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1.前言

本篇目的：Android图形系统中，HWC特别的复杂，特别是HWComposer、ComposerImpl、Composer、Hwc2::Composer之间的关系，有种剪不断理还乱的感觉，通过几个演化例子，看清楚它本来的面目。

2.HWComposer、ComposerImpl、Composer、Hwc2::Composer定义与实现

1.HWComposer实现

frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.h

<1>.android::HWComposer定义

namespace Hwc2 {
class Composer;
}namespace android {
class HWComposer {virtual ~HWComposer();virtual void setCallback(HWC2::ComposerCallback*) = 0;....
};
}

<2>.impl::HWComposer定义(继承自android::HWComposer)

namespace impl {class HWComposer final : public android::HWComposer {
public:explicit HWComposer(std::unique_ptr<Hwc2::Composer> composer);explicit HWComposer(const std::string& composerServiceName);~HWComposer() override;
};
}

<3>.impl::HWComposer实现

frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

namespace impl {HWComposer::HWComposer(std::unique_ptr<Hwc2::Composer> composer): mComposer(std::move(composer)),mMaxVirtualDisplayDimension(static_cast<size_t>(sysprop::max_virtual_display_dimension(0))),mUpdateDeviceProductInfoOnHotplugReconnect(sysprop::update_device_product_info_on_hotplug_reconnect(false)) {}HWComposer::HWComposer(const std::string& composerServiceName): HWComposer(std::make_unique<Hwc2::impl::Composer>(composerServiceName)) {}HWComposer::~HWComposer() {mDisplayData.clear();
}
}

HWComposer构造函数通过它的委托构造函数，通过std::make_uniqueHwc2::impl::Composer(composerServiceName)实例画，那么Hwc2::impl::Composer的实现在哪呢？

2.ComposerImpl实现（Hwc2::Composer是它的别名）

hardware/interfaces/graphics/composer/2.1/utils/hal/include/composer-hal/2.1/Composer.h

<1>.Composer定义，它是ComposerImpl的别名

using Composer = detail::ComposerImpl<IComposer, ComposerHal>;
template <typename Interface, typename Hal>
class ComposerImpl : public Interface {public:static std::unique_ptr<ComposerImpl> create(std::unique_ptr<Hal> hal) {