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SwipeBackLayout源码解析

Github: SwipeBackLayout 分析版本： e4ddae6

SwipeBackLayout 是一个仿 IOS 通过手势退出界面的开源库。

SwipeBackLayout

SwipeBackLayout源码解析

SwipeBackLayout 可以通过在左、右和下边缘来拖动整个 Activity 达到退出 Activity 的效果。

使用

添加到 Gradle ：

compile 'me.imid.swipebacklayout.lib:library:1.0.0'

继承 SwipeBackActivity ：

public class DemoActivity extends SwipeBackActivity {
}

onCreate 中 setContentView() 照常使用
可以通过 getSwipeBackLayout() 定制 SwipeBackLayout

在 styles.xml 中的主题中添加：

<item name="android:windowIsTranslucent">true</item>

注意

需要在项目中添加最新的 supportV4 包

demo

public class DemoActivity extends SwipeBackActivity implements View.OnClickListener {
    private int[] mBgColors;

    private static int mBgIndex = 0;

    private String mKeyTrackingMode;

    private RadioGroup mTrackingModeGroup;

    private SwipeBackLayout mSwipeBackLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_demo);
        changeActionBarColor();
        findViews();
        mKeyTrackingMode = getString(R.string.key_tracking_mode);
        mSwipeBackLayout = getSwipeBackLayout();

        mTrackingModeGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(RadioGroup group, int checkedId) {
                int edgeFlag;
                switch (checkedId) {
                    case R.id.mode_left:
                        edgeFlag = SwipeBackLayout.EDGE_LEFT;
                        break;
                    case R.id.mode_right:
                        edgeFlag = SwipeBackLayout.EDGE_RIGHT;
                        break;
                    case R.id.mode_bottom:
                        edgeFlag = SwipeBackLayout.EDGE_BOTTOM;
                        break;
                    default:
                        edgeFlag = SwipeBackLayout.EDGE_ALL;
                }
                mSwipeBackLayout.setEdgeTrackingEnabled(edgeFlag);
                saveTrackingMode(edgeFlag);
            }
        });
    }
...

源码

SwipeBackActivity

public class SwipeBackActivity extends AppCompatActivity implements SwipeBackActivityBase {
    private SwipeBackActivityHelper mHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mHelper = new SwipeBackActivityHelper(this);
        mHelper.onActivityCreate();
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        mHelper.onPostCreate();
    }

    @Override
    public View findViewById(int id) {
        View v = super.findViewById(id);
        if (v == null && mHelper != null)
            return mHelper.findViewById(id);
        return v;
    }

    @Override
    public SwipeBackLayout getSwipeBackLayout() {//SwipeBackActivityBase接口中的方法
        return mHelper.getSwipeBackLayout();
    }

    @Override
    public void setSwipeBackEnable(boolean enable) {//SwipeBackActivityBase接口中的方法
        getSwipeBackLayout().setEnableGesture(enable);
    }

    @Override
    public void scrollToFinishActivity() {//SwipeBackActivityBase接口中的方法
        Utils.convertActivityToTranslucent(this);
        getSwipeBackLayout().scrollToFinishActivity();
    }
}

在 SwipeBackActivity 中实现了 SwipeBackActivityBase 接口，在 Activity 的生命周期函数 onCreate() 中创建了 SwipeBackActivityHelper 对象，该类的作用是设置 Activity 的透明和在 DecorView 中替换 SwipeBackLayout 。 onPostCreate() 是在 Activity 完全运行起来之后才会被调用。其中 findViewById() 方法进行了判断，首先在 Activity 的 contentView 中获取，获取不到再到 SwipeBackLayout 中获取。

SwipeBackActivityHelper

在 SwipeBackActivity 的 onCreate() 中的调用方法：

public class SwipeBackActivityHelper {
 
 private SwipeBackLayout mSwipeBackLayout;

    public SwipeBackActivityHelper(Activity activity) {
        mActivity = activity;
    }

