Question

错误 mAppBarLayout.addOnOffsetChangedListener（this）;

错误显示：

引起：java.lang.NullPointerException：尝试调用虚拟方法'无效 android.support.design.widget.AppBarLayout.addOnOffsetChangedListener（android.support.design.widget.AppBarLayout $ OnOffsetChangedListener）” 在空对象引用上

这是我的 user_profile.java

package com.example.farid.codersappdemo;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import android.support.design.widget.AppBarLayout;
import android.support.design.widget.CollapsingToolbarLayout;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.View;
import android.view.animation.AlphaAnimation;
import android.widget.FrameLayout;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;

public class user_profile extends AppCompatActivity
    implements AppBarLayout.OnOffsetChangedListener{

    private static final float PERCENTAGE_TO_SHOW_TITLE_AT_TOOLBAR  = 0.9f;
    private static final float PERCENTAGE_TO_HIDE_TITLE_DETAILS     = 0.3f;
    private static final int ALPHA_ANIMATIONS_DURATION              = 200;

    private boolean mIsTheTitleVisible          = false;
    private boolean mIsTheTitleContainerVisible = true;

    private LinearLayout mTitleContainer;
    private TextView mTitle;
    private AppBarLayout mAppBarLayout;
    private Toolbar mToolbar;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        bindActivity();

        mAppBarLayout.addOnOffsetChangedListener(this);

        mToolbar.inflateMenu(R.menu.profile_menu);
        startAlphaAnimation(mTitle, 0, View.INVISIBLE);
    }

    private void bindActivity() {
        mToolbar        = findViewById(R.id.profile_toolbar);
        mTitle          = findViewById(R.id.profile_textview_title);
        mTitleContainer = findViewById(R.id.profile_linearlayout_title);
        mAppBarLayout   = findViewById(R.id.profile_appbar);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.profile_menu, menu);
        return true;
    }

    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int offset) {
        int maxScroll = appBarLayout.getTotalScrollRange();
        float percentage = (float) Math.abs(offset) / (float) maxScroll;

        handleAlphaOnTitle(percentage);
        handleToolbarTitleVisibility(percentage);
    }

    private void handleToolbarTitleVisibility(float percentage) {
        if (percentage >= PERCENTAGE_TO_SHOW_TITLE_AT_TOOLBAR) {

            if(!mIsTheTitleVisible) {
                startAlphaAnimation(mTitle, ALPHA_ANIMATIONS_DURATION, View.VISIBLE);
                mIsTheTitleVisible = true;
            }

        } else {

            if (mIsTheTitleVisible) {
                startAlphaAnimation(mTitle, ALPHA_ANIMATIONS_DURATION, View.INVISIBLE);
                mIsTheTitleVisible = false;
            }
        }
    }

    private void handleAlphaOnTitle(float percentage) {
        if (percentage >= PERCENTAGE_TO_HIDE_TITLE_DETAILS) {
            if(mIsTheTitleContainerVisible) {
                startAlphaAnimation(mTitleContainer, ALPHA_ANIMATIONS_DURATION, View.INVISIBLE);
                mIsTheTitleContainerVisible = false;
            }

        } else {

            if (!mIsTheTitleContainerVisible) {
                startAlphaAnimation(mTitleContainer, ALPHA_ANIMATIONS_DURATION, View.VISIBLE);
                mIsTheTitleContainerVisible = true;
            }
        }
    }

    public static void startAlphaAnimation (View v, long duration, int visibility) {
        AlphaAnimation alphaAnimation = (visibility == View.VISIBLE)
                ? new AlphaAnimation(0f, 1f)
                : new AlphaAnimation(1f, 0f);

        alphaAnimation.setDuration(duration);
        alphaAnimation.setFillAfter(true);
        v.startAnimation(alphaAnimation);
    }
}

这里是 activity_user_profile.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".user_profile"
    tools:ignore="RtlHardcoded">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/profile_appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

        <android.support.design.widget.CollapsingToolbarLayout
            android:id="@+id/profile_collapsing_toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">

            <ImageView
                android:id="@+id/profile_cover_img"
                android:layout_width="match_parent"
                android:layout_height="300dp"
                android:scaleType="centerCrop"
                android:src="@drawable/material_background"
                android:tint="#11000000"
                app:layout_collapseMode="parallax"
                app:layout_collapseParallaxMultiplier="0.9"
                />
            <FrameLayout
                android:id="@+id/profile_framelayout_title"
                android:layout_width="match_parent"
                android:layout_height="100dp"
                android:layout_gravity="bottom|center_horizontal"
                android:background="@color/colorPrimary"
                android:orientation="vertical"
                app:layout_collapseMode="parallax"
                app:layout_collapseParallaxMultiplier="0.3"
                >

                <LinearLayout
                    android:id="@+id/profile_linearlayout_title"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
                    android:orientation="vertical"
                    >

                    <TextView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:layout_gravity="center_horizontal"
                        android:gravity="bottom|center"
                        android:text="@string/quila_name"
                        android:textColor="@android:color/white"
                        android:textSize="30sp"
                        />

                    <TextView
                        android:layout_width="wrap_content"
                        android:layout_height="wrap_content"
                        android:layout_gravity="center_horizontal"
                        android:layout_marginTop="4dp"
                        android:text="@string/quila_tagline"
                        android:textColor="@android:color/white"
                        />

                </LinearLayout>
            </FrameLayout>
        </android.support.design.widget.CollapsingToolbarLayout>
    </android.support.design.widget.AppBarLayout>

    <android.support.v4.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="none"
        app:behavior_overlapTop="30dp"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"

        >

        <android.support.v7.widget.CardView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_margin="8dp"
            app:cardElevation="8dp"
            app:contentPadding="16dp"
            >

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:lineSpacingExtra="8dp"
                android:text="@string/lorem"
                android:textSize="18sp"
                />
        </android.support.v7.widget.CardView>


    </android.support.v4.widget.NestedScrollView>

    <android.support.v7.widget.Toolbar
        android:id="@+id/profile_toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="@color/colorPrimary"
        app:layout_anchor="@id/profile_framelayout_title"
        app:theme="@style/ThemeOverlay.AppCompat.Dark"
        app:title=""
        >

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:orientation="horizontal"
            >

            <Space
                android:layout_width="@dimen/image_final_width"
                android:layout_height="@dimen/image_final_width"
                />

            <TextView
                android:id="@+id/profile_textview_title"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:layout_marginLeft="8dp"
                android:gravity="center_vertical"
                android:text="@string/quila_name2"
                android:textColor="@android:color/white"
                android:textSize="20sp"
                />

        </LinearLayout>
    </android.support.v7.widget.Toolbar>

    <de.hdodenhof.circleimageview.CircleImageView
        android:layout_width="@dimen/image_width"
        android:layout_height="@dimen/image_width"
        android:layout_gravity="center_horizontal"
        android:src="@drawable/man"
        app:civ_border_color="@android:color/white"
        app:civ_border_width="2dp"
        app:finalHeight="@dimen/image_final_width"
        app:finalYPosition="2dp"
        app:layout_behavior="behavior.AvatarImageBehavior"
        app:startHeight="2dp"
        app:startToolbarPosition="2dp"
        app:startXPosition="2dp"
        />

</android.support.design.widget.CoordinatorLayout>

java.lang.NullPointerException：尝试调用虚方法'void android.support.design.widget.AppBarLayout.addOnOffsetChangedListener

0 个答案: