View的渐变动画效果（代码实现）

渐变动画（也叫消失）通常指渐渐的淡出某个UI组件，同时同步地淡入另一个。当App想切换内容或View的情况下，这种动画很有用。渐变简短不易察觉，同时又提供从一个界面到下一个之间流畅的转换。如果在需要转换的时候没有使用任何动画效果，这会使得转换看上去感到生硬而仓促。

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1. xml布局

<LinearLayout 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:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/click_btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="点击按钮"
        />

        <TextView
            android:id="@+id/content"
            style="?android:textAppearanceMedium"
            android:lineSpacingMultiplier="1.2"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="aaaaaaaaaaaaaaaaaaaaaa"
            android:padding="16dp" />

        <ProgressBar android:id="@+id/loading_spinner"
            style="?android:progressBarStyleLarge"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"

            />

</LinearLayout>

效果是点击按钮后，渐渐隐藏ProgressBar ， 显示 TextView

public class MainActivity extends AppCompatActivity {

    private View mContentView;
    private View mLoadingView;
    private int mShortAnimationDuration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mContentView = findViewById(R.id.content);
        mLoadingView = findViewById(R.id.loading_spinner);
        mContentView.setVisibility(View.GONE);

        // Retrieve and cache the system's default "short" animation time.
        mShortAnimationDuration = getResources().getInteger(android.R.integer.config_shortAnimTime);

        findViewById(R.id.click_btn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //渐变动画
                crossfade();
            }
        });
    }


    private void crossfade() {

        // Set the content view to 0% opacity but visible, so that it is visible
        // (but fully transparent) during the animation.
        mContentView.setAlpha(0f);
        mContentView.setVisibility(View.VISIBLE);

        // Animate the content view to 100% opacity, and clear any animation
        // listener set on the view.
        mContentView.animate()
                .alpha(1f)
                .setDuration(2000)
                .setListener(null);

        // Animate the loading view to 0% opacity. After the animation ends,
        // set its visibility to GONE as an optimization step (it won't
        // participate in layout passes, etc.)
        mLoadingView.animate()
                .alpha(0f)
                .setDuration(2000)
                .setListener(new AnimatorListenerAdapter() {
                    @Override
                    public void onAnimationEnd(Animator animation) {
                        mLoadingView.setVisibility(View.GONE);
                    }
                });
    }