    @SuppressWarnings("deprecation")
    public void onActivityCreate() {
        //设置Window的background为透明
        mActivity.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
        //设置decorView没有background
        mActivity.getWindow().getDecorView().setBackgroundDrawable(null);
        //inflate一个SwipeBackLayout出来
        mSwipeBackLayout = (SwipeBackLayout) LayoutInflater.from(mActivity).inflate(
                me.imid.swipebacklayout.lib.R.layout.swipeback_layout, null);
        //设置手势滑动监听器
        mSwipeBackLayout.addSwipeListener(new SwipeBackLayout.SwipeListener() {
            @Override
            public void onScrollStateChange(int state, float scrollPercent) {
            }

            @Override
            public void onEdgeTouch(int edgeFlag) {
                //当有边界触摸的时候设置成透明的
                Utils.convertActivityToTranslucent(mActivity);
            }

            @Override
            public void onScrollOverThreshold() {

            }
        });
    }
}

在 onActivityCreate 中主要就是将 window 、 decorView 的背景设置为透明的。

在 SwipeBackActivity 的 onPostCreate() 中的调用方法：

public class SwipeBackActivityHelper {
 public void onPostCreate() {
        mSwipeBackLayout.attachToActivity(mActivity);
    }
}

在 attachToActivity 中的操作就是将 decorView 中的 childView 换成 SwipeBackLayout ，然后将 childView 添加到 SwipeBackLayout 中。

其他的方法：

public class SwipeBackActivityHelper {
    public View findViewById(int id) {
        if (mSwipeBackLayout != null) {
            return mSwipeBackLayout.findViewById(id);
        }
        return null;
    }

    public SwipeBackLayout getSwipeBackLayout() {
        return mSwipeBackLayout;
    }
}

SwipeBackLayout

SwipeBackLayout 是一个 View ，可以从构造函数开始看：

public class SwipeBackLayout extends FrameLayout {
  
 /**
     * Minimum velocity that will be detected as a fling
     */
    private static final int MIN_FLING_VELOCITY = 400; // dips per second
  
    private static final int[] EDGE_FLAGS = {
            EDGE_LEFT, EDGE_RIGHT, EDGE_BOTTOM, EDGE_ALL
    };
 private int mEdgeFlag;
  
 private ViewDragHelper mDragHelper;

 public SwipeBackLayout(Context context) {
        this(context, null);
    }

    public SwipeBackLayout(Context context, AttributeSet attrs) {
        this(context, attrs, R.attr.SwipeBackLayoutStyle);
    }

    public SwipeBackLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs);
        mDragHelper = ViewDragHelper.create(this, new ViewDragCallback());

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.SwipeBackLayout, defStyle,
                R.style.SwipeBackLayout);

        //与边缘可拖动的距离
        int edgeSize = a.getDimensionPixelSize(R.styleable.SwipeBackLayout_edge_size, -1);
        if (edgeSize > 0) {
            //设置给ViewDragHelper
            setEdgeSize(edgeSize);
        }
        //边缘模式，分为EDGE_LEFT, EDGE_RIGHT, EDGE_BOTTOM, EDGE_ALL
        int mode = EDGE_FLAGS[a.getInt(R.styleable.SwipeBackLayout_edge_flag, 0)];
        //设置给ViewDragHelper
        setEdgeTrackingEnabled(mode);
        //边缘滑动的时候的阴影
        int shadowLeft = a.getResourceId(R.styleable.SwipeBackLayout_shadow_left,
                R.drawable.shadow_left);
        int shadowRight = a.getResourceId(R.styleable.SwipeBackLayout_shadow_right,
                R.drawable.shadow_right);
        int shadowBottom = a.getResourceId(R.styleable.SwipeBackLayout_shadow_bottom,
                R.drawable.shadow_bottom);
        setShadow(shadowLeft, EDGE_LEFT);
        setShadow(shadowRight, EDGE_RIGHT);
        setShadow(shadowBottom, EDGE_BOTTOM);
        a.recycle();
        //得到密度
        final float density = getResources().getDisplayMetrics().density;
        //手势滑动最小速度
        final float minVel = MIN_FLING_VELOCITY * density;
        //设置给ViewDragHelper
        mDragHelper.setMinVelocity(minVel);
        mDragHelper.setMaxVelocity(minVel * 2f);
    }
  
