異常資訊：

java.lang.NoClassDefFoundError: android.support.v4.view.KeyEventCompat
    at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:526)
    at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
    at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:319)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1962)
    at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:3876)
    at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3850)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3423)
    at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3473)
    at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3442)
    at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3549)
    at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3450)
    at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3606)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3423)
    at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3473)
    at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3442)
    at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3450)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3423)
    at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3473)
    at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3442)
    at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3582)
    at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3742)
    at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2010)
    at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1704)
    at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1695)
    at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:1987)
    at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
    at android.os.MessageQueue.nativePollOnce(Native Method)
    at android.os.MessageQueue.next(MessageQueue.java:138)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:5019)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    at dalvik.system.NativeStart.main(Native Method)
 

為啥出現：

因為當support_version 27，support-v4中去除了keyeventcompat類。我們可以看到，在appcompatactivity的dispatchkeyevent方法種執行to keyeventcompat.isctrlpressed（事件），因為不能找到the class，所以導致APP崩潰

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        if (KeyEventCompat.isCtrlPressed(event) &&
                event.getUnicodeChar(event.getMetaState() & ~KeyEvent.META_CTRL_MASK) == '<') {
            // Capture the Control-< and send focus to the ActionBar
            final int action = event.getAction();
            if (action == KeyEvent.ACTION_DOWN) {
                final ActionBar actionBar = getSupportActionBar();
                if (actionBar != null && actionBar.isShowing() && actionBar.requestFocus()) {
                    mEatKeyUpEvent = true;
                    return true;
                }
            } else if (action == KeyEvent.ACTION_UP && mEatKeyUpEvent) {
                mEatKeyUpEvent = false;
                return true;
            }
        }
        return super.dispatchKeyEvent(event);
    }
 

1、在你的工程下新建一個包：android.support.v4.view

2、複製KeyEventCompat 和 KeyEventCompatHoneycomb 兩個類到這個包下面

3、Rebuild project

其中 KeyEventCompat ：

/*
 * Copyright (C) 2011 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.
 */

package android.support.v4.view;

import android.view.KeyEvent;
import android.view.View;

/**
 * Helper for accessing features in {@link KeyEvent} introduced after
 * API level 4 in a backwards compatible fashion.
 */
public final class KeyEventCompat {
    /**
     * Interface for the full API.
     */
    interface KeyEventVersionImpl {
        int normalizeMetaState(int metaState);
        boolean metaStateHasModifiers(int metaState, int modifiers);
        boolean metaStateHasNoModifiers(int metaState);
        boolean isCtrlPressed(KeyEvent event);
    }

    /**
     * Interface implementation that doesn't use anything about v4 APIs.
     */
    static class BaseKeyEventVersionImpl implements KeyEventVersionImpl {
        private static final int META_MODIFIER_MASK =
                KeyEvent.META_SHIFT_ON | KeyEvent.META_SHIFT_LEFT_ON | KeyEvent.META_SHIFT_RIGHT_ON
                        | KeyEvent.META_ALT_ON | KeyEvent.META_ALT_LEFT_ON | KeyEvent.META_ALT_RIGHT_ON
                        | KeyEvent.META_SYM_ON;

        // Mask of all lock key meta states.
        private static final int META_ALL_MASK = META_MODIFIER_MASK;

        private static int metaStateFilterDirectionalModifiers(int metaState,
                                                               int modifiers, int basic, int left, int right) {
            final boolean wantBasic = (modifiers & basic) != 0;
            final int directional = left | right;
            final boolean wantLeftOrRight = (modifiers & directional) != 0;

            if (wantBasic) {
                if (wantLeftOrRight) {
                    throw new IllegalArgumentException("bad arguments");
                }
                return metaState & ~directional;
            } else if (wantLeftOrRight) {
                return metaState & ~basic;
            } else {
                return metaState;
            }
        }

        @Override
        public int normalizeMetaState(int metaState) {
            if ((metaState & (KeyEvent.META_SHIFT_LEFT_ON | KeyEvent.META_SHIFT_RIGHT_ON)) != 0) {
                metaState |= KeyEvent.META_SHIFT_ON;
            }
            if ((metaState & (KeyEvent.META_ALT_LEFT_ON | KeyEvent.META_ALT_RIGHT_ON)) != 0) {
                metaState |= KeyEvent.META_ALT_ON;
            }
            return metaState & META_ALL_MASK;
        }

