Android 可折叠式标题栏的实现

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.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=".activity.FruitActivity">
    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/app_bar"
        android:layout_width="match_parent"
        android:layout_height="250dp">
        <com.google.android.material.appbar.CollapsingToolbarLayout
            android:id="@+id/collapsing"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
            app:contentScrim="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|exitUntilCollapsed">
            <ImageView
                android:id="@+id/iv_image"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:scaleType="centerCrop"
                android:src="@drawable/head"
                app:layout_collapseMode="parallax" />
            <androidx.appcompat.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                app:layout_collapseMode="pin" />
        </com.google.android.material.appbar.CollapsingToolbarLayout>
    </com.google.android.material.appbar.AppBarLayout>
    <androidx.core.widget.NestedScrollView
        android:id="@+id/nested_scrollView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">
            <androidx.cardview.widget.CardView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginLeft="15dp"
                android:layout_marginTop="35dp"
                android:layout_marginRight="15dp"
                android:layout_marginBottom="15dp"
                app:cardBackgroundColor="@color/white"
                app:cardCornerRadius="4dp">
                <TextView
                    android:id="@+id/tv_text"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_margin="10dp"
                    android:text="我这里是一个卡片布局！" />
            </androidx.cardview.widget.CardView>
        </LinearLayout>
    </androidx.core.widget.NestedScrollView>
    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/floating"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:background="#00000000"
        android:src="@drawable/comment"
        app:layout_anchor="@id/app_bar"
        app:layout_anchorGravity="bottom|end"></com.google.android.material.floatingactionbutton.FloatingActionButton>
</androidx.coordinatorlayout.widget.CoordinatorLayout>

public class FruitActivity extends AppCompatActivity {
    private CollapsingToolbarLayout collapsing;
    private Toolbar toolbar;
    private FloatingActionButton floating;
    private TextView tv_text;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fruit);
        collapsing = findViewById(R.id.collapsing);
        toolbar = findViewById(R.id.toolbar);
        floating = findViewById(R.id.floating);
        tv_text = findViewById(R.id.tv_text);
        setSupportActionBar(toolbar);
        ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) {
            actionBar.setDisplayHomeAsUpEnabled(true);
        }
        collapsing.setTitle("这是CollapsingToolbarLayout");
        String text = "努力努力再努力";
        tv_text.setText(generateText(text));
        floating.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(FruitActivity.this, "您点击了悬浮按钮哦！", Toast.LENGTH_SHORT).show();
            }
        });
    }
    private String generateText(String text) {
        StringBuilder stringBuilder = new StringBuilder("");
        for (int i = 0; i < 500; i++) {
            stringBuilder.append(text);
        }
        return stringBuilder.toString();
    }
    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()) {
          //Toolbar左上角默认有一个返回的箭头，含义是返回上一个活动
          //这个按钮叫做HomeAsUp按钮，这个按钮的id永远都是android.R.id.home
            case android.R.id.home:
                finish();
                break;
        }
        return true;
    }
}