    /**
     * Set the size of an edge. This is the range in pixels along the edges of
     * this view that will actively detect edge touches or drags if edge
     * tracking is enabled.
     *
     * @param size The size of an edge in pixels
     */
    public void setEdgeSize(int size) {
        mDragHelper.setEdgeSize(size);
    }
  
    /**
     * Enable edge tracking for the selected edges of the parent view. The
     * callback's
     * {@link me.imid.swipebacklayout.lib.ViewDragHelper.Callback#onEdgeTouched(int, int)}
     * and
     * {@link me.imid.swipebacklayout.lib.ViewDragHelper.Callback#onEdgeDragStarted(int, int)}
     * methods will only be invoked for edges for which edge tracking has been
     * enabled.
     *
     * @param edgeFlags Combination of edge flags describing the edges to watch
     * @see #EDGE_LEFT
     * @see #EDGE_RIGHT
     * @see #EDGE_BOTTOM
     */
    public void setEdgeTrackingEnabled(int edgeFlags) {
        mEdgeFlag = edgeFlags;
        mDragHelper.setEdgeTrackingEnabled(mEdgeFlag);
    }
  
 public void setShadow(int resId, int edgeFlag) {
        setShadow(getResources().getDrawable(resId), edgeFlag);
    }
  
 /**
     * Set a drawable used for edge shadow.
     *
     * @param shadow    Drawable to use
     * @param edgeFlag Combination of edge flags describing the edge to set
     * @see #EDGE_LEFT
     * @see #EDGE_RIGHT
     * @see #EDGE_BOTTOM
     */
    public void setShadow(Drawable shadow, int edgeFlag) {
        if ((edgeFlag & EDGE_LEFT) != 0) {
            mShadowLeft = shadow;
        } else if ((edgeFlag & EDGE_RIGHT) != 0) {
            mShadowRight = shadow;
        } else if ((edgeFlag & EDGE_BOTTOM) != 0) {
            mShadowBottom = shadow;
        }
        invalidate();
    }
  
   //处理ViewDragHelper
   @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        if (!mEnable) {
            return false;
        }
        try {
            return mDragHelper.shouldInterceptTouchEvent(event);
        } catch (ArrayIndexOutOfBoundsException e) {
            // FIXME: handle exception
            // issues #9
            return false;
        }
    }
  
   //处理ViewDragHelper
 @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (!mEnable) {
            return false;
        }
        mDragHelper.processTouchEvent(event);
        return true;
    }
}

SwipeBackLayout 继承自 FrameLayout ，其中手势的操作是通过 ViewDragHelper 来实现的。在构造函数中一些必要的参数设置给 ViewDragHelper 。

public class SwipeBackLayout extends FrameLayout {
   /**
     * Edge flag indicating that the left edge should be affected.
     */
    public static final int EDGE_LEFT = ViewDragHelper.EDGE_LEFT;

    /**
     * Edge flag indicating that the right edge should be affected.
     */
    public static final int EDGE_RIGHT = ViewDragHelper.EDGE_RIGHT;

    /**
     * Edge flag indicating that the bottom edge should be affected.
     */
    public static final int EDGE_BOTTOM = ViewDragHelper.EDGE_BOTTOM;

    /**
     * Edge flag set indicating all edges should be affected.
     */
    public static final int EDGE_ALL = EDGE_LEFT | EDGE_RIGHT | EDGE_BOTTOM;
 /**
     * Default threshold of scroll
     * 超过0.3f的屏幕比例的距离之后可以滑动出去了，临界值是0.3f
     */
    private static final float DEFAULT_SCROLL_THRESHOLD = 0.3f;

   private static final int OVERSCROLL_DISTANCE = 10;

 private float mScrimOpacity;

    /**
     * Edge being dragged
     */
    private int mTrackingEdge;
  
   //滑动了距离和整个屏幕的的百分比
   private float mScrollPercent;

    private int mContentLeft;

    private int mContentTop;
  
 /**
     * Threshold of scroll, we will close the activity, when scrollPercent over
     * this value;
     */
    private float mScrollThreshold = DEFAULT_SCROLL_THRESHOLD;

    private class ViewDragCallback extends ViewDragHelper.Callback {
        private boolean mIsScrollOverValid;

