缩放视图

这节课展示如何实现触摸缩放动画,它是比较实用的,例如在相册中用动画将缩略图扩展到全屏尺寸的图片。

下面是一个触摸缩放动画,内容是把缩略图展开到全屏幕显示。

缩放动画
点击设备屏幕来重新播放视频

如果你想提前查看完整的工作示例,请点击下载并运行示例应用程序,然后选择缩放示例。请参见下面文件的实现代码:

  • src/TouchHighlightImageButton.java(一个简单的帮助类,当按下图像按钮时会显示蓝色的触摸高亮。)

  • src/ZoomActivity.java

  • layout/activity_zoom.xml

创建视图

为想要缩放的内容创建布局文件,包含小版本和大版本的内容。下面的示例创建了一个缩略图并可点击的 ImageButton 和一个展示图片大图的 ImageView

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:padding="16dp">

        <ImageButton
            android:id="@+id/thumb_button_1"
            android:layout_width="100dp"
            android:layout_height="75dp"
            android:layout_marginRight="1dp"
            android:src="@drawable/thumb1"
            android:scaleType="centerCrop"
            android:contentDescription="@string/description_image_1" />

    </LinearLayout>

    <!-- This initially-hidden ImageView will hold the expanded/zoomed version of
         the images above. Without transformations applied, it takes up the entire
         screen. To achieve the "zoom" animation, this view's bounds are animated
         from the bounds of the thumbnail button above, to its final laid-out
         bounds.
         -->

    <ImageView
        android:id="@+id/expanded_image"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="invisible"
        android:contentDescription="@string/description_zoom_touch_close" />

</FrameLayout>

设置缩放动画

当应用布局后,需要设置事件用来触发缩放动画。下面的例子为 ImageButton 添加了一个 View.OnClickListener,当用户点击按钮时执行缩放动画。


public class ZoomActivity extends FragmentActivity {
    // Hold a reference to the current animator,
    // so that it can be canceled mid-way.
    private Animator mCurrentAnimator;

    // The system "short" animation time duration, in milliseconds. This
    // duration is ideal for subtle animations or animations that occur
    // very frequently.
    private int mShortAnimationDuration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_zoom);

        // Hook up clicks on the thumbnail views.

        final View thumb1View = findViewById(R.id.thumb_button_1);
        thumb1View.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                zoomImageFromThumb(thumb1View, R.drawable.image1);
            }
        });

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

缩放视图

你需要在合适的时候进行缩放动画。一般来说,使用动画时需要按照边界来把正常尺寸的视图扩展到大尺寸的视图,下面的方法展示了如何实现缩放动画:

  1. 把高清图像资源设置到已经被隐藏的“放大版”的 ImageView 中。下面的示例在 UI 线程上加载一个大的图像资源以简化操作。但是我们需要在一个单独的线程中来加载以免阻塞UI线程,然后再回到UI线程中设置。理想状况下,图片不要大过屏幕尺寸。

  2. 计算 ImageView 开始和结束时的边界。

  3. 从起始边到结束边同步地动态改变四个点的位置和大小属性 XYSCALE_XSCALE_Y)。这四个动画被加入到了 AnimatorSet,所以它们可以同一时间开始。

  4. 缩小已经被放大的图片,当用户点击时只需要执行类似的相反动画。我们可以在 ImageView 中添加一个 View.OnClickListener 来实现它。当点击时,ImageView 缩回到原来缩略图的大小,然后设置它的 visibility 为 GONE 来隐藏。


private void zoomImageFromThumb(final View thumbView, int imageResId) {
    // If there's an animation in progress, cancel it
    // immediately and proceed with this one.
    if (mCurrentAnimator != null) {
        mCurrentAnimator.cancel();
    }

    // Load the high-resolution "zoomed-in" image.
    final ImageView expandedImageView = (ImageView) findViewById(
            R.id.expanded_image);
    expandedImageView.setImageResource(imageResId);

    // Calculate the starting and ending bounds for the zoomed-in image.
    // This step involves lots of math. Yay, math.
    final Rect startBounds = new Rect();
    final Rect finalBounds = new Rect();
    final Point globalOffset = new Point();

