Snackbar简单使用及源码浅析

snackbar的简单用法效果图：

贴上面效果的代码

public void onClick(View view){        Snackbar snackbar = Snackbar.make(btn,"长沙下雨了",Snackbar.LENGTH_SHORT)                .setAction("确定", new View.OnClickListener() {                    @Override                    public void onClick(View view) {                        Toast.makeText(MainActivity.this,"点击确定了",Toast.LENGTH_SHORT).show();                    }                })                .setCallback(new Snackbar.Callback(){                    @Override                    public void onShown(Snackbar sb) {                        super.onShown(sb);                    }                    @Override                    public void onDismissed(Snackbar transientBottomBar, int event) {                        super.onDismissed(transientBottomBar, event);                    }                })                .setActionTextColor(Color.YELLOW);        View v = snackbar.getView();        TextView tv = v.findViewById(R.id.snackbar_text);        tv.setTextColor(Color.BLUE);        snackbar.show();    }

通过Snackbar的make(@NonNull View view, @NonNull CharSequence text,
@Duration int duration)方法来创建一个实例，第一个参数是点击的view，第二个参数是要提示的字符串，第三个参数是snackbar要显示的时间，duration的取值有三个：LENGTH_SHORT较短时间，LENGTH_LONG较长时间，LENGTH_INDEFINITE不会消失（如果没有调用dismiss方法或者没有其他的snackbar需要显示），创建完成后可以用show()方法进行显示。

也可以通过setAction(CharSequence text, final View.OnClickListener listener)方法设置一个用于交互的按钮，例如上面的确定按钮。

通过setActionTextColor(@ColorInt int color)方法来设置交互按钮文字的颜色。

还可以通过setCallback(Callback callback)方法设置回调，实现onShown和onDismissed方法，就是在snackbar显示和消失时的回调，我们可以做一些我们想做的事情。

View v = snackbar.getView();
TextView tv = v.findViewById(R.id.snackbar_text);
tv.setTextColor(Color.BLUE);
这段代码是为了设置snackbar设置要显示的文字的颜色，（例如上面的“长沙下雨了”）
Snackbar是没有提供给我们改变文字颜色的方法的。我们可以在源码中可以发现这个从下面弹出来的snackbar的布局是由左边一个TextView，右边一个Button组成。我们通过id，直接获取TextView进行设置。

下面对Snackbar的源码就行浅浅的分析：
Snackbar继承BaseTransientBottomBar。
入口Snackbar.make()方法。

@NonNull    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.");        }        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;    }

这一行代码
final ViewGroup parent = findSuitableParent(view);
通过findSuitableParent()方法找到顶层View，这里的顶层View不是DecorView，而是CoordinatorLayout或者FrameLayout，看这个方法的实现就知道为什么了：

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;    }

这个方法里面是个do while循环 ，通过view.getParent()获取父布局，并再复制给view，直到找到CoordinatorLayout，然后return，或者找到id为android.R.id.content的FrameLayout布局。
那它为什么要找到这个布局呢，其实就是要在后面把我们的snackbar的实质布局添加到这上面显示出来。

看完这个方法，再回到Snackbar的make()方法里接着向下走，

final SnackbarContentLayout content =(SnackbarContentLayout) inflater.inflate( R.layout.design_layout_snackbar_include, parent, false);

这行代码就是创建了Snackbar显示的布局SnackbarContentLayout，SnackbarContentLayout是继承LinearLayout的，我们可以看一下这个layout布局：

<view    xmlns:android="http://schemas.android.com/apk/res/android"    class="android.support.design.internal.SnackbarContentLayout"    android:theme="@style/ThemeOverlay.AppCompat.Dark"    android:layout_width="match_parent"    android:layout_height="wrap_content"    android:layout_gravity="bottom">    <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:minWidth="48dp"        android:visibility="gone"        android:textColor="?attr/colorAccent"        style="?attr/borderlessButtonStyle"/></view>

里面有一个id为snackbar_text的TextView，和一个id为snackbar_action的Button。所以我们可以直接通过findviewbyid的方式拿到TextView和Button来改变它们的属性。

好，接下来再回到make方法向下走，
final Snackbar snackbar = new Snackbar(parent, content, content);
将找到的parent，和解析出的布局SnackbarContentLayout通过Snackbar的构造函数传进去，创建Snackbar实例，然后返回。这就是make()的实现。我们还需要再看一下Snackbar的创建，

private Snackbar(ViewGroup parent, View content, ContentViewCallback contentViewCallback) {        super(parent, content, contentViewCallback);    }