       //如果可拖动则返回true 否则为false
        @Override
        public boolean tryCaptureView(View view, int i) {//i是pointerId
           //是否touch到了边缘
            boolean ret = mDragHelper.isEdgeTouched(mEdgeFlag, i);
           //哪个边缘被touch了
            if (ret) {
                if (mDragHelper.isEdgeTouched(EDGE_LEFT, i)) {
                    mTrackingEdge = EDGE_LEFT;
                } else if (mDragHelper.isEdgeTouched(EDGE_RIGHT, i)) {
                    mTrackingEdge = EDGE_RIGHT;
                } else if (mDragHelper.isEdgeTouched(EDGE_BOTTOM, i)) {
                    mTrackingEdge = EDGE_BOTTOM;
                }
               //回调出去
                if (mListeners != null && !mListeners.isEmpty()) {
                    for (SwipeListener listener : mListeners) {
                        listener.onEdgeTouch(mTrackingEdge);
                    }
                }
                mIsScrollOverValid = true;
            }
            boolean directionCheck = false;
           //是否达到了滑动的门槛
            if (mEdgeFlag == EDGE_LEFT || mEdgeFlag == EDGE_RIGHT) {
                directionCheck = !mDragHelper.checkTouchSlop(ViewDragHelper.DIRECTION_VERTICAL, i);
            } else if (mEdgeFlag == EDGE_BOTTOM) {
                directionCheck = !mDragHelper
                        .checkTouchSlop(ViewDragHelper.DIRECTION_HORIZONTAL, i);
            } else if (mEdgeFlag == EDGE_ALL) {
                directionCheck = true;
            }
            return ret & directionCheck;
        }

       //返回指定View在横向上能滑动的最大距离
        @Override
        public int getViewHorizontalDragRange(View child) {
            return mEdgeFlag & (EDGE_LEFT | EDGE_RIGHT);
        }

       //返回指定View在纵向上能滑动的最大距离
        @Override
        public int getViewVerticalDragRange(View child) {
            return mEdgeFlag & EDGE_BOTTOM;
        }

       //当子视图位置变化时，会回调这个函数
        @Override
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            super.onViewPositionChanged(changedView, left, top, dx, dy);
           //计算当前滑动比例
            if ((mTrackingEdge & EDGE_LEFT) != 0) {
                mScrollPercent = Math.abs((float) left
                        / (mContentView.getWidth() + mShadowLeft.getIntrinsicWidth()));
            } else if ((mTrackingEdge & EDGE_RIGHT) != 0) {
                mScrollPercent = Math.abs((float) left
                        / (mContentView.getWidth() + mShadowRight.getIntrinsicWidth()));
            } else if ((mTrackingEdge & EDGE_BOTTOM) != 0) {
                mScrollPercent = Math.abs((float) top
                        / (mContentView.getHeight() + mShadowBottom.getIntrinsicHeight()));
            }
            mContentLeft = left;
            mContentTop = top;
            invalidate();
           //当滑动比例小于可滑动出去的时候，且mIsScrollOverValid已经为false的时候
            if (mScrollPercent < mScrollThreshold && !mIsScrollOverValid) {
                mIsScrollOverValid = true;
            }
            if (mListeners != null && !mListeners.isEmpty()
                    && mDragHelper.getViewDragState() == STATE_DRAGGING
                    && mScrollPercent >= mScrollThreshold && mIsScrollOverValid) {
                mIsScrollOverValid = false;
               //回调出去，已经达到可以滑出结束Activity的标准了
                for (SwipeListener listener : mListeners) {
                    listener.onScrollOverThreshold();
                }
            }
   //当比例大于等于1的时候，就可以关闭掉Activity了
            if (mScrollPercent >= 1) {
                if (!mActivity.isFinishing()) {
                    mActivity.finish();
                    mActivity.overridePendingTransition(0, 0);
                }
            }
        }
 
