[移动开发]Android 11 toast居中显示

????????Android 11以后，系统Toast的text toast被设置为固定在屏幕下方显示，通过setGravity已经不会生效。

????????为了解决这个问题，去看了Toast源码。从show()方法开始，

????????

    /**
     * Show the view for the specified duration.
     */
    public void show() {
        if (Compatibility.isChangeEnabled(CHANGE_TEXT_TOASTS_IN_THE_SYSTEM)) {
            checkState(mNextView != null || mText != null, "You must either set a text or a view");
        } else {
            if (mNextView == null) {
                throw new RuntimeException("setView must have been called");
            }
        }

        INotificationManager service = getService();
        String pkg = mContext.getOpPackageName();
        TN tn = mTN;
        tn.mNextView = mNextView;
        final int displayId = mContext.getDisplayId();

        try {
            if (Compatibility.isChangeEnabled(CHANGE_TEXT_TOASTS_IN_THE_SYSTEM)) {
                if (mNextView != null) {
                    // It's a custom toast
                    service.enqueueToast(pkg, mToken, tn, mDuration, displayId);
                } else {
                    // It's a text toast
                    ITransientNotificationCallback callback =
                            new CallbackBinder(mCallbacks, mHandler);
                    service.enqueueTextToast(pkg, mToken, mText, mDuration, displayId, callback);
                }
            } else {
                service.enqueueToast(pkg, mToken, tn, mDuration, displayId);
            }
        } catch (RemoteException e) {
            // Empty
        }
    }

到TN的handleShow()方法，

        public void handleShow(IBinder windowToken) {
            if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
                    + " mNextView=" + mNextView);
            // If a cancel/hide is pending - no need to show - at this point
            // the window token is already invalid and no need to do any work.
            if (mHandler.hasMessages(CANCEL) || mHandler.hasMessages(HIDE)) {
                return;
            }
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                mPresenter.show(mView, mToken, windowToken, mDuration, mGravity, mX, mY,
                        mHorizontalMargin, mVerticalMargin,
                        new CallbackBinder(getCallbacks(), mHandler));
            }
        }

最终跟踪到ToastPresenter的show()方法，

    /**
     * Shows the toast in {@code view} with the parameters passed and callback {@code callback}.
     */
    public void show(View view, IBinder token, IBinder windowToken, int duration, int gravity,
            int xOffset, int yOffset, float horizontalMargin, float verticalMargin,
            @Nullable ITransientNotificationCallback callback) {
        checkState(mView == null, "Only one toast at a time is allowed, call hide() first.");
        mView = view;
        mToken = token;

        adjustLayoutParams(mParams, windowToken, duration, gravity, xOffset, yOffset,
                horizontalMargin, verticalMargin);
        if (mView.getParent() != null) {
            mWindowManager.removeView(mView);
        }
        try {
            mWindowManager.addView(mView, mParams);
        } catch (WindowManager.BadTokenException e) {
            // Since the notification manager service cancels the token right after it notifies us
            // to cancel the toast there is an inherent race and we may attempt to add a window
            // after the token has been invalidated. Let us hedge against that.
            Log.w(TAG, "Error while attempting to show toast from " + mPackageName, e);
            return;
        }
        trySendAccessibilityEvent(mView, mPackageName);
        if (callback != null) {
            try {
                callback.onToastShown();
            } catch (RemoteException e) {
                Log.w(TAG, "Error calling back " + mPackageName + " to notify onToastShow()", e);
            }
        }
    }

对比android11之前的源码，发现基本没有变动，都是通过WindowManager的addView方法添加toast，而且LayoutParams也有设置Gravity属性，

    /**
     * Customizes {@code params} according to other parameters, ready to be passed to {@link
     * WindowManager#addView(View, ViewGroup.LayoutParams)}.
     */
    private void adjustLayoutParams(WindowManager.LayoutParams params, IBinder windowToken,
            int duration, int gravity, int xOffset, int yOffset, float horizontalMargin,
            float verticalMargin) {
        Configuration config = mResources.getConfiguration();
        int absGravity = Gravity.getAbsoluteGravity(gravity, config.getLayoutDirection());
        params.gravity = absGravity;
        if ((absGravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
            params.horizontalWeight = 1.0f;
        }
        if ((absGravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
            params.verticalWeight = 1.0f;
        }
        params.x = xOffset;
        params.y = yOffset;
        params.horizontalMargin = horizontalMargin;
        params.verticalMargin = verticalMargin;
        params.packageName = mContext.getPackageName();
        params.hideTimeoutMilliseconds =
                (duration == Toast.LENGTH_LONG) ? LONG_DURATION_TIMEOUT : SHORT_DURATION_TIMEOUT;
        params.token = windowToken;
    }

但是Gravity属性为什么没有生效呢？

? ? ? ? 考虑到是不是Toast的默认view的布局文件设置了match_parent属性，导致Gravity属性不生效。去Toast默认view的布局文件查看（默认view初始化在Toast类的makeText方法里，最终走的是ToastPresenter的getTextToastView方法），果然，发现默认view的布局文件宽高属性，均为match_parent，

    /**
     * Make a standard toast to display using the specified looper.
     * If looper is null, Looper.myLooper() is used.
     *
     * @hide
     */
    public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
            @NonNull CharSequence text, @Duration int duration) {
        if (Compatibility.isChangeEnabled(CHANGE_TEXT_TOASTS_IN_THE_SYSTEM)) {
            Toast result = new Toast(context, looper);
            result.mText = text;
            result.mDuration = duration;
            return result;
        } else {
            Toast result = new Toast(context, looper);
            View v = ToastPresenter.getTextToastView(context, text);
            result.mNextView = v;
            result.mDuration = duration;

            return result;
        }
    }

    /**
     * Returns the default text toast view for message {@code text}.
     */
    public static View getTextToastView(Context context, CharSequence text) {
        View view = LayoutInflater.from(context).inflate(TEXT_TOAST_LAYOUT, null);
        TextView textView = view.findViewById(com.android.internal.R.id.message);
        textView.setText(text);
        return view;
    }

<?xml version="1.0" encoding="utf-8"?>
<!--
/* //device/apps/common/res/layout/transient_notification.xml
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/
-->

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="?android:attr/toastFrameBackground">

    <TextView
        android:id="@android:id/message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:layout_marginHorizontal="24dp"
        android:layout_marginVertical="15dp"
        android:layout_gravity="center_horizontal"
        android:textAppearance="@style/TextAppearance.Toast"
        android:textColor="@color/primary_text_default_material_light"
        />

</LinearLayout>

即虽然我们看到屏幕上的toast只是一个小小的文字部分，但实际其view是布满全局的，这个时候设置gravity就没有任何意义了，因为无论你设置gravity是什么值，view都是铺满全屏的。

? ? ? ? 解决方案：用到的解决方案就是给toast设置自己的布局文件，当然，自己的布局文件要注意不是铺满全屏的，然后把样式设置成跟系统自带的一样就可以了，此时，设置gravity为Gravity.CENTER就可以生效了（注意，不设置的话，默认还是在屏幕下方），参考代码

        Toast toast = new Toast(getApplication());
        View view = LayoutInflater.from(getApplication()).inflate(R.layout.layout_toast, null);
        //根据自己需要对view设置text或其他样式
        toast.setView(view);
        toast.setGravity(Gravity.CENTER, 0, 0);
        toast.setDuration(Toast.LENGTH_SHORT);
        toast.show();

参考布局文件

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="72dp"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:background="#66000000"
    android:paddingStart="40dp"
    android:paddingEnd="40dp"
    android:paddingTop="18dp"
    android:paddingBottom="18dp">

    <TextView
        android:id="@+id/tv_toast_message"
        android:layout_width="wrap_content"
        android:layout_height="36dp"
        android:gravity="center_vertical"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        android:textSize="24sp"
        android:textColor="#FFFFFF"
        android:text="测试toast"/>

</androidx.constraintlayout.widget.ConstraintLayout>

题外话：这次Android11修改toast，不仅把setGravity设置成不可用，还把setView设置成废弃方法了，但实际上我测试发现，无论android11还是android12，调用setView方法后，再设置setGravity还是可以用的，使用默认的setView时不可以用，最终什么时候这个方法不可用无法保证。