        @Override
        public boolean metaStateHasModifiers(int metaState, int modifiers) {
            metaState = normalizeMetaState(metaState) & META_MODIFIER_MASK;
            metaState = metaStateFilterDirectionalModifiers(metaState, modifiers,
                    KeyEvent.META_SHIFT_ON, KeyEvent.META_SHIFT_LEFT_ON, KeyEvent.META_SHIFT_RIGHT_ON);
            metaState = metaStateFilterDirectionalModifiers(metaState, modifiers,
                    KeyEvent.META_ALT_ON, KeyEvent.META_ALT_LEFT_ON, KeyEvent.META_ALT_RIGHT_ON);
            return metaState == modifiers;
        }

        @Override
        public boolean metaStateHasNoModifiers(int metaState) {
            return (normalizeMetaState(metaState) & META_MODIFIER_MASK) == 0;
        }

        @Override
        public boolean isCtrlPressed(KeyEvent event) {
            return false;
        }
    }

    /**
     * Interface implementation for devices with at least v11 APIs.
     */
    static class HoneycombKeyEventVersionImpl extends BaseKeyEventVersionImpl {
        @Override
        public int normalizeMetaState(int metaState) {
            return KeyEventCompatHoneycomb.normalizeMetaState(metaState);
        }

        @Override
        public boolean metaStateHasModifiers(int metaState, int modifiers) {
            return KeyEventCompatHoneycomb.metaStateHasModifiers(metaState, modifiers);
        }

        @Override
        public boolean metaStateHasNoModifiers(int metaState) {
            return KeyEventCompatHoneycomb.metaStateHasNoModifiers(metaState);
        }

        @Override
        public boolean isCtrlPressed(KeyEvent event) {
            return KeyEventCompatHoneycomb.isCtrlPressed(event);
        }
    }

    /**
     * Select the correct implementation to use for the current platform.
     */
    static final KeyEventVersionImpl IMPL;
    static {
        if (android.os.Build.VERSION.SDK_INT >= 11) {
            IMPL = new HoneycombKeyEventVersionImpl();
        } else {
            IMPL = new BaseKeyEventVersionImpl();
        }
    }

    // -------------------------------------------------------------------

    public static int normalizeMetaState(int metaState) {
        return IMPL.normalizeMetaState(metaState);
    }

    public static boolean metaStateHasModifiers(int metaState, int modifiers) {
        return IMPL.metaStateHasModifiers(metaState, modifiers);
    }

    public static boolean metaStateHasNoModifiers(int metaState) {
        return IMPL.metaStateHasNoModifiers(metaState);
    }

    public static boolean hasModifiers(KeyEvent event, int modifiers) {
        return IMPL.metaStateHasModifiers(event.getMetaState(), modifiers);
    }

    public static boolean hasNoModifiers(KeyEvent event) {
        return IMPL.metaStateHasNoModifiers(event.getMetaState());
    }

    /**
     * @deprecated Call {@link KeyEvent#startTracking()} directly. This method will be removed in a
     * future release.
     */
    @Deprecated
    public static void startTracking(KeyEvent event) {
        event.startTracking();
    }

    /**
     * @deprecated Call {@link KeyEvent#isTracking()} directly. This method will be removed in a
     * future release.
     */
    @Deprecated
    public static boolean isTracking(KeyEvent event) {
        return event.isTracking();
    }

    /**
     * @deprecated Call {@link View#getKeyDispatcherState()} directly. This method will be removed
     * in a future release.
     */
    @Deprecated
    public static Object getKeyDispatcherState(View view) {
        return view.getKeyDispatcherState();
    }

    /**
     * @deprecated Call
     * {@link KeyEvent#dispatch(KeyEvent.Callback, KeyEvent.DispatcherState, Object)} directly.
     * This method will be removed in a future release.
     */
    @Deprecated
    public static boolean dispatch(KeyEvent event, KeyEvent.Callback receiver, Object state,
                                   Object target) {
        return event.dispatch(receiver, (KeyEvent.DispatcherState)state, target);
    }

    public static boolean isCtrlPressed(KeyEvent event) {
        return IMPL.isCtrlPressed(event);
    }

    private KeyEventCompat() {}
}
 

/*
 * Copyright (C) 2011 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.
 */

package android.support.v4.view;

import android.support.annotation.RequiresApi;
import android.annotation.TargetApi;
import android.view.KeyEvent;

/**
 * Implementation of key event compatibility that can call Honeycomb APIs.
 */

@RequiresApi(11)
@TargetApi(11)
class KeyEventCompatHoneycomb {
    public static int normalizeMetaState(int metaState) {
        return KeyEvent.normalizeMetaState(metaState);
    }

    public static boolean metaStateHasModifiers(int metaState, int modifiers) {
        return KeyEvent.metaStateHasModifiers(metaState, modifiers);
    }

    public static boolean metaStateHasNoModifiers(int metaState) {
        return KeyEvent.metaStateHasNoModifiers(metaState);
    }

    public static boolean isCtrlPressed(KeyEvent event) {
        return event.isCtrlPressed();
    }
}