       //当手指从子视图松开时，会调用这个函数，同时返回在x轴和y轴上当前的速度
        @Override
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            final int childWidth = releasedChild.getWidth();
            final int childHeight = releasedChild.getHeight();

            int left = 0, top = 0;
            if ((mTrackingEdge & EDGE_LEFT) != 0) {//左边边缘
               //速度满足>=0且已经滑过了临界点0.3f，滑到最右边，不然滑到0的位置
                left = xvel > 0 || xvel == 0 && mScrollPercent > mScrollThreshold ? childWidth + mShadowLeft.getIntrinsicWidth() + OVERSCROLL_DISTANCE : 0;
            } else if ((mTrackingEdge & EDGE_RIGHT) != 0) {//右边边缘
    //速度满足>=0且已经滑过了临界点0.3f，滑到最左边，不然滑到0的位置
                left = xvel < 0 || xvel == 0 && mScrollPercent > mScrollThreshold ? -(childWidth + mShadowLeft.getIntrinsicWidth() + OVERSCROLL_DISTANCE) : 0;
            } else if ((mTrackingEdge & EDGE_BOTTOM) != 0) {//上边边缘
    //速度满足>=0且已经滑过了临界点0.3f，滑到最下边，不然滑到0的位置
                top = yvel < 0 || yvel == 0 && mScrollPercent > mScrollThreshold ? -(childHeight + mShadowBottom.getIntrinsicHeight() + OVERSCROLL_DISTANCE) : 0;
            }
   //移动View
            mDragHelper.settleCapturedViewAt(left, top);
           //刷新View
            invalidate();
        }
   
       //返回一个值，告诉Helper，这个view能滑动的最大（或者负向最大）的横向坐标
        @Override
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            int ret = 0;
            if ((mTrackingEdge & EDGE_LEFT) != 0) {
                ret = Math.min(child.getWidth(), Math.max(left, 0));
            } else if ((mTrackingEdge & EDGE_RIGHT) != 0) {
                ret = Math.min(0, Math.max(left, -child.getWidth()));
            }
            return ret;
        }

       //返回一个值，告诉Helper，这个view能滑动的最大（或者负向最大）的纵向坐标
        @Override
        public int clampViewPositionVertical(View child, int top, int dy) {
            int ret = 0;
            if ((mTrackingEdge & EDGE_BOTTOM) != 0) {
                ret = Math.min(0, Math.max(top, -child.getHeight()));
            }
            return ret;
        }

       //当边缘开始拖动的时候，会调用这个回调
        @Override
        public void onViewDragStateChanged(int state) {
            super.onViewDragStateChanged(state);
            if (mListeners != null && !mListeners.isEmpty()) {
                for (SwipeListener listener : mListeners) {
                    listener.onScrollStateChange(state, mScrollPercent);
                }
            }
        }
    }
  
    @Override
    public void computeScroll() {
       //调用mDragHelper.settleCapturedViewAt(left, top)之后会进到这里
        mScrimOpacity = 1 - mScrollPercent;
        if (mDragHelper.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }
  
 @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        mInLayout = true;
        if (mContentView != null) {
            mContentView.layout(mContentLeft, mContentTop,
                    mContentLeft + mContentView.getMeasuredWidth(),
                    mContentTop + mContentView.getMeasuredHeight());
        }
        mInLayout = false;
    }
    
    @Override
    public void requestLayout() {
        if (!mInLayout) {
            super.requestLayout();
        }
    }
}

在 ViewDragHelper.Callback 的手势判断中，处理的主要逻辑主要在 tryCaptureView 、 onViewPositionChanged 、 onViewReleased 三个方法中，分别是在准备滑动、滑动时、和放手的时候的逻辑。

在 tryCaptureView 中主要进行了边缘的判断，以及是否满足滑动条件；在 onViewPositionChanged 中计算了当前滑动距离与整个 ContentView 的距离的比例，是否超越临界值等；在 onViewReleased 中处理了手抬起之后的操作，比如将 View 滑归位或者滑出去等。

现在基本上了解了滑动的机制了，那么回过头来看看 attachToActivity ：

public class SwipeBackLayout extends FrameLayout {
  
    private View mContentView;

    public void attachToActivity(Activity activity) {
        mActivity = activity;
        TypedArray a = activity.getTheme().obtainStyledAttributes(new int[]{
                android.R.attr.windowBackground
        });
        int background = a.getResourceId(0, 0);
        a.recycle();

