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45 lines
3.2 KiB
45 lines
3.2 KiB
7 months ago
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# Pointer Capture in InputFlinger
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## Introduction
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[Pointer Capture](https://developer.android.com/training/gestures/movement#pointer-capture) is a feature that was introduced to the Android input pipeline in Android 8.0 (Oreo). Pointer Capture can be enabled or disabled for an `InputWindow` through requests to `InputManagerService`. Enabling Pointer Capture performs the following changes related to the mouse cursor and the devices that control it:
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- The position of the mouse cursor is fixed to its location before Pointer Capture was enabled.
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- The mouse cursor is hidden.
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- Events from a mouse will be delivered with the source `SOURCE_MOUSE_RELATIVE`, and their `AXIS_X` and `AXIS_Y` will report relative position changes.
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- Events from a touchpad will be delivered with the source `SOURCE_TOUCHPAD`, and their `AXIS_X` and `AXIS_Y` will report the absolute position of each of the pointers on the touchpad.
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- Events from mouse and touchpad devices are dispatched to the focused `InputWindow`.
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- Events from devices that do not normally control the mouse cursor are not affected.
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`InputWindow`s can only gain Pointer Capture if they have window focus. If a window with Pointer Capture loses focus, Pointer Capture is disabled.
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## Pointer Capture pipeline in InputFlinger
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`InputDispatcher` is responsible for controlling the state of Pointer Capture. Since the feature requires changes to how events are generated, Pointer Capture is configured in `InputReader`.
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### Enabling Pointer Capture
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There are four key steps that take place when Pointer Capture is enabled:
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1. Requests to enable Pointer Capture are forwarded from `InputManagerService` to `InputDispatcher`.
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2. If the window that makes the request has focus, `InputDispatcher` enables the Pointer Capture state in `InputReader` through the `InputDispatcherPolicy`.
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3. When `InputReader` is successfully configured, it notifies `InputDispatcher` through the `InputListener` interface.
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4. `InputDispatcher` then notifies the `InputWindow` that Pointer Capture has been enabled by sending a special `CAPTURE` event through the `InputChannel`.
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### Disabling Pointer Capture
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Pointer Capture can be disabled in two ways: by a request through `InputManagerService`, and as a result of the `InputWindow` losing focus.
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When Pointer Capture is disabled by a request from the application, it follows the same pipeline as when Pointer Capture is enabled.
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#### Window loses Pointer Capture when it loses focus
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When an `InputWindow` with Pointer Capture loses focus, Pointer Capture is disabled immediately. The `InputWindow` receives a `CAPTURE` event through the `InputChannel`, followed by a `FOCUS` event to notify loss of focus.
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## Pointer Capture in `InputDispatcher`
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`InputDispatcher` tracks two pieces of state information regarding Pointer Capture:
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- `mFocusedWindowRequestedPointerCapture`: Whether or not the focused window has requested Pointer Capture. This is updated whenever the Dispatcher receives requests from `InputManagerService`.
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- `mWindowTokenWithPointerCapture`: The Binder token of the `InputWindow` that currently has Pointer Capture. This is only updated during the dispatch cycle. If it is not `nullptr`, it signifies that the window was notified that it has Pointer Capture.
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