    // The start bounds are the global visible rectangle of the thumbnail,
    // and the final bounds are the global visible rectangle of the container
    // view. Also set the container view's offset as the origin for the
    // bounds, since that's the origin for the positioning animation
    // properties (X, Y).
    thumbView.getGlobalVisibleRect(startBounds);
    findViewById(R.id.container)
            .getGlobalVisibleRect(finalBounds, globalOffset);
    startBounds.offset(-globalOffset.x, -globalOffset.y);
    finalBounds.offset(-globalOffset.x, -globalOffset.y);

    // Adjust the start bounds to be the same aspect ratio as the final
    // bounds using the "center crop" technique. This prevents undesirable
    // stretching during the animation. Also calculate the start scaling
    // factor (the end scaling factor is always 1.0).
    float startScale;
    if ((float) finalBounds.width() / finalBounds.height()
            > (float) startBounds.width() / startBounds.height()) {
        // Extend start bounds horizontally
        startScale = (float) startBounds.height() / finalBounds.height();
        float startWidth = startScale * finalBounds.width();
        float deltaWidth = (startWidth - startBounds.width()) / 2;
        startBounds.left -= deltaWidth;
        startBounds.right += deltaWidth;
    } else {
        // Extend start bounds vertically
        startScale = (float) startBounds.width() / finalBounds.width();
        float startHeight = startScale * finalBounds.height();
        float deltaHeight = (startHeight - startBounds.height()) / 2;
        startBounds.top -= deltaHeight;
        startBounds.bottom += deltaHeight;
    }

    // Hide the thumbnail and show the zoomed-in view. When the animation
    // begins, it will position the zoomed-in view in the place of the
    // thumbnail.
    thumbView.setAlpha(0f);
    expandedImageView.setVisibility(View.VISIBLE);

    // Set the pivot point for SCALE_X and SCALE_Y transformations
    // to the top-left corner of the zoomed-in view (the default
    // is the center of the view).
    expandedImageView.setPivotX(0f);
    expandedImageView.setPivotY(0f);

    // Construct and run the parallel animation of the four translation and
    // scale properties (X, Y, SCALE_X, and SCALE_Y).
    AnimatorSet set = new AnimatorSet();
    set
            .play(ObjectAnimator.ofFloat(expandedImageView, View.X,
                    startBounds.left, finalBounds.left))
            .with(ObjectAnimator.ofFloat(expandedImageView, View.Y,
                    startBounds.top, finalBounds.top))
            .with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_X,
            startScale, 1f)).with(ObjectAnimator.ofFloat(expandedImageView,
                    View.SCALE_Y, startScale, 1f));
    set.setDuration(mShortAnimationDuration);
    set.setInterpolator(new DecelerateInterpolator());
    set.addListener(new AnimatorListenerAdapter() {
        @Override
        public void onAnimationEnd(Animator animation) {
            mCurrentAnimator = null;
        }

        @Override
        public void onAnimationCancel(Animator animation) {
            mCurrentAnimator = null;
        }
    });
    set.start();
    mCurrentAnimator = set;

    // Upon clicking the zoomed-in image, it should zoom back down
    // to the original bounds and show the thumbnail instead of
    // the expanded image.
    final float startScaleFinal = startScale;
    expandedImageView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (mCurrentAnimator != null) {
                mCurrentAnimator.cancel();
            }

            // Animate the four positioning/sizing properties in parallel,
            // back to their original values.
            AnimatorSet set = new AnimatorSet();
            set.play(ObjectAnimator
                        .ofFloat(expandedImageView, View.X, startBounds.left))
                        .with(ObjectAnimator
                                .ofFloat(expandedImageView,
                                        View.Y,startBounds.top))
                        .with(ObjectAnimator
                                .ofFloat(expandedImageView,
                                        View.SCALE_X, startScaleFinal))
                        .with(ObjectAnimator
                                .ofFloat(expandedImageView,
                                        View.SCALE_Y, startScaleFinal));
            set.setDuration(mShortAnimationDuration);
            set.setInterpolator(new DecelerateInterpolator());
            set.addListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    thumbView.setAlpha(1f);
                    expandedImageView.setVisibility(View.GONE);
                    mCurrentAnimator = null;
                }

                @Override
                public void onAnimationCancel(Animator animation) {
                    thumbView.setAlpha(1f);
                    expandedImageView.setVisibility(View.GONE);
                    mCurrentAnimator = null;
                }
            });
            set.start();
            mCurrentAnimator = set;
        }
    });
}

翻译:@misparking
审核:@iOnesmile
原始文档:https://developer.android.google.cn/training/animation/zoom.html#animate