        ViewGroup decor = (ViewGroup) activity.getWindow().getDecorView();
  // 拿到decorView的第一个子view
        ViewGroup decorChild = (ViewGroup) decor.getChildAt(0);
        decorChild.setBackgroundResource(background);
       //把这个decorChild从decorView删除掉
        decor.removeView(decorChild);
       //将decorView添加到SwipeBackLayout中
        addView(decorChild);
       //将decorChild赋值给成员变量mContentView
        setContentView(decorChild);
        // 在DecorView下增加SwipeBackLayout
        decor.addView(this);
    }
  
 /**
     * Set up contentView which will be moved by user gesture
     *
     * @param view
     */
    private void setContentView(View view) {
        mContentView = view;
    }
}

通过 attachToActivity 将 decorView 中的 contentView 换成了 SwipeBackLayout ，而 contentView 则被添加到了 SwipeBackLayout 中。与正常的相比，之间多了一个 SwipeBackLayout 。

在滑动的时候哪些阴影是怎么出现的呢：

public class SwipeBackLayout extends FrameLayout {
  
 private static final int DEFAULT_SCRIM_COLOR = 0x99000000;

    private float mScrimOpacity;
  
    private int mScrimColor = DEFAULT_SCRIM_COLOR;
  
 private float mScrollPercent;
  
 private Drawable mShadowLeft;
    private Drawable mShadowRight;
    private Drawable mShadowBottom;
  
   private Rect mTmpRect = new Rect();
  
 @Override
    public void computeScroll() {
        mScrimOpacity = 1 - mScrollPercent;
        if (mDragHelper.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }
  
    @Override
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        final boolean drawContent = child == mContentView;

        boolean ret = super.drawChild(canvas, child, drawingTime);
        if (mScrimOpacity > 0 && drawContent
                && mDragHelper.getViewDragState() != ViewDragHelper.STATE_IDLE) {
            drawShadow(canvas, child);
            drawScrim(canvas, child);
        }
        return ret;
    }
  
    private void drawScrim(Canvas canvas, View child) {
       //得到alpha值
        final int baseAlpha = (mScrimColor & 0xff000000) >>> 24;
       //得到新的alpha值
        final int alpha = (int) (baseAlpha * mScrimOpacity);
       //得到新的color
        final int color = alpha << 24 | (mScrimColor & 0xffffff);
  //绘制
        if ((mTrackingEdge & EDGE_LEFT) != 0) {
            canvas.clipRect(0, 0, child.getLeft(), getHeight());
        } else if ((mTrackingEdge & EDGE_RIGHT) != 0) {
            canvas.clipRect(child.getRight(), 0, getRight(), getHeight());
        } else if ((mTrackingEdge & EDGE_BOTTOM) != 0) {
            canvas.clipRect(child.getLeft(), child.getBottom(), getRight(), getHeight());
        }
        canvas.drawColor(color);
    }
 
   
    private void drawShadow(Canvas canvas, View child) {
        final Rect childRect = mTmpRect;
       //得到当前View的位置
        child.getHitRect(childRect);

        if ((mEdgeFlag & EDGE_LEFT) != 0) {
           //给drawable设置位置
            mShadowLeft.setBounds(childRect.left - mShadowLeft.getIntrinsicWidth(), childRect.top, childRect.left, childRect.bottom);
           //设置透明度
            mShadowLeft.setAlpha((int) (mScrimOpacity * FULL_ALPHA));
           //画到canvas上
            mShadowLeft.draw(canvas);
        }
  //给drawable设置位置、设置透明度、画到canvas上
        if ((mEdgeFlag & EDGE_RIGHT) != 0) {
            mShadowRight.setBounds(childRect.right, childRect.top, childRect.right + mShadowRight.getIntrinsicWidth(), childRect.bottom);
            mShadowRight.setAlpha((int) (mScrimOpacity * FULL_ALPHA));
            mShadowRight.draw(canvas);
        }
  //给drawable设置位置、设置透明度、画到canvas上
        if ((mEdgeFlag & EDGE_BOTTOM) != 0) {
            mShadowBottom.setBounds(childRect.left, childRect.bottom, childRect.right, childRect.bottom + mShadowBottom.getIntrinsicHeight());
            mShadowBottom.setAlpha((int) (mScrimOpacity * FULL_ALPHA));
            mShadowBottom.draw(canvas);
        }
    }
}

