Android T多屏多显——应用双屏间拖拽移动功能（更新中）

功能以及显示效果简介

需求：在双屏显示中，把启动的应用从其中一个屏幕中移动到另一个屏幕中。
操作：通过双指按压应用使其移动，如果移动的距离过小，我们就不移动到另一屏幕，否则移动到另一屏。

功能分析

多屏中移动应用至另一屏本质就是Task的移动。
从窗口层级结构的角度来说，就是把Display1中的DefaultTaskDisplayArea上的Task，移动到Display2中的DefaultTaskDisplayArea上

关键代码知识点

移动Task至另一屏幕

代码路径：frameworks/base/services/core/java/com/android/server/wm/RootWindowContainer.java

    /**
     * Move root task with all its existing content to specified display.
     *
     * @param rootTaskId Id of root task to move.
     * @param displayId  Id of display to move root task to.
     * @param onTop      Indicates whether container should be place on top or on bottom.
     */
    void moveRootTaskToDisplay(int rootTaskId, int displayId, boolean onTop) {
        final DisplayContent displayContent = getDisplayContentOrCreate(displayId);
        if (displayContent == null) {
            throw new IllegalArgumentException("moveRootTaskToDisplay: Unknown displayId="
                    + displayId);
        }

        moveRootTaskToTaskDisplayArea(rootTaskId, displayContent.getDefaultTaskDisplayArea(),
                onTop);
    }
    
    
    
    /**
     * Move root task with all its existing content to specified task display area.
     *
     * @param rootTaskId      Id of root task to move.
     * @param taskDisplayArea The task display area to move root task to.
     * @param onTop           Indicates whether container should be place on top or on bottom.
     */
    void moveRootTaskToTaskDisplayArea(int rootTaskId, TaskDisplayArea taskDisplayArea,
            boolean onTop) {
        final Task rootTask = getRootTask(rootTaskId);
        if (rootTask == null) {
            throw new IllegalArgumentException("moveRootTaskToTaskDisplayArea: Unknown rootTaskId="
                    + rootTaskId);
        }

        final TaskDisplayArea currentTaskDisplayArea = rootTask.getDisplayArea();
        if (currentTaskDisplayArea == null) {
            throw new IllegalStateException("moveRootTaskToTaskDisplayArea: rootTask=" + rootTask
                    + " is not attached to any task display area.");
        }

        if (taskDisplayArea == null) {
            throw new IllegalArgumentException(
                    "moveRootTaskToTaskDisplayArea: Unknown taskDisplayArea=" + taskDisplayArea);
        }

        if (currentTaskDisplayArea == taskDisplayArea) {
            throw new IllegalArgumentException("Trying to move rootTask=" + rootTask
                    + " to its current taskDisplayArea=" + taskDisplayArea);
        }
        rootTask.reparent(taskDisplayArea, onTop);

        // Resume focusable root task after reparenting to another display area.
        rootTask.resumeNextFocusAfterReparent();

        // TODO(multi-display): resize rootTasks properly if moved from split-screen.
    }