmlns:android="http://schemas.android.com/apk/res/android"      class="android.support.design.widget.Snackbar$SnackbarLayout"      android:layout_width="match_parent"      android:layout_height="wrap_content"      android:layout_gravity="bottom"      style="@style/Widget.Design.Snackbar" />

注意到class了吗，没错，此View是Snackbar中定义的SnackbarLayout，继承自BaseTransientBottomBar中的SnackbarBaseLayout，而SnackbarBaseLayout继承自FrameLayout。SnackbarLaout中只重新了onMeasure方法，其他实现都在SnackbarBaseLayout中。
还有一个关键的地方，layout_gravity被设置成了bottom，这也是为什么Snackbar总显示在底部的原因。

到这里我们已经知道了Snackbar的布局实际上是一个FrameLayout，其内容是一个LinearLayout。BaseTransientBottomBar提供了getView方法来获取mView，mView即为Snackbar的根布局FrameLayout。既然能获取到根布局，那往此布局中addView肯定是没问题了，之前提到的问题3也可以利用这一点解决了。

show方法

为什么不是action，而是show。因为我关心的是Snackbar如何显示的。

public void show() {        SnackbarManager.getInstance().show(mDuration, mManagerCallback);    }

看到这里是不是有点蒙蔽了，怎么到了SnackbarManager的show方法了，不着急，我们先来看看mManagerCallback:

final SnackbarManager.Callback mManagerCallback = new SnackbarManager.Callback() {        @Override        public void show() {            sHandler.sendMessage(sHandler.obtainMessage(MSG_SHOW, BaseTransientBottomBar.this));        }        @Override        public void dismiss(int event) {            sHandler.sendMessage(sHandler.obtainMessage(MSG_DISMISS, event, 0,                    BaseTransientBottomBar.this));        }    };

mManagerCallback内部使用了Handler来控制show和dismiss，最终sHandler会调用showView：

final void showView() {    if (mView.getParent() == null) {        final ViewGroup.LayoutParams lp = mView.getLayoutParams();        if (lp instanceof CoordinatorLayout.LayoutParams) {            // 如果LayoutParams是CoordinatorLayout的，就设置Behavior            final CoordinatorLayout.LayoutParams clp = (CoordinatorLayout.LayoutParams) lp;            final Behavior behavior = new Behavior();            behavior.setStartAlphaSwipeDistance(0.1f);            behavior.setEndAlphaSwipeDistance(0.6f);            //设置SwipeDismissBehavior，具体作用就是滑动删除view  behavior.setSwipeDirection(SwipeDismissBehavior.SWIPE_DIRECTION_START_TO_END);            behavior.setListener(new SwipeDismissBehavior.OnDismissListener() {                @Override                public void onDismiss(View view) {                    view.setVisibility(View.GONE);                    dispatchDismiss(BaseCallback.DISMISS_EVENT_SWIPE);                }                @Override                public void onDragStateChanged(int state) {                    switch (state) {                        case SwipeDismissBehavior.STATE_DRAGGING:                        case SwipeDismissBehavior.STATE_SETTLING:                            // If the view is being dragged or settling, pause the timeout                            SnackbarManager.getInstance().pauseTimeout(mManagerCallback);                            break;                        case SwipeDismissBehavior.STATE_IDLE:                            // If the view has been released and is idle, restore the timeout                            SnackbarManager.getInstance()                                    .restoreTimeoutIfPaused(mManagerCallback);                            break;                    }                }            });            clp.setBehavior(behavior);            // Also set the inset edge so that views can dodge the bar correctly            clp.insetEdge = Gravity.BOTTOM;        }        //重点来了，mView被添加到了mTargetParent中，之前向上遍历view获取的ViewGroup        mTargetParent.addView(mView);    }    mView.setOnAttachStateChangeListener(            new BaseTransientBottomBar.OnAttachStateChangeListener() {            @Override            public void onViewAttachedToWindow(View v) {}            @Override            public void onViewDetachedFromWindow(View v) {                if (isShownOrQueued()) {                    // If we haven't already been dismissed then this event is coming from a                    // non-user initiated action. Hence we need to make sure that we callback                    // and keep our state up to date. We need to post the call since                    // removeView() will call through to onDetachedFromWindow and thus overflow.                    sHandler.post(new Runnable() {                        @Override                        public void run() {                            onViewHidden(BaseCallback.DISMISS_EVENT_MANUAL);                        }                    });                }            }        });    if (ViewCompat.isLaidOut(mView)) {        if (shouldAnimate()) {            // If animations are enabled, animate it in            animateViewIn();        } else {            // Else if anims are disabled just call back now            onViewShown();        }    } else {        // Otherwise, add one of our layout change listeners and show it in when laid out        mView.setOnLayoutChangeListener(new BaseTransientBottomBar.OnLayoutChangeListener() {            @Override            public void onLayoutChange(View view, int left, int top, int right, int bottom) {                mView.setOnLayoutChangeListener(null);                if (shouldAnimate()) {                    // If animations are enabled, animate it in                    animateViewIn();                } else {                    // Else if anims are disabled just call back now                    onViewShown();                }            }        });    }}

通过showView中的代码，终于了解到Snackbar是如何显示的。Snackbar被直接添加到mTargetParent中，就是make方法传递进来的View的父CoordinatorLayout或根布局。
根据Snackbar的布局文件我们知道其layout_gravity为bottom，也就是会显示在mTargetParent的底部。那我们是不是只要将一个有固定高度的CoordinatorLayout传递个给Snackbar的make，就可以改变Snackbar的显示位置了？答案是肯定的！
到这里我们的问题1和问题2都明了了。那问题4应该如何解决呢？

SnackbarManager

根据上面的分析，我们知道show方法会调用SnackbarManager的show方法，那我们就来看看SnackBarManager的源码：

class SnackbarManager {    static final int MSG_TIMEOUT = 0;    private static final int SHORT_DURATION_MS = 1500;    private static final int LONG_DURATION_MS = 2750;    private static SnackbarManager sSnackbarManager;    //单例模式    static SnackbarManager getInstance() {        if (sSnackbarManager == null) {            sSnackbarManager = new SnackbarManager();        }        return sSnackbarManager;    }    private final Object mLock;    private final Handler mHandler;    //用来存储当前显示Snackbar的duration和Callback    private SnackbarRecord mCurrentSnackbar;    //用来存储接下来要显示的Snackbar的duration和Callback    private SnackbarRecord mNextSnackbar;    private SnackbarManager() {        mLock = new Object();        mHandler = new Handler(Looper.getMainLooper(), new Handler.Callback() {            @Override            public boolean handleMessage(Message message) {                switch (message.what) {                    case MSG_TIMEOUT:                        handleTimeout((SnackbarRecord) message.obj);                        return true;                }                return false;            }        });    }    interface Callback {        void show();        void dismiss(int event);    }    public void show(int duration, Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback)) { //判断是否是当前显示的Snackbar，更新duration                // Means that the callback is already in the queue. We'll just update the duration                mCurrentSnackbar.duration = duration;                // If this is the Snackbar currently being shown, call re-schedule it's                // timeout                mHandler.removeCallbacksAndMessages(mCurrentSnackbar);//移除Callback，避免内存泄露                scheduleTimeoutLocked(mCurrentSnackbar);//重新关联设置duration和Callback                return;            } else if (isNextSnackbarLocked(callback)) { //判断是否是接下来要显示的Snackbar，更新duration                // We'll just update the duration                mNextSnackbar.duration = duration;            } else {                // Else, we need to create a new record and queue it                mNextSnackbar = new SnackbarRecord(duration, callback);            }            if (mCurrentSnackbar != null && cancelSnackbarLocked(mCurrentSnackbar,                    Snackbar.Callback.DISMISS_EVENT_CONSECUTIVE)) {                // If we currently have a Snackbar, try and cancel it and wait in line                return;            } else {                // Clear out the current snackbar                mCurrentSnackbar = null;                // Otherwise, just show it now                showNextSnackbarLocked();            }        }    }    public void dismiss(Callback callback, int event) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback)) {                cancelSnackbarLocked(mCurrentSnackbar, event);            } else if (isNextSnackbarLocked(callback)) {                cancelSnackbarLocked(mNextSnackbar, event);            }        }    }    /**     * Should be called when a Snackbar is no longer displayed. This is after any exit     * animation has finished.     */    public void onDismissed(Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback)) {                // If the callback is from a Snackbar currently show, remove it and show a new one                mCurrentSnackbar = null;                if (mNextSnackbar != null) {                    showNextSnackbarLocked();                }            }        }    }    /**     * Should be called when a Snackbar is being shown. This is after any entrance animation has     * finished.     */    public void onShown(Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback)) {                scheduleTimeoutLocked(mCurrentSnackbar);            }        }    }    public void pauseTimeout(Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback) && !mCurrentSnackbar.paused) {                mCurrentSnackbar.paused = true;                mHandler.removeCallbacksAndMessages(mCurrentSnackbar);            }        }    }    public void restoreTimeoutIfPaused(Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback) && mCurrentSnackbar.paused) {                mCurrentSnackbar.paused = false;                scheduleTimeoutLocked(mCurrentSnackbar);            }        }    }    public boolean isCurrent(Callback callback) {        synchronized (mLock) {            return isCurrentSnackbarLocked(callback);        }    }    public boolean isCurrentOrNext(Callback callback) {        synchronized (mLock) {            return isCurrentSnackbarLocked(callback) || isNextSnackbarLocked(callback);        }    }    private static class SnackbarRecord {        final WeakReference<Callback> callback;        int duration;        boolean paused;        SnackbarRecord(int duration, Callback callback) {            this.callback = new WeakReference<>(callback);            this.duration = duration;        }        boolean isSnackbar(Callback callback) {            return callback != null && this.callback.get() == callback;        }    }    private void showNextSnackbarLocked() {        if (mNextSnackbar != null) {            mCurrentSnackbar = mNextSnackbar;            mNextSnackbar = null;            final Callback callback = mCurrentSnackbar.callback.get();            if (callback != null) {                callback.show();            } else {                // The callback doesn't exist any more, clear out the Snackbar                mCurrentSnackbar = null;            }        }    }    //具体的cancel方法，回调callback的dismiss    private boolean cancelSnackbarLocked(SnackbarRecord record, int event) {        final Callback callback = record.callback.get();        if (callback != null) {            // Make sure we remove any timeouts for the SnackbarRecord            mHandler.removeCallbacksAndMessages(record);            callback.dismiss(event);            return true;        }        return false;    }    private boolean isCurrentSnackbarLocked(Callback callback) {        return mCurrentSnackbar != null && mCurrentSnackbar.isSnackbar(callback);    }    private boolean isNextSnackbarLocked(Callback callback) {        return mNextSnackbar != null && mNextSnackbar.isSnackbar(callback);    }    private void scheduleTimeoutLocked(SnackbarRecord r) {        if (r.duration == Snackbar.LENGTH_INDEFINITE) {            // If we're set to indefinite, we don't want to set a timeout            return;        }        int durationMs = LONG_DURATION_MS;        if (r.duration > 0) {            durationMs = r.duration;        } else if (r.duration == Snackbar.LENGTH_SHORT) {            durationMs = SHORT_DURATION_MS;        }        mHandler.removeCallbacksAndMessages(r);        mHandler.sendMessageDelayed(Message.obtain(mHandler, MSG_TIMEOUT, r), durationMs);    }    void handleTimeout(SnackbarRecord record) {        synchronized (mLock) {            if (mCurrentSnackbar == record || mNextSnackbar == record) {                cancelSnackbarLocked(record, Snackbar.Callback.DISMISS_EVENT_TIMEOUT);            }        }    }}

单例模式，持有两个SnackbarRecord对象来存储当前显示和接下来要显示的Snackbar的duration和Callback，也就是说最多只会保有两个Snackbar的管理。
看其中的show方法可知，当mCurrentSnackbar不为null的时候，后面来的Snackbar都会存储在mNextSnackbar中。只有当当前显示的Snackbarduration到了后，调用onDismissed方法，清空mCurrentSnackbar，然后才会显示下一个Snackbar。
也就是说，当一个Snackbar显示期间，多次创建其他Snackbar show，到当前显示的Snackbar结束后，也只会显示最后一个创建的Snackbar。
到这里，问题4也清楚了。至于问题5，涉及到CoordinatorLayout Behavior，可阅读我的另一篇文章——《CoordinatorLayout源码解析》。