就这样，阴影就绘制出来了。

再看看 scrollToFinishActivity ：

public class SwipeBackLayout extends FrameLayout {
 /**
     * Scroll out contentView and finish the activity
     */
    public void scrollToFinishActivity() {
       //得到contentView的宽高
        final int childWidth = mContentView.getWidth();
        final int childHeight = mContentView.getHeight();
  //要移动到的位置
        int left = 0, top = 0;
        if ((mEdgeFlag & EDGE_LEFT) != 0) {
            left = childWidth + mShadowLeft.getIntrinsicWidth() + OVERSCROLL_DISTANCE;
            mTrackingEdge = EDGE_LEFT;
        } else if ((mEdgeFlag & EDGE_RIGHT) != 0) {
            left = -childWidth - mShadowRight.getIntrinsicWidth() - OVERSCROLL_DISTANCE;
            mTrackingEdge = EDGE_RIGHT;
        } else if ((mEdgeFlag & EDGE_BOTTOM) != 0) {
            top = -childHeight - mShadowBottom.getIntrinsicHeight() - OVERSCROLL_DISTANCE;
            mTrackingEdge = EDGE_BOTTOM;
        }

        mDragHelper.smoothSlideViewTo(mContentView, left, top);
        invalidate();
    }
  
 @Override
    public void computeScroll() {
       //调用mDragHelper.smoothSlideViewTo(mContentView, left, top);之后进到这里
        mScrimOpacity = 1 - mScrollPercent;
        if (mDragHelper.continueSettling(true)) {
            ViewCompat.postInvalidateOnAnimation(this);
        }
    }
}

Utils

public class Utils {
    private Utils() {
    }
  
 /**
     * Convert a translucent themed Activity
     * {@link android.R.attr#windowIsTranslucent} back from opaque to
     * translucent following a call to
     * {@link #convertActivityFromTranslucent(android.app.Activity)} .
     * <p>
     * Calling this allows the Activity behind this one to be seen again. Once
     * all such Activities have been redrawn
     * <p>
     * This call has no effect on non-translucent activities or on activities
     * with the {@link android.R.attr#windowIsFloating} attribute.
     */
    public static void convertActivityToTranslucent(Activity activity) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            convertActivityToTranslucentAfterL(activity);
        } else {
            convertActivityToTranslucentBeforeL(activity);
        }
    }
  
 /**
     * Calling the convertToTranslucent method on platforms before Android 5.0
     */
    public static void convertActivityToTranslucentBeforeL(Activity activity) {
        try {
            Class<?>[] classes = Activity.class.getDeclaredClasses();
            Class<?> translucentConversionListenerClazz = null;
            for (Class clazz : classes) {
                if (clazz.getSimpleName().contains("TranslucentConversionListener")) {
                    translucentConversionListenerClazz = clazz;
                }
            }
            Method method = Activity.class.getDeclaredMethod("convertToTranslucent",
                    translucentConversionListenerClazz);
            method.setAccessible(true);
            method.invoke(activity, new Object[] {
                null
            });
        } catch (Throwable t) {
        }
    }

    /**
     * Calling the convertToTranslucent method on platforms after Android 5.0
     */
    private static void convertActivityToTranslucentAfterL(Activity activity) {
        try {
            Method getActivityOptions = Activity.class.getDeclaredMethod("getActivityOptions");
            getActivityOptions.setAccessible(true);
            Object options = getActivityOptions.invoke(activity);

            Class<?>[] classes = Activity.class.getDeclaredClasses();
            Class<?> translucentConversionListenerClazz = null;
            for (Class clazz : classes) {
                if (clazz.getSimpleName().contains("TranslucentConversionListener")) {
                    translucentConversionListenerClazz = clazz;
                }
            }
            Method convertToTranslucent = Activity.class.getDeclaredMethod("convertToTranslucent",
                    translucentConversionListenerClazz, ActivityOptions.class);
            convertToTranslucent.setAccessible(true);
            convertToTranslucent.invoke(activity, null, options);
        } catch (Throwable t) {
        }
    }
}