调用了super，看一下父类实现，

protected BaseTransientBottomBar(@NonNull ViewGroup parent, @NonNull View content,            @NonNull ContentViewCallback contentViewCallback) {        if (parent == null) {            throw new IllegalArgumentException("Transient bottom bar must have non-null parent");        }        if (content == null) {            throw new IllegalArgumentException("Transient bottom bar must have non-null content");        }        if (contentViewCallback == null) {            throw new IllegalArgumentException("Transient bottom bar must have non-null callback");        }        mTargetParent = parent;        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);        ViewCompat.setAccessibilityLiveRegion(mView,                ViewCompat.ACCESSIBILITY_LIVE_REGION_POLITE);        ViewCompat.setImportantForAccessibility(mView,                ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_YES);        // Make sure that we fit system windows and have a listener to apply any insets        ViewCompat.setFitsSystemWindows(mView, true);        ViewCompat.setOnApplyWindowInsetsListener(mView,                new android.support.v4.view.OnApplyWindowInsetsListener() {                    @Override                    public WindowInsetsCompat onApplyWindowInsets(View v,                            WindowInsetsCompat insets) {                        // Copy over the bottom inset as padding so that we're displayed                        // above the navigation bar                        v.setPadding(v.getPaddingLeft(), v.getPaddingTop(),                                v.getPaddingRight(), insets.getSystemWindowInsetBottom());                        return insets;                    }                });        mAccessibilityManager = (AccessibilityManager)                mContext.getSystemService(Context.ACCESSIBILITY_SERVICE);    }

首先将找到的父布局parent赋值给mTargetParent，然后给mView赋值，通过mView = (SnackbarBaseLayout) inflater.inflate(R.layout.design_layout_snackbar, mTargetParent, false);这行代码，再将content布局添加到mView里。

到这里make()方法结束。
下面就是调用show()方法来显示了；
在看show方法前，先看两个初始化的操作，在Snackbar的父类BaseTransientBottomBar里有一个static静态代码块，初始化了一个Handler，用来发消息。

static {        sHandler = new Handler(Looper.getMainLooper(), new Handler.Callback() {            @Override            public boolean handleMessage(Message message) {                switch (message.what) {                    case MSG_SHOW:                        ((BaseTransientBottomBar) message.obj).showView();                        return true;                    case MSG_DISMISS:                        ((BaseTransientBottomBar) message.obj).hideView(message.arg1);                        return true;                }                return false;            }        });    }

还有一个是SnackbarManager的Callback的初始化。

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));        }    };

show()和dismiss()方法都会通过Handler发消息，分别调用对用的BaseTransientBottomBar的showView()和hideView()方法。

好下面我们来看show方法，

/**     * Show the {@link BaseTransientBottomBar}.     */    public void show() {        SnackbarManager.getInstance().show(mDuration, mManagerCallback);    }

public void show(int duration, Callback callback) {        synchronized (mLock) {            if (isCurrentSnackbarLocked(callback)) {                // 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);                scheduleTimeoutLocked(mCurrentSnackbar);                return;            } else if (isNextSnackbarLocked(callback)) {                // 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();            }        }    }

在第一个if else里面如果snackbar是第一次显示，会走到最后一个else里面mNextSnackbar = new SnackbarRecord(duration, callback);
在SnackbarRecord的构造方法里就是简单的赋值操作

SnackbarRecord(int duration, Callback callback) {            this.callback = new WeakReference<>(callback);            this.duration = duration;        }

然后再往下执行，就到了showNextSnackbarLocked()方法；

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;            }        }    }

这里的callback就是前面传入并封装到SnackbarRecord里的SnackbarManager的内部类实例mManagerCallback，调用了callback的show方法。然后就衔接到了上面一开始初始化Hanlder和Callback的地方，show方法里Handler发消息调用BaseTransientBottomBar的showView方法。

final void showView() {        if (mView.getParent() == null) {            final ViewGroup.LayoutParams lp = mView.getLayoutParams();            if (lp instanceof CoordinatorLayout.LayoutParams) {                // If our LayoutParams are from a CoordinatorLayout, we'll setup our Behavior                final CoordinatorLayout.LayoutParams clp = (CoordinatorLayout.LayoutParams) lp;                final Behavior behavior = new Behavior();                behavior.setStartAlphaSwipeDistance(0.1f);                behavior.setEndAlphaSwipeDistance(0.6f);                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;            }            mTargetParent.addView(mView);        }

showView方法最后会执行mTargetParent.addView(mView);结束end。

这样snackbar显示的布局就添加到了所谓的顶层布局FrameLayout的最上层了，而不会被我们的contentView布局所覆盖。

Snackbar是 Material Design Support 8大控件之一，它和Dialog，Toast都有类似之处，
Dialog略显笨拙，会阻断用户的连续性，体验会打折扣，交互性太强。而Dialog的生命周期跟Snackbar一样，都是跟随当前Activity. 而Toast与它们两不同，跟当前的Activity的无关，因为Toast是通过Window实现的。Toast与Snackbar的不同处还有就是Toast不会一直显示，Snackbar可以通过设置Snackbar.LENGTH_INDEFINITE不消失，还有Toast只是用于显示没有交互性，Snackbar可以交互。

